CLINICAL PICTURE

In dogs, malignant lymphoma clinically manifests in different ways, depending on the form and location of the disease. The frequent form is multicentric, followed in decreasing order by the anterior mediastinal, gastrointestinal, and cutaneous forms; extralymphonodular forms: in the eye and the central nervous system.

The general clinical signs develop progressively and they are non-specific: weight loss, anorexia and adynamia.

The multicentric form starts with lymphadenopathy in the anterior regions of the body, with generalization in a short time, sometimes exaggerated hypertrophy, 2–10-fold higher than normal, with smooth or irregular surface, with slightly mobile lymph nodes, firm and non-painful on palpation. Hepato- and splenomegaly can be found. This form is found in 80% of malignant lymphoma cases in dogs.

The anterior mediastinal form determines clinical respiratory signs (dyspnea, cough, intolerance to exercise) due to tumor masses from lymph nodes, hypertrophy and thoracic effusions.

The gastrointestinal form determines malabsorption syndrome, weight loss that can result in cachexia. Mesenteric lymph nodes, the liver and/or spleen are frequently involved.

Cutaneous malignant lymphoma forms can mimic dermatological lesions, such as pyodermitis, fungal infections, demodex, autoimmune dermatosis, seborrhea, or other types of tumor proliferation.

Ocular lymphoma manifests by exophthalmia and blindness, optic nerve compressions, diffuse infiltrations of the anterior eye segments [30].

Central nervous system lymphoma has different manifestations depending on the multifocal aspect or on medullary compressions with pain, pareses or paralyses [10, 20, 42].

Atypical malignant lymphoma can be located in different organs or tissues such as the kidney, lung, liver, heart and testis [17, 38].

Lymphomatoid granulomas have been diagnosed in the lung, under the form of nodules 0.5–3 cm in diameter, with the involvement of mediastinal lymph nodes that are large, pale and firm. Metastases have been identified in the liver, kidney, spleen, pancreas and adrenal glands. Microscopically, neoplasms are formed by anaplastic mononuclear cells, with slightly eosinophilic cytoplasm, vesicular nuclei, with a moderate to high mitotic rate; giant binucleated and multinucleated cells. Lung tissue is infiltrated with pleomorphic cells, with necrosis and perivascular fibrinoid deposition [19].

Paraneoplastic syndromes in malignant lymphomas manifest by anemia, hypercalcemia, blood hyperviscosity, cachexia and other less frequent forms. Multiple myeloma, complicated by hyperviscosity syndrome, was diagnosed by BUBLOT et al. (1985) in a 9-year-old dog showing on necropsy fluids in the cavities and moderate hypertrophy of the spleen and liver, with an intensely hemorrhagic, orange size tumor in the spleen and kidney. Microscopy identified histioplasmacytic tumor cells in the bone marrow, liver and spleen. The more differentiated cells had a small, round, heterochromatin-rich nucleus and clearly basophilic cytoplasm, sometimes with a clear perinuclear area. Less differentiated cells had less dense, medium size nuclei, obvious nucleoli, perinuclear heterochromatin, and vacuolated cytoplasm, indistinct outline. In the bone marrow, differentiated tumor cells represented 90%. The kidneys were markedly changed, having tumor cells in the glomerules, PAS-positive material in the Bowman capsule and basal membranes. In the spleen, tumor cells infiltrated structures diffusely, with the presence of megakaryocytes.

Blood serum electrophoresis, performed at the time of consultation, showed “slightly monoclonal” β-globulins. The presence of glaucoma is known to be specific for the hyperviscosity syndrome.

Anemia is extremely frequent in dogs with malignant lymphomas, manifesting as normochromic, normocytic and aregenerative anemia. An inhibition of iron release from the mononuclear phagocyte system is frequently found, which opposes hemoglobin synthesis [30]. Extremely severe hemolytic anemia may occur, associated with secondary hypersplenism, lymphoid splenic infiltration or anemia as a result of disseminated intravascular coagulation [31].

Hypercalcemia is a paraneoplastic syndrome, relatively frequent in dogs with malignant lymphoma, being estimated at 10–40% of cases, more frequently in anterior mediastinal forms. There are many possible explanations for this: ectopic (tumor) parathormone secretion; ectopic prostaglandin PGE 2 secretion; ectopic antivitamin D sterol secretion; ectopic secretion of a “PTH-like” peptide that stimulates bone resorption called “osteoclast activation factor” (OAF). This factor would be a lymphokine produced by lymphocytes as an immune response to the neoplasm, or even by neoplastic lymphocytes. The dog can manifest nervous symptoms (stupor, coma, seizures), urinary symptoms (polyuria-polydipsia), digestive symptoms (anorexia, vomiting, diarrhea), muscular symptoms (weakness, muscle atrophy).

Monoclonal gammopathy is induced by an immunoglobulin secretion by the B lymphocytes involved in the cancer process, which sometimes develops in dogs with malignant lymphoma and leads to paraneoplastic syndromes of blood hyperviscosity and Bence-Jones proteinuria. The blood hyperviscosity syndrome is sometimes responsible for hemorrhagic diathesis (epistaxis and intestinal hemorrhage), through platelet aggregation defects and a diminution in platelet factor III, or for the involvement of the central nervous system (dementia, ataxia or coma) through cerebral hypoxia. The ocular fundus examination is generally characteristic of this syndrome with changes in the venous pathways and tubular hemorrhage [5].

Cancer cachexia is frequently found in dogs with advanced malignant lymphoma, due to anorexia, digestive disorders and malabsorption secondary to chemotherapy.

Paraneoplastic syndromes that are more rarely found can be: polyglobulia induced by the abnormal secretion of erythropoietin or an “erythropoietin-like” substance; hypoglycemia, produced by an abnormal secretion of an “insulin-like” substance or by the excessive use of glucose by the tumor mass; appearance of glomerulonephritis [5].

PATHOMORPHOLOGICAL PICTURE

The incidence of lesions of different lymphoid organs or tissues can be hierarchized as follows: lymph nodes, 100%; spleen, 43–100%; tonsils, 40–95%; liver, 41–93%; lung, 11–68%; kidneys, 21–77%; stomach, 8–63%; thyroid, 22–57%; salivary glands, 45–53%; pancreas, 26–56%; adrenal glands, 22–25%; bone marrow, 63–83%; testes, 10%; heart, 4–14% [13].

Lymph nodes significantly increase in volume, sometimes even 10-fold more than normal; they are little adherent to tissues, the capsule is intact and highly distended. A homogeneous, succulent, white-gray tumor tissue appears in section, with frequent necrotic hemorrhagic foci, distributed throughout the whole lymph node mass. Structures are homogenized so that the cortical and medullary areas are no longer differentiated.

Microscopically, architecture is totally or partially changed, but the cortex and the medulla cannot be distinguished. The capsule is infiltrated and occasionally fragmented [52]. Hemorrhagic and/or necrotic foci are frequently found in the center of lymph nodes. Two types of proliferations are microscopically recognized: diffuse and follicular.

The diffuse form is characterized at onset by tumor cell proliferation in the cortical or paracortical area. In advanced forms, the proliferation extends to the lymph node, inducing the atrophy and disappearance of lymphoid follicles and the disorganization of normal structures that can no longer be recognized (Fig. 17.4. and 17.6.).

Fig. 17.4

Malignant lymphoma, lymphocytic, lymph node.

Fig. 17.6

Malignant lymphoma, histioblastic, lymph node.

The follicular or nodular form: at onset, the lesion proliferation has a follicular pattern; the marginal sinuses, paracortical and medullary areas are gradually obliterated, and finally, the capsule infiltration occurs. This follicular form seems to be associated with gastrointestinal lesions or the multicentric form.

The spleen is constantly hypertrophied, and in section it has white-gray spots in a heterogeneous reddish pulp. Encephaloid tumor nodules that deform the capsule are frequent.

Microscopically, in early forms, lymphoid formations are affected and subsequently invade the parenchyma, resulting in the effacement of cords and sinuses. Tumor infiltration may be linear-diffuse, periarteriolar to the white pulp, or multifocal peritrabecular and periarteriolar proliferation can occur (Fig. 17.19.).

Fig. 17.19

Myeloid neutrophilic leukemia, spleen.

The thymus is frequently affected in young, sometimes even adult subjects. Hypertrophy may be generalized or partial, lobulation is not maintained, consistency is firm, in section color is white-gray and the gland is wet [45].

Microscopically, neoplastic cell infiltration causes the destruction of all normal structures, Hassall bodies being unrecognizable (Fig. 17.25.–17.27.).

Fig. 17.25

Thymoma, predominantly epithelial.

Fig. 17.26

Timom, predominant epitelial. Thymoma, predominantly epithelial.

Fig. 17.27

Thymoma, predominantly lymphocytic.

Tonsils, as well as lymphoid formations from the digestive tract are increased in volume, white-gray, with small prominent nodules and clear peripheral delimitation. In section, white-gray micronodules are identified, which confer a mixed gray aspect.

The bone marrow, as well as blood, is affected at a later stage, the tumor process being only microscopically diagnosed (Fig. 17.15.).

Fig. 17.15

Multiple myeloma, bone marrow.

The liver appears enlarged, having a mixed gray aspect both at the surface and in section. White-gray encephaloid nodular formations are found in more advanced stages. Microscopically, the tumor formation starts in the portal spaces, but the lesion gradually extends to the sinusoids and all structures. Tumor nodules replace the hepatic parenchyma, and produce compression atrophy at the periphery (Fig. 17.18.).

Fig. 17.18

Myeloid neutrophilic leukemia, liver.

Kidneys are enlarged, with irregular outline and firm consistency. Infiltration is diffuse and located in the cortex, and hemorrhage occurs in the medulla [14]. Microscopically, initial lesions are located in the perivascular cortical regions, subsequently, they extend, and invade the whole parenchyma, inducing the progressive dissociation of nephrons. Vascular lesions cause atrophy, hemorrhage, and necrosis. The renal capsule is generally not affected by tumor infiltration, while the medulla or even the pelvis is penetrated by the neoplasm, with the presence of microhemorrhage (Fig. 17.16.).

Fig. 17.16

Multiple myeloma, kidney.

Skin in chronic evolution shows cutaneous lesions located in the back and flanks. Hairless erythematous areas initially appear, and painful pruriginous nodules gradually form, with generalized lymph node hypertrophy. Proliferative nodular formations are white-gray, nacreous, with a necrosed center, sometimes adherent to the underlying musculature [27].

Microscopically, the tumor lesion is associated with inflammatory reaction. The neoplastic process affects the epidermis, adnexal glands, pilosebaceous follicles and subcutaneous connective tissue [52]. Cutaneous tumor lesions are formed by cells of different types and are not identical in the lesions of a same affected subject.

Cutaneous malignant lymphoma is considered by recent classifications as mycosis fungoides, known in humans.

The lung may present a homogeneous diffuse infiltrative lesion, which confers a gray, lardaceous color and firm consistency, with atelectatic islands along with emphysema. Clear or serohemorrhagic fluid may appear in the thoracic cavity.

Microscopically, the infiltration is perivascular and peribronchial, with the occasional presence of small compact tumor formations. Tumor cells may extend to the interalveolar walls and the alveoli.

The central nervous system can present diffuse or nodular formations, gray matter degeneration, white matter demyelination and infiltration in the dura mater.

The eye may show nodular or diffuse lesions, corneal edema, neovascularization, ulcerative keratitis, anterior chamber hemorrhage, uveitis, tumor cell infiltration and iris discoloration, glaucoma. Retrobulbar tumor masses produce exophthalmia, compressions of the optic nerve and the involvement of ocular muscles [14].

In 1986, CARTER, VALLI and LUMSDEN proposed some cytologic, histological criteria, and the predominance of a certain cellular type in the classification of malignant lymphomas in dogs. This classification is internationally recommended and is based on the following criteria:

• – architecture, which can be follicular or diffuse; in dogs, proliferations have a diffuse character, totally abolishing lymph node architecture;
• – the size of tumor cell nuclei, which is a parameter in the assessment of malignancy;
• – the mitotic index, which is estimated as weak for 0–2 mitoses per microscopic field, with a 40 x magnification; moderate for 3–5 mitoses per microscopic field, and high in the case of 6 and more cells in mitosis;
• – nuclei can be cleaved or not;
• – number and appearance of nucleoli.

Depending on these criteria and the cellular type, malignant lymphomas may be:

1. low malignancy lymphomas;
2. intermediate malignancy lymphomas;
3. high malignancy lymphomas.

Low malignancy lymphomas

Diffuse small lymphocytic lymphoma is characterized by a diffuse architecture and a low mitotic index. Nuclei are erythrocyte sized, round, with a uniform periphery, generally lacking nucleoli. Sometimes, some cells may have a small central nucleolus. The cytoplasm is scant and weakly staining. These cells are similar to normal small, well differentiated lymphocytes, but are different through their high number and architectural type, which allows the making of diagnosis. The coexistence of this malignant lymphoma type with chronic lymphoid leukemia is frequent in humans, occasional in dogs.

There are two variants of diffuse lymphocytic lymphoma: plasmacytoid and intermediate.

Diffuse lymphocytic lymphoma of plasmacytoid type is characterized by the eccentric arrangement of nuclei, with extremely abundant, predominantly eosinophilic staining cytoplasm.

Diffuse lymphocytic lymphoma of intermediate type has been identified in humans and dogs, the cells presenting larger nuclei, with a diameter that can be 1.5 times larger than that of an erythrocyte. The nuclear membrane shows many irregularities and an obvious central nucleolus.

Follicular small cleaved cell lymphoma has not been reported in dogs, and tumors that present a pseudofollicular architecture and small cleaved cell types are usually catalogued as diffuse small cleaved cell lymphomas.

Follicular mixed cell lymphoma has a follicular architecture and a low mitotic index. Cells are probably of diffuse mixed type, being associated with small nucleus cells and large nucleus cells (Fig. 17.3.).

Fig. 17.3

Malignant lymphoma, prolymphocytic, lymph node.

Intermediate malignancy lymphomas

Follicular large cell lymphoma has a follicular architecture and a high mitotic index. Cells are large, with large nuclei and numerous nucleoli.

Diffuse small cleaved cell lymphoma has a diffuse architecture and a low mitotic index, which can sometimes be moderate or even high. Cell nuclei have the same or a smaller size than erythrocytes and show deep indentations of the nuclear membrane that confer them an irregular angular appearance. Nuclei are extremely dense and hyperchromatic, with coarse heterochromatin and 1–2 small nucleoli. The cytoplasm is scant.

Diffuse mixed cell lymphoma has a mixed architecture and a variable, most frequently low or moderate mitotic index. This lymphoma shows two distinct cell populations: small cleaved cells and large cleaved or non-cleaved cells. Small cleaved cells are identical to cells found in diffuse small cleaved cell lymphoma. Large cells contain nuclei with vesicular chromatin, with numerous nucleoli; nuclei can be cleaved or non-cleaved. Cleaved nuclei from large cells are round to elongated and they can present irregularities of the nuclear outline, but without deep linear fissures characteristic of cleaved nuclei. The cytoplasm of large cells is generally abundant but weakly stained, and cellular limits are unclear (Fig. 17.1.).

Fig. 17.1

Malignant lymphoma, poorly differentiated, lymph node.

In dogs, almost all diffuse mixed large cells show non-cleaved nuclei.

The diagnosis of diffuse large cell lymphoma is made when one third of cells are large.

Diffuse large cell lymphoma has a diffuse architecture, a variable, generally high mitotic index. Almost all cell nuclei are non-cleaved, with a diameter twice as large as that of an erythrocyte, and they present vesicular chromatin, with numerous nucleoli, most frequently with a peripheral arrangement, near the nuclear membrane. The number of these nucleoli can be 5–6. The cytoplasm can be abundant or moderate.

High malignancy lymphomas

Immunoblastic malignant lymphoma is characterized by a diffuse architecture and a high mitotic index. Cells have an indistinct outline, with clear plasmacytoid cytoplasm. Nuclei are voluminous, of round or slightly oval shape, vesicular, with a single large central nucleolus.

Malignant lymphoblastic lymphoma is characterized by a diffuse architecture and an extremely high mitotic index (up to 35 mitoses/field). Cells have scant, poorly delimited, pale cytoplasm. Nuclei are round, oval or slightly irregular, with homogeneous chromatin and 1–3 nucleoli (Fig. 17.2.).

Fig. 17.2

Malignant lymphoma, lymphoblastic, lymph node.

Small non-cleaved cell lymphoma has a diffuse architecture and a generally high mitotic index. Two subtypes of this lymphoma are described: Burkitt-like and non-Burkitt-like small non-cleaved cell lymphoma.

Burkitt-like cell lymphoma has been sporadically found in dogs, and is characterized by homogeneous shapes and nucleus structures, which are round, vesicular, with small multiple nucleoli. Cells have a well delimited cytoplasm and a compact arrangement.

Non-Burkitt-like cell lymphoma is characterized by small, non-cleaved shapes, larger nuclei than those of normal small lymphocytes but which never reach the size of nuclei from diffuse large cell lymphoma. Nuclei have a round, rarely ovoid shape, with a thickened, sometimes irregular, non-cleaved membrane. Heterochromatin is coarse, peripheral and intensely colored, euchromatin is clear, and one or more obvious nucleoli can be identified. Cells have a scant pale cytoplasm, being well delimited, like a ring surrounding the nucleus.

Diagnostic criteria in lymphohematopoietic tumors

Clinical criteria (according to WHO). Following clinical examination and complementary investigations, the clinical grade is established:

• – stage I: limited invasion of a single lymph node or a single lymphoid tissue from a single organ (except for bone marrow);
• – stage II: regional invasion of several lymph nodes with or without the involvement of tonsils;
• – stage III: generalized invasion of lymph nodes;
• – stage IV: invasion of the liver and/or spleen, with or without the generalized involvement of lymph nodes;
• – stage V: invasion of blood, bone marrow and/or other organs, with or without changes of the criteria corresponding to stages I to IV.

Two aspects can be assessed for each stage:

• – the absence or presence of several clinical signs: moderate fever, anorexia, adynamia and normal calcemia values;
• – severe general symptoms: weight loss, anemia, leukopenia, vomiting, diarrhea, and changes in the bioclinical profile.

In dogs, stages III, IV and V are the most frequently diagnosed, probably due to an aggressive evolution of malignant lymphomas and/or late diagnosing of the disease. A favorable response to treatment is mentioned for stages I and II, while stages III–IV manifest no difference regarding treatment [17].

From an epidemiological point of view, age and sex do not seem to be risk factors, but a longer survival has been found in females, without this being argued.

Radiological diagnosis may bring clarifications regarding metastases in the thoracic and abdominal cavities. Hyperplasias of deep (trache-obronchial, substernal, lumbo-aortic, mesenteric) lymph nodes, symmetric hypertrophy of the anterior mediastinum, abnormal pulmonary density, thoracic and/or abdominal effusions, hepatomegaly and/or splenomegaly can be identified.

Laboratory investigations can bring additional and certainty data in positive diagnosis. Hematologic changes in malignant lymphomas are inconsistent or even contradictory. Affected dogs with malignant lymphoma show hematologic constants within normal limits. Affected subjects with lymphocytosis or lymphopenia; leukocytosis with neutrophilia or monocytosis; in advanced forms, anemia and/or thrombopenia are mentioned. In most cases (80%), dogs with malignant lymphoma present aleukemic forms, and in advanced stages, leukemia occurs in 30% of cases [32].

The complete biochemical examination allows the detection of the affected organs and/or an underlying paraneoplastic syndrome. The data obtained cannot be generalized, on the contrary, they vary from one subject to another, with the possible appearance of hyper- or hypoproteinemia, hypercalcemia and increased hepatic enzyme activity.

Calcemia should be evaluated depending on albuminemia and it requires the calculation of conjugated calcium levels (Ca) in relation to albuminemia (Alb) or total protein levels (TP), according to a formula proposed by VIERA et al. (1985):

Corrected Ca = Ca − Alb − 35

Corrected Ca = Ca − (0.4 × TP) + 33

Histological and cytological diagnosis of lymphomas in dogs according to CARTER, VALLI and LUMSDEN, 1986

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DSL = Diffuse small lymphocytic	DM = Diffuse mixed
FSC = Follicular small cleaved	DL = Diffuse large
FM = Follicular mixed	IB = Immunoblastic
FL = Follicular large	LB = Lymphoblastic
DSC = Diffuse small cleaved	SNC = Small non-cleaved

Fine needle punctures of the lymph nodes, liver, hypertrophied spleen, bone marrow, thoracic or abdominal effusions allow for immediate cytological analyses. The observation of a homogeneous population of abnormal lymphoid cells ensures an extremely rapid diagnosis of quasi-certainty.

The exeresis of a hypertrophied lymph node represents a complementary examination that is essential in the suspicion of canine malignant lymphoma. Thus, a histological and cytological investigation, the evaluation of architecture, cell morphology, and the determination of the malignancy grade of malignant lymphoma can be performed.

Malignant lymphoma therapy. Chemotherapy in canine malignant lymphoma is particularly effective, having a highly practical character. Cytotoxic agents act by different mechanisms, a potentiality between these agents can be achieved, as well as the choice of the administration rate depending on the cell cycle. In order to understand the action and results obtained by chemotherapy, some terms or medical expressions should be defined [5].

Complete remission defines the recovery to normal of the size of lymph nodes, liver and spleen.

Partial remission is a reduction by more than 50% of peripheral lymphadenopathy, hepatomegaly and splenomegaly.

Remission period is the duration of response to chemotherapy.

Survival duration corresponds to the time period between the diagnosis of malignant lymphoma and natural death or euthanasia. In order to obtain an optimum survival period, complete remission is necessary. In the case of complete remission, in 75% of cases, the mean survival time is 10–14 months, and in the absence of treatment, the mean survival time does not exceed 6 weeks.

The majority of therapeutic protocols use vincristine, cyclophosphamide and prednisone, for the induction of remission. Some authors add 1-asparaginase or doxorubicin. Based on the existing tested chemotherapeutic drugs, various protocols have been devised, without the possibility of establishing an ideal formula, correlating efficacy, toxicity and cost. Regardless of the treatment instituted, the leukocyte formula should be monitored, and treatment should be suppressed when the number of leukocytes decreases below 3000/mm³ or when the number of blood platelets decreases below 100000/mm³; antibiotherapy is mandatory.

In fighting malignant lymphomas, adjuvant therapy is considered in addition to specific targeted therapy.

Immunotherapy is used in order to increase immune response. Thus, non-specific immunomodulation with levamisole or bacterial endotoxin is envisaged, although efficacy is uncertain, sometimes lacking, at other times response being favorable. The use of vaccines carried out with autogenous tumor cell parts, associated with chemotherapy, gives good results.

Blood transfusion, the administration of heparinate plasma and fibronectin have allowed the obtaining of short remissions. The mechanism of action is unknown and the large blood amount makes this treatment unusable in current practice.

Autologous bone marrow transplants after effective chemotherapy and radiotherapy have resulted in long remissions in a significant number of dogs treated in this way.

Hypercalcemia can be treated by the administration of glucocorticoids, such as prednisone, in dogs with malignant lymphoma. Calcitonin, which inhibits bone resorption, can be used. Hypercalcemia is always an aggravating prognostic factor [53].

In all cases, therapy should be individualized and adopted depending on the animal responses to treatment.

The REAL system for the classification of lymphomas was chosen as a basis of use because of the need to distinguish between lymphomas of bone marrow (B) and thymic-dependent (T) type that may have similar morphology but differ in histological behavior and response to therapy.

17.1.1. B-cell lymphoid neoplasms

17.1.1.1. Precursor B-cell neoplasms

B-cell lymphoblastic leukemia/lymphoma, a rapidly progressive neoplasm of B-cell lymphoblasts involving the bone marrow and/or peripheral tissues in varying degrees. Cytologically, the cells are of moderate size with densely stained, finely distributed chromatin, indistinct nucleoli, and minimal cytoplasm (Fig. 17.41, 17.42). Histologically, these tumors are characterized by intermediate nuclear size, diffuse architecture, and a high mitotic rate, with chromosomes less distinctly stained and visualized than in lymphomas of other types. The nuclei are round or have shallow indentations (convoluted type), and, as in cytological preparations, nucleoli are small and obscured or absent. Some tumors of this type will have a “starry-sky” appearance at low magnification due to a high level of tingible-body macrophages, similar to high-grade B-cell lymphoma, Burkitt-like.

Fig. 17.41

B-cell lymphoblastic leukemia. The nuclei are one and a half to two red cells in diameter, generally round with densely stained, finely distributed chromatin with nucleoli generally absent. L1 cells have a high NC ratio with moderate cytoplasmic basophilia. (more...)

Fig. 17.42

B-cell lymphoblastic lymphoma. The needle aspirate is highly cellular with a background of basophilic cytoplasmic fragments (lymphoglandular bodies). The nuclei are round with shallow indentations. Nucleoli are largely obscured, but are evident in the bare (more...)

Acute lymphoblastic leukemia occurs in all species, but is most frequently seen in cats, dogs, and calves. Three cytologic types of lymphoblastic leukemias are described in humans and designated L1–L3. All three types occur in animals, with the L2 type the most common.

B-cell lymphoblastic lymphoma is a relatively rare tumor recognized primarily in dogs and cats. Lymphoblastic lymphomas of B-cell or T-cell types are not distinguishable morphologically without staining to identify phenotipic differentiation. Dogs are usually seen with generalized, rapidly progressive lymphadenopathy. Lymphoblastic neoplasms have been inadequately studied for immunophenotype, but it appears most lymphoblastic leukemias are B-cell type and most lymphoblastic lymphomas are T-cell type. The lymphoblastic lymphomas, present with hypercalcemia, are usually of T-cell type [55].

17.1.1.2. Mature B-cell neoplasms

B-cell chronic lymphocytic leukemia/lymphoma, a slowly progressive disease of lymphocyte accumulation rather than proliferation characterized by a very low mitotic rate and involving bone marrow and peripheral tissues to varying degrees. Small lymphocytic leukemias (SLL) and lymphomas are uncommon tumors that are seen most frequently in the cat, dog, and cow. The leukemic presentation usually has a very high lymphocytosis of small, round nuclei (slightly larger than the diameter of an rbc); large, dense chromocenters; absent nucleoli; and inapparent cytoplasm. The small lymphocytic lymphomas are usually of B-cell type and often occur in the intestinal tract of most domestic species [55]. (Fig. 17.43, 17.44).

Fig. 17.43

B-cell chronic lymphocytic leukemia. The majority of the lymphocytes are small with compact chromatin and a narrow rim of cytoplasm. The large cell with cytoplasmic granules is likely a benign promyelocyte released due to phthisis of normal marrow and (more...)

Fig. 17.44

B-cell chronic lymphocytic leukemia. The majority of the lymphocytes are small with compact chromatin and a narrow rim of cytoplasm. The large cell with cytoplasmic granules is likely a benign promyelocyte released due to phthisis of normal marrow and (more...)

B-cell lymphocytic lymphoma intermediate type (LLI), a slowly progressive lymphoma characterized by nuclei slightly larger than those of chronic lymphocytic leukemia/lymphoma, but with scant cytoplasm. The intermediate type of B-cell lymphocytic lymphoma commonly occurs in most domestic species. The nuclei of the “intermediate” lymphocyte type are slightly larger and more vesicular than those of chronic lymphocytic leukemia/lymphoma and about equal in size to nuclei of the small-cleaved cell of the Working Formulation terminology or the centrocyte of the Kiel classification. The small-cleaved or centrocyte-type lymphocyte morphology is typical of the small B lymphocytes of germinal centers. While T cells may have a similar morphology, the terms small-cleaved cell and centrocyte are reserved for B cells. T-cell lymphomas with similar cytology are identified by the site of involvement (ie, cutaneous T-cell lymphoma and intestinal T-cell lymphoma). As in chronic lymphocytic leukemia and lymphoma, the mitotic rate in LLI is low, and the lesions have a relatively indolent rate of progression and respond poorly to chemotherapeutic agents that are cell-cycle dependent [55]. (Fig. 17.45).

Fig. 17.45

B-cell lymphocytic lymphoma intermediary type. Architecturally, the lymph-node was uniformly enlarged with the peripheral capsule thinned and taut and generally with a compressed but intact peripheral sinus. Cytologically, the nuclei tend to be round (more...)

Lymphoplasmacytic lymphoma (LPL), a slowly progressive disease characterized by diffuse architecture and, cytologically, by a low mitotic rate and round nuclei slightly larger than those of chronic lymphocytic lymphoma without nucleoli and with a moderate amount of densely stained, eccentrically placed cytoplasm. Small lymphocytic lymphomas of plasmacytoid type are relatively common tumors of domestic species that share characteristics of chronic lymphocytic lymphoma, such as the absence of nucleoli and a very low mitotic rate, and have the same nuclear characteristics as lymphocytic lymphoma of intermediate type. They are characterized by more abundant cytoplasm than small lymphocytic or cleaved cell lymphoma and that which is present is usually eccentrically placed about the nucleus. These tumors are characteristically of B-cell type [55]. (Fig. 17.46).

Fig. 17.46

Lymphoplasmacytic lymphoma. Cytologically, the nuclei are round to oval with occasional shallow indentation. The nuclei are slightly larger than small lymphocytic lymphoma with greater internal nuclear detail. There are numerous small chromocenters and (more...)

Follicular lymphomas, slowly progressive tumors derived from lymph node germinal centers that are recognized by characteristic follicular architecture and fading mantle cell cuffs. Cytologically, follicular lymphomas consist of small cleaved lymphocytes (centrocytes) with densely stained chromocenters and absent nucleoli and mitoses admixed with a variable proportion of large cells (centroblasts) with round or deeply clefted nuclei, peripheralized chromatin, one to three prominent nucleoli that are often peripheral and impinge on nuclear membranes, frequent mitoses, and a moderate amount of cytoplasm.

The National Cancer Institute Working Formulation recognized three types of follicular lymphoma depending on cell type, which, in order of lesion progression rate in human disease, were composed of small cleaved cells, mixed small cleaved and large cells, and finally large cells. In the revised European American lymphoma system, follicular lymphomas are graded I to III, based on the proportions of large and small cells involved. By this definition, grade I follicular lymphomas are primarily composed of small cleaved cells, or centrocytes by the KIEL classification, and only one to five larger cells, or centroblasts per 400x field. Grade II follicular lymphomas have between 6 and 15 centroblasts per 400x field, and grade III follicular lymphomas have 16 or greater large cells or centroblasts at the same magnification. Follicular lymphomas likely occur in all domestic species but are most commonly recognized in the cat and cow and less commonly in the dog. In perspective, follicular lesions make up as much as 25% of lymphomas in humans, while less than 5% of animal lymphomas are of follicular type, assuming all of them are recognized. Typically, these low-grade or indolent types of lymphomas may reach quite a large size, exceeding several centimeters in longest dimension, in both the cat and dog before they spread to other areas of the body.

Mantle cell lymphoma (MCL), a slowly progressive tumor of characteristic follicular architecture with proliferation and coalescence of mantle cell cuffs. Cytologically, the nuclei are of intermediate-cell type with a low mitotic rate and a mixture of round and shallow indented (cleaved) membranes; it is a rare tumor of domestic animals and is rarely recognized as a specific entity. Mantle cell lymphomas are B-cell tumors that arise from the mantle cuff of small, dark lymphocytes that surround germinal centers [55]. (Fig. 17.47).

Fig. 17.47

Mantle cell lymphoma. Nuclei are round or have shallow indentation and characteristically are quite deeply stained with increased intranuclear detail, similar to that of small lymphocytic intermediate lymphoma. Nucleoli are generally absent, and the mitotic (more...)

Follicular center cell lymphoma I, a slowly progressive tumor with follicular architecture composed primarily of small mature cleaved cells (WF) or centrocytes (Kiel classification). Mitoses are infrequently observed [55]. (Fig. 17.48).

Fig. 17.48

Follicular center cell lymphoma I. detail of the lymph node in figure 11. Cytologically, the follicular center cells are primarily small-cleaved lymphocytes or centrocytes. Bar = 10 microns.

Follicular center cell lymphoma II, a slowly progressive tumor with progressive architecture composed primarily of small, mature cleaved cells (WF) or centrocytes (Kiel classification) and large cleaved and noncleaved cells (WF) or centroblasts (Kiel classification); mitoses are present in most follicles [55]. (Fig. 17.49).

Fig. 17.49

Follicular center cell lymphoma II. Cytologically, the majority of cells are centrocytes. A single centroblast (center) is present.

Follicular center cell lymphoma III, a slowly progressive tumor with follicular architecture composed primarily of large cleaved and noncleaved cells (WF) or centroblasts (Kiel classification); mitoses are numerous in all follicles [55]. (Fig. 17.50).

Fig. 17.50

Follicular center cell lymphoma III. The majority of the cells are centroblasts and consist of larger cells with vesicular nuclei, irregularly thickened nuclear membranes, a fine granular chromatin pattern with irregular parachromatin clearing, and one (more...)

Nodal marginal zone lymphoma, a slowly progressive proliferation of B cells with abundant, relatively clear cytoplasm that encircles follicles outside of the mantle cell cuff. Marginal zone lymphoma is a relatively recently recognized entity in human and veterinary pathology; this type has been recognized in the cat and dog and likely occurs in all species; the disease appears to be rare. Marginal zone cells in the benign state occur in relatively small numbers adjacent to the peripheral and trabecular sinuses, and in larger numbers in some types of inflammatory reactions. Controversy over nomenclature of this lesion persists in human pathology with the cell type in question referred to as monocytoid B cells. The lesions in spleen and node are referred to as marginal zone tumors and mucosal lesions as mucosa-associated lymphoid tumors or MALT lymphomas [55]. (Fig. 17.51, 17.52).

Fig. 17.51

Nodal marginal zone lymphoma. The fading germinal center and narrow rim of residual mantle cells surrounded by enlarging rim of marginal zone cells. Bar = 50 microns.

Fig. 17.52

Nodal marginal zone lymphoma. Cytologically, the marginal zone cells have medium-sized nuclei that are round to irregularly indented and have irregularly thickened nuclear membranes, residual large chromocenters, and irregular parachromatin clearing. (more...)

Splenic marginal zone lymphoma, a slowly progressive tumor of follicles characterized by three cellular zones consisting of a germinal center, a mantle, and an outer rim of neoplastic marginal zone cells accompanied by atrophy of thymic-dependent periarteriolar sheaths. Splenic germinal centers form around a terminal branch of the small sheathed arteries and are surrounded by a mantle of small T lymphocytes that is continuous with the outer sheath of the parent muscular artery. Outside of this mantle cell cuff, the marginal zone is usually occupied by B cells representing those in circulation. The monocytoid B cell with reniform nucleus and abundant cytoplasm that forms the marginal zone in lymph nodes and spleen (or a cell with similar appearance) forms a continuous layer around and outside of the mantle cells in this malignancy. Consequently, it is felt that the development of three distinct cellular zones involving the splenic follicle is indicative of marginal zone lymphoma. In animals, as in humans, marginal zone lymphoma may appear first in the spleen and later in the liver and other organs, or the initial diagnosis may be made on the basis of an enlarged peripheral node [5]. (Fig. 17.53, 17.54).

Fig. 17.53

Splenic marginal zone lymphoma. Spleen was noted to have a nodular splenic mass after a prolonged history of recurrent immune mediated thrombocytopenia. The mass is made up of coalescing foci of marginal zone cells that appear to have arisen around germinal (more...)

Fig. 17.54

Splenic marginal zone lymphoma prepared with the CD79a reagent to mark B cells. The marginal zone cells are forming cuffs around a central paler area, giving the individual lesion a "bulls-eye" or target appearance. Bar= 1.0mm.

Extranodal marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma), a low-grade B-cell lymphoma usually arising in a background of chronic inflammation and associated with a mucosal surface of the respiratory or enteric system, or associated glands. Lymphomas involving mucosal surfaces likely occur in all species of animals, but are most frequently recognized in mature dogs and cats. In humans, tumors of this type most commonly involve the salivary glands, stomach, thyroid, and lung, while in animals most are found involving the stomach and small intestine. Cytologically, MALT tumors are usually composed of small lymphocytes of intermediate type with indented nuclei, or there may be a more mixed pattern with some larger lymphoid cells similar to the centrocytes and centroblasts of a benign germinal center. The characteristic lesion is of follicular hyperplasia in a submucosal site, with progression to a monophorphic cell population that invades the mucosa in a “lymphoepithelial lesion” that destroys deep glands and ulcerates the surface epithelium. Because this type of lymphoma appears to arise in a background of chronic immune stimulation, these two types of reactions are frequently found to coexist, and even low- and high-grade B-cell lymphomas may be found within the same lesion. The importance of recognizing MALT lymphomas is that they are surgically resectable lesions, to be differentiated from high-grade lymphomas that tend to rapidly involve the adjacent lymph nodes. In human pathology, MALT tumors of the small-cell type have been referred to as monocytoid B-cell lymphoma, and their similarity to the cells of marginal zone lymphoma and hairy cell leukemia of the spleen has been noted, due to their relatively abundant and lightly stained cytoplasm providing pale areas of cellular expansion on low-power histological examination [55]. (Fig. 17.55).

Fig. 17.55

Extranodal marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma). Deep mucosal area with a "lymphoepithelial lesion" characterized by glandular destruction by invading lymphocytes. Bar = 20 microns.

Hairy cell leukemia, a form of chronic B-cell leukemia consisting of cells with nuclei of intermediate size, rare mitoses, small nucleoli, and abundant clear cytoplasm. On wet-mount examination and in blood films, there are thin filiform cytoplasmic extrusions projecting outward from the cell margins. Hairy cell leukemia is not generally recognized in animals. A leukemic disease of cats, described some years ago and originally termed reticuloendotheliosis, resembles hairy cell leukemia as it is currently described in humans. Typically, in human peripheral blood, the hairy cells are characterized by very fine cytoplasmic protrusions that project at right angles to the surface of the cells and are best seen in wet-mount preparations using phase contrast. In humans, the disease is considered a chronic lymphocytic leukemia of mature-cell type that stains for B-cell markers and expresses surface immunoglobulin. In the blood, the cells are also referred to as villous lymphocytes, because of the cytoplasmic extrusions, and, cytochemically, the cells are uniquely positive for the tartrate-resistant acid phosphatase (TRAP) reaction. It is possible that the advent of the feline leukemia virus vaccine has altered the pattern of this disease as it currently occurs in cats [55]. (Fig. 17.56).

Fig. 17.56

Hairy cell leukemia. Blood from a 41-year-old human male with history of splenomegaly and thrombocytopenia. The nuclei are small and compact with very abundant, water-clear cytoplasm that displaces adjacent red cells. Bar = 20 microns.

Plasmacytic tumors usually occur in mature animals and are most frequently seen in dogs, rarely in cats. Plasmacytomas may occur as cutaneous lesions, or anywhere along the gastrointestinal tract. Focal and diffuse lesions can be found in the spleen and in the bone marrow. Plasmacytoma of the lymph node is a rare event. In animals, plasmacytomas may occur in well-differentiated forms that appear to be benign tumors, or if they appear anaplastic and malignant, they are very slowly progressive and rarely metastasize. In animals, anaplastic plasmacytomas occur that have malignant histologic characteristics, both cytologically and architecturally [55, 58].

Indolent plasmacytoma, an indolent, well-differentiated plasma cell tumor. Histologically, these tumors are characterized by diffuse architecture consisting of a relatively uniform population of cells with deeply stained nuclei that are round to oval, of intermediate size, and occasionally binucleate, with rare or absent mitoses and large, dense chromocenters; small or absent nucleoli; and a moderate amount of deeply stained and eccentrically placed cytoplasm. Golgi zones are irregularly evident [55, 58]. (Fig. 17.57).

Fig. 17.57

Indolent plasmacytoma. Neoplastic cells have round to irregularly compressed, sickle-shaped (helmet-shape), eccentric nuclei with often clumped chromatin and a moderate amount of eosinophilic cytoplasm. There are scattered binucleate cells (lower center). (more...)

Anaplastic plasmacytoma, a locally aggressive plasma cell tumor often arising in a background of chronic benign lymphoproliferation and characterized cytologically by hyperchromicity, anisokaryosis, binucleation, and a moderate level of mitosis. Cytoplasm is deeply stained, variably abundant, and eccentrically placed. The presence of a fine stromal background with some packeting of cells is seen in some plasmacytomas with anaplastic morphology. These tumors are very slowly progressive and rarely metastasize [55, 58]. (Fig. 17.58).

Fig. 17.58

Anaplastic plasmacytoma. Cytologically, the cells in the areas of deeper penetration of the gastric lamina propria have marked nuclear hyperchromicity and anisokaryosis wit5h frequent binucleation. Multiple nucleoli are present in the larger nuclei, and (more...)

Plasma cell myeloma, a slowly progressive but highly malignant, generally multifocal (multiple myeloma) plasma cell tumor of the marrow cavity characterized by gammopathy, clinical hyperviscosity, focal osteolysis, myelophthisis, and bone pain. Multiple myeloma is a rare disease in the dog, and very rare or unrecognized in other species. In the dog, myeloma is very similar to the human disease, with multifocal osteolytic lesions involving the axial skeleton, which are usually identified radiographically in dogs presenting for other signs, including lameness. The diagnosis is readily made by a marrow core or aspirate directed at focal areas of osteolysis. Cytologically, the nuclei are large, round to oval, and hyperchromatic with occasional binucleation and, frequently, multiple nucleoli, but very few mitoses. Characteristically, myeloma cells have very abundant deeply and evenly stained cytoplasm with variably distinct cell boundaries. Hypergammaglobulinemia and hyperviscosity are seen in the dog and may be problematic during anesthesia induced for diagnostic purposes because of attendant hypotension, sludging of blood cells, and fatal shock. Bence Jones-like proteins in the urine appear to be found less commonly in dogs with myeloma than in the human counterpart. The low mitotic rate of myelomas presents a therapeutic challenge [55]. (Fig. 17.59).

Fig. 17.59

Plasma cell myeloma. Regressing marrow elements with pyknosis of megakaryocytes that are surrounded by cells with massive amounts of quite densely eosinophilic cytoplasm. The nuclei are moderately hyperchromatic and round to oval with moderate anisokaryosis (more...)

Large B-cell lymphomas, of diffuse architecture composed of large B cells, constitute a spectrum of lesions in both humans and animals. Large cell lymphomas, presumptively of B-cell type, are the most commonly occurring tumors in the cow, cat, horse and pig. The large cell lymphomas of diffuse (nonfollicular) architecture were subdivided in the Working Formulation into cleaved cell and noncleaved cell categories and in the Kiel classification into centroblastic and immunoblastic categories. The REAL classification added phenotypic definition to the category termed B-cell large cell lymphoma and eliminated the subclassifications on the basis of no real difference in biological behavior. In cattle, diffuse large-cleaved cell lymphoma, analogous to the DLCCL of the Working Formulation, makes up more than one third of the lymphoma cases seen and tends to be associated with bovine leukemia virus infection. Diffuse large cell lymphomas of cattle of the noncleaved variety are only slightly less common, with these two categories together making up two thirds of lymphoma types commonly observed in mature animals. In the dog and pig, diffuse large non-cleaved cell lymphomas make up 20% and 60%, respectively, of cases seen, with the large cleaved-cell type rarely observed. In the cat and horse, the diffuse large non-cleaved cell lymphomas make up about 10% and 20%, respectively, with the cleaved-cell type of about 13% in the cat and 2% in the horse [51, 55].

T-cell-rich B-cell lymphoma, a slowly progressive lymphoma characterized by fine diffuse reticulin sclerosis and a mixed population of small cleaved-cell T cells and large neoplastic B cells; the atypical B cells may constitute 5% or less of the total cellular population. A category of lymphoma characterized by a mixed population of small and large cells was termed diffuse mixed type in the Working Formulation and centroblastic-centrocytic diffuse in the Kiel classification. When phenotypic definition is added to lymphomas of mixed-cell type, two entities are identified. One is a diffuse tumor of large and small (centroblastic and centrocytic) B lymphocytes, as occurs in follicular lymphoma. In classifications of lymphomas in animals, these tumors have largely been included in the category of large B-cell lymphomas, if the large cells are even focally the predominant population. The second entity in the diffuse mixed category recognized by the REAL classification consists of a predominant population of small, reactive T lymphocytes with compact, densely stained chromatin and shallow nuclear indentations. The larger cells may constitute as few as 5% to 10% of the total population, have large vesicular nuclei with peripheralized chromatin and prominent nucleoli, and phenotypically are B cells. This category is termed T-cell-rich B-cell lymphoma. A further distinction between these two mixed cell tumors - one related to follicular center cells and the second of mixed T- and B-cell types - is that the latter category is consistently accompanied by fine diffuse sclerosis. The reticulin framework eliminates the artefactual “cracking” of the tissue, as occurs in the follicular and diffuse B-cell lymphomas. Lymphomas of the diffuse mixed and presumably T-cell-rich B-cell type are commonly seen in horses (35%) and cats (7%), but constitute only 2% to 3% of lymphomas in the cow, dog, and pig.

In animals, the T-cell-rich B-cell lymphoma has the most characteristic presentation in the horse, where there may be hundreds of subcutaneous tumors, round and flattened disclike lesions that appear to be arising along subcutaneous lymphatics. Affected animals remain outwardly healthy for periods of several years and are often euthanized because they are unsightly or cannot be saddled due to widespread lesions. In other species, the lesions generally involve lymph nodes and may appear as a marked enlargement of a single peripheral node. In humans, the lung lesion known as lymphomatoid granulomatosis has been compared to the T-cell-rich B-cell lymphoma; the latter term may be even more appropriate in animals, as angioinvasive tendencies are less characteristic of the disease in dogs [55]. (Fig. 17.60).

Fig. 17.60

T-cell-rich B-cell lymphoma. Large atypical cells with vesiculated nuclei, a branched chromatin pattern with irregular areas of parachromatin clearing, and prominent, often single central, nucleoli. The cells have abundant cytoplasm and are frequently (more...)

Large cell immunoblastic lymphoma, a moderately aggressive lymphoid tumor of diffuse architecture consisting of large cells with round-to-oval or cleaved nuclei with frequent mitoses, thick nuclear membranes, branched chromatin with parachromatin clearing, and, typically, a prominent single central nucleolus. Cytoplasm is variable in amount and often deeply stained. Tingible-body macrophages are present and may be numerous [55]. (Fig. 17.61).

Fig. 17.61

Large cell immunoblastic lymphoma. The mass is composed of large atypical cells with irregularly clefted and indented nuclei and very prominent nucleoli. There is irregular thickening of the nuclear membranes with a very fine granular chromatin pattern (more...)

Diffuse large B-cell (noncleaved, cleaved) lymphoma, a moderately aggressive lymphoid tumor of diffuse arhitecture consisting of a relatively uniform population of large lymphoid cells with vesicular nuclei, branched chromatin, and two to three prominent nucleoli that typically impinge on the nuclear membranes. Mitoses are frequent as are tingible-body macrophages. (Fig. 17.62).

Fig. 17.62

Diffuse large B-cell (noncleaved) lymphoma. The nuclei are large and hyperchromatic with irregular thickening of nuclear membranes, a branched chromatin pattern with residual small, dark chromocenters, and irregular parachromatin clearing. The cells have (more...)

Thymic B-cell lymphoma (mediastinal B), a rare lymphoid neoplasm of the mediastinum most frequently recognized in young mature large-breed dogs. The lesions are slowly developing, and animals usually present due to vomition after eating, apparently due to esophageal compression. Lesions are typically heavily encapsulated and traversed by broad collagen bands, giving a gross appearance of thymoma. Cytologically, the tumor cells are large. Nuclei are vesicular with prominent nucleoli and may be deeply clefted and occasionally large and multilobulated.

The thymic large cell lymphomas are usually B-cell type, similar to their human counterpart. The masses may be 7 cm or more in diameter at the time of presentation, providing a large target for fine needle aspiration, which may be variably productive due to the high level of coarse and fine intratumoral sclerosis [55]. (Fig. 17.63).

Fig. 17.63

Thymic B-cell lymphoma. Histologically, a triangular projection on the mass was fibrous and septate and represented residual thymus, while the bulk of the mass was a diffuse nonseptate lymphoma. A diffuse area of the mass consisted of large cell lymphoma (more...)

Intravascular large B-cell lymphoma, a systemic disease characterized by proliferation of lymphoid cellular clusters within the lumen of small veins and, to a lesser extent, of the arteries of the heart, lung, and central nervous system. Intravascular lymphoma is a rare disease of dogs and likely other species that may present as sudden onset of syncope and death in an outwardly normal animal [55]. (Fig. 17.64).

Fig. 17.64

Intravascular large B-cell lymphoma. Colonization of venous lumen by medium-sized and large mononuclear cells with occasional binucleation and relatively abundant cytoplasm. Bar = 10 microns.

Burkitt-type lymphoma, a rapidly progressive lymphoma consisting of very uniform cells with round-to-oval, intermediate-sized vesicular nuclei with multiple small nucleoli with a narrow rim of cytoplasm. These aggressive lymphomas are composed of cells with nuclei slightly larger than those of lymphocytic lymphoma intermediate type and smaller than those of large cell lymphoma. They have high mitotic and apoptotic rates. The presence of numerous tingible-body macrophages on low-power examination gives a “starry-sky” appearance. In morphometric studies, this lymphoma can be distinguished from small cell lymphoma of the intermediate type. A major diagnostic characteristic of the Burkitt-type lymphomas in humans and animals is a monotonous uniformity in nuclear size. The nuclei are round or slightly oval and approximately one and a half red cells in diameter. There is thickening of the nuclear membranes and parachromatin clearing with two to five small but well-defined nucleoli. The cytoplasm is moderate in amount and cell boundaries are distinct and closely interfaced [55]. (Fig. 17.65).

Fig. 17.65

Burkitt-type lymphoma. The tumor is composed of relatively small cells with vesicular nuclei, peripheralized chromatin, and multiple small nucleoli. The cytoplasm is minimal, and the cells present a uniform appearance on histological examination. Tangible-body macrophages (more...)

High-grade B-cell lymphoma, Burkitt-like, a rapidly progressive lymphoma with high mitotic and apoptotic rates similar to Burkitt-type lymphomas, but with moderate variation in nuclear size.

In the Working Formulation, a highly aggressive lymphoma with nuclei similar in size to precursor B- or T-cell or lymphoblastic lymphoma, but with thickened nuclear membranes, parachromatin clearing, and prominent multiple small nucleoli was designated small none leaved cell lymphoma (SNCCL). Like Burkitt lymphoma, SNCCL was recognized to be of B-cell lineage. In the REAL classification, the aggressive lymphomas intermediate in size between small and large cell lymphomas that lacked the uniform chromatin pattern of precursor or lymphoblastic lymphomas were termed Burkitt if the nuclei were of uniform size and Burkitt-like if there was significant anisokaryosis. In practice, the diagnosis of Burkitt-type lymphoma in animals is relatively easy because of the uniformly medium-sized nuclei with irregular chromatin distribution, multiple small nucleoli, and high mitotic and apoptotic rates. In contrast, the diagnosis of high-grade B-cell lymphoma Burkitt-like is more difficult because, with the greater variation in nuclear size, there is overlap with the smaller nuclei in large B-cell lymphoma, which is the major differential diagnosis [55]. (Fig. 17.66).

Fig. 17.66

High grade B-cell lymphoma, Bukitt-like. The nuclei have irregular chromatin distribution and small nucleoli. The striking feature of the tissue is the presence of many large macrophages containing nuclear debris (tingible bodies) of apoptotic tumor cells. (more...)

17.1.2. T-cell and NK-cell lymphoid neoplasms

17.1.2.1. Precursor T-cell neoplasms

T-cell lymphoblastic leukemia/lymphoma, a rapidly progressive neoplasm of T-cell lymphoblasts involving bone marrow and/or peripheral tissue in varying degrees. Cytologically, in tissue or blood, the cells are similar and of moderate size with densely stained, finely distributed chromatin, indistinct or absent nucleoli, and minimal cytoplasm. Histologically, acute lymphoblastic leukemia or lymphoblastic lymphoma is characterized by intermediate nuclear size, diffuse architecture, and a high mitotic rate with chromosomes less-distinctly stained and visualized than in lymphomas of other types. The nuclei are round or have shallow indentations (convoluted type), and, as in cytological preparations, nucleoli are small and obscured or absent. Some tumors of this type will have a “starry-sky” appearance at low magnification due to a high level of tingible-body macrophages, similar to high-grade B-cell lymphoma, Burkitt-like.

Lymphoblastic leukemia needs to be differentiated from benign reactive lymphocytosis. Classic examples of benign reactive lymphocytosis are the persistent B-cell lymphocytosis of cattle with chronic but benign bovine leukemia virus infection or young dogs following vaccinations.

T-cell lymphoblastic lymphoma is most often seen in the dog and is relatively rare in other animals. Lymphomas of the acute T-lymphoblastic cell type and related leukemias are the lesions most frequently associated with paraneoplastic hypercalcemia.

The cells of B- and T-cell lymphoblastic leukemia or lymphoma are not distinguishable cytologically or histologically. Immunophenotypically, most are T-cell lymphomas with a high proliferative rate. Nuclei are of intermediate size, similar to Burkitt-type lymphoma. In contrast to that latter disease, which is characterized by vesicular nuclei with peripheralized chromatin, the T-lymphoblastic cells have a very fine diffuse chromatin distribution, and, despite their aggressive character and name, the nucleoli are absent or inapparent. These neoplasms have a high proliferative and apoptotic rate, but a “starry-sky” pattern is less apparent than with Burkitt-type tumors. Mitotic figures in lymphoblastic lymphoma are less sharply delineated than in other aggressive lymphomas and must be carefully sought in order to recognize these tumors as high-grade lesions. The nuclei may be deeply indented and are referred to as convoluted, or may be round and entire. If observed early in the course of involvement of a single node, the pattern is of diffuse proliferation in paracortical areas, which may compress the medulla, while coexisting with residual germinal centers in the outer cortex. Tumor cells rapidly invade and destroy the peripheral sinus and may fix the lymph nodes to surrounding tissues [55]. (Fig. 17.67, 17.68).

Fig. 17.67

T-cell lymphoblastic leukemia. There is a predominant population of small to medium-sized lymphoblasts with round to oval nuclei and a narrow rim of highly basophilic cytoplasm, corresponding to the FAB L1 classification. Bar = 10 microns.

Fig. 17.68

T-cell lymphoblastic lymphoma. Cytologically, the tumor is of lymphoblastic convoluted type with irregularly and often deeply indented nuclear membranes. The chromatin is finely distributed with some irregular thickening of nuclear membranes. The nucleoli (more...)

17.1.2.2. Mature T-cell and NK-cell neoplasms

Large granular lymphoproliferative disorders (LGL)

T-cell chronic lymphocytic leukemia, a slowly progressive T-cell disease of relatively small cells with a mature or dense chromatin distribution. Chronic lymphocytic leukemia (CLL) is a disease of accumulation rather than proliferation with a low mitotic and death rate of the malignant cells. As a consequence, they do not respond well to cytoreductive therapy, which is directed at cells in mitotic cycle, in contrast to the acute lymphoblastic leukemias that are characterized by both a high mitotic and apoptotic rate. In both humans and animals, CLL may be discovered incidentally by a routine blood examination in the absence of clinical signs. With progression, there is weight loss, anemia, and increased susceptibility to bruising and infection due to declining platelet and neutrophil counts. Ultimately, after a subclinical course that may last for 1 to 2 years, CLL may enter an accelerated phase in which the cells become larger and more typical of lymphoblastic leukemia. While lymphoblastic leukemia is more common among young humans and animals, CLL is most commonly diagnosed in mature dogs and cats. Clinically, well-developed CLL is characterized by splenomegaly and usually hepatomegaly and diffuse marrow colonization. In contrast, the lymph nodes may be atrophic.

In contrast to CLL in humans, which is generally of B-cell type (greater than 95%), more than half, or as much as 70%, of canine CLL is of T-cell type. In dogs, more than half of the cases of T-cell type CLL have fine to large azurophilic granules, are of the large granular lymphocyte (LGL) lineage, and, similarly to the Fischer rat, are primarily of splenic origin. These cells react positively with CD2 and CD8, which distinguishes them from CLL of the NK-cell type.

The cells of the prolymphocytic variant of T-CLL have larger nuclei, approximately two red cells in diameter, that are characteristically round and have large, deeply stained chromocenters outlined by prominent parachromatin clearing [55]. (Fig. 17.69).

Fig. 17.69

T-cell chronic lymphocytic leukemia of large granular lymphocyte type. Lymphocytes have small nuclei with mature, coarsely aggregated chromatin and inapparent nucleoli. There is a moderate volume of pale cytoplasm that contains a variable number of fine (more...)

T-cell LGL lymphoma/leukemia, a slowly progressive leukemic disease of dogs and cattle and aggressive lymphoma of cats, each characterized by cells with large azurophilic cytoplasmic granules. Leukemias of lymphocytes with large cytoplasmic granules are of either T-cell or natural killer (NK)-cell type in humans and likely have a similar origin in animals. Cytologically, the two cells types are not distinguishable; however, large granular lymphoid proliferations that are CD3 positive are of T-cell type, and, if nonreactive with CD3, may be presumed to be of natural killer-cell type. In cattle, the disease tends to progress with only moderate levels of leukemia, generally less than 50,000 per cubic microliter of blood. The diseases tend to involve the bone marrow and spleen, where the tumor cells colonize the sinusoids of the red pulp. Atrophy of the splenic germinal centers and periarteriolar lymphoid sheaths appears to be less common than in the more aggressive leukemias. Cytologically, the tumor cells are medium-sized to large and the nuclei may be round and entire, or deeply clefted and even binucleated. Less commonly, the disease in dogs may have larger cells and progress rapidly, more like the usual presentation in cats. In cattle, medium and large lymphocytes with one to three large azurophilic granules are frequently seen in the peripheral blood during inflammatory diseases. In contrast, in cats, the LGL lymphomas are usually primary in the enteric tract and rapidly peripheralize to regional nodes, liver, spleen, and kidneys with variable levels of leukemia on presentation [55]. (Fig. 17.70).

Fig. 17.70

T-cell LGL lymphoma/leukemia. Impression smears were made of a tru-cut biopsy of the liver. Hepatocytes are displaced by medium to large lymphocytes with round and indented nuclei that have a finely uniform chromatin pattern, inapparent nucleoli, and (more...)

NK-cell chronic lymphocytic leukemia, a chronic lymphocytic leukemia that overlaps T-cell chronic lymphocytic leukemia in morphology, but does not mark with CDS. NK-cell leukemias and lymphomas are not sufficiently well recognized in animal pathology for adequate description. Pathologically, the LGL leukemias of NK type are similar to the other T-cell lymphomas with gamma/delta receptors that present with hepatosplenomegaly, but usually not lymphadenopathy [55]. (Fig. 17.71).

Fig. 17.71

NK-cell type chronic lymphocytic leukemia. The lymphocytes were negative for CD3 (no T-cell receptor) and for the pan B-cell marker CD-79 alpha. There is currently no definitive NK-cell marker in dogs; however, the observed constellation of antigen expression (more...)

Cutaneous T-cell neoplasms

Cutaneous epitheliotropic lymphoma (CEL)

CEL, mycosis fungoides type, a slowly progressive, often multifocal and locally extensive cutaneous and mucocutaneous epitheliotropic lymphoma with peripheral lesions that wax and wane with time and progress to a chronic leukemic phase. Epitheliotropic lymphomas of the mycosis fungoides type (MF) occur in dogs, cats, cattle, and rarely, in horses. In cattle, the disease occurs as the sporadic lymphoma of “skin type”, typically in 2- to 3-year-olds. The disease appears as focal raised fungating areas of depilation from 2.5 to 12.5 cm in diameter and may be mistaken for ringworm in the early stages. In comparison to other lymphomas of cattle, the disease is relatively indolent and the skin lesions may wax and wane over a period of a year to 18 months before the animal succumbs to deep organ involvement. In dogs and cats, the lesions are usually focal and multifocal with prominent lymphoid infiltration into the adnexa progressing to focal areas of ulceration. Intraepithelial nests of lymphocytes, similar to Pautrier microabscess of the human condition, occur in cats, dogs, and cattle. In horses, the disease is rare and has been seen without apparent skin lesions but with brick-red oral mucosa and diffuse lymphoid infiltration of the affected papillary dermis. In all species, the tumor cells tend to be small lymphocytes of intermediate type with irregularly indented nuclei and relatively abundant, often water-clear cytoplasm. Where ultrathin sectioning has been carried out, the nuclei in cattle, and apparently in the horse, are deeply and multiply indented, similar to the human form of the disease, and are referred to as “cerebriform” in outline. If animals are allowed to survive the chronic epitheliotropic skin disease, a leukemic phase develps, particularly in the cow and horse, with the leukemic cells having the characteristic nuclear membrane irregularities. In human medicine, this phase is referred to as Sezary syndrome and the characteristic leukemic structure as the Sezary cell. In the human counterpart of this disease, the Sezary cells have been identified as predominantly of “helper” T-cell type, which might explain the tendency for animals, particularly cattle, to injure the lesions by rubbing, possibly due to itching. In dogs, cutaneous epitheliotropic lymphoma cells are usually CD8+ [55, 59]. (Fig. 17.72, 17.73).

Fig. 17.72

Cutaneous epitheliotropic lymphoma, mycosis fungoides type. There is heavy infiltration by small dark cells into the papillary dermis and basilar areas of the epithelium. A single intraepithelial "cystic" area of rumor cells in the centre. Bar = 50 microns. (more...)

Fig. 17.73

Cutaneous epitheliotropic lymphoma, mycosys fungoides type. Imunohistochemical reaction for CD3 stains numerous invading cells in the papillary dermis and epidermis of the same lesion depicted in figure 49. bar = 50microns.

CEL, pagetoid reticulosis (Woringer-Kolopp) type, a variant of epitheliotropic lymphoma in which malignant gamma/delta T lymphocytes colonize the epidermis. This type of cutaneous lymphoma occurs rarely in dogs and likely in other species. The human disease is known as Pagetoid reticulosis or Woringer-Kolopp disease. Clinically, the lesions present as areas of hyperkeratosis and become moist and ulcerate. Histologically, the malignant cells are largely above the basement membrane. The thickening of the epidermal layer is only partially due to the high level of lymphocytic infiltration (characteristic hyperkeratosis contributes). Like the cells of mycosis fungoides, the tumor nuclei are frequently surrounded by abundant, water-clear cytoplasm [55]. (Fig. 17.74, 17.75).

Fig. 17.74

Cutaneous epitheliotropic lymphoma, pagetoid reticulosis type. There is a dense infiltration of small cells in the papillary dermis and deeper layers of the epithelium. The small, clear areas around the nuclei indicate abundant clear cytoplasm of the (more...)

Fig. 17.75

Cutaneous epitheliotropic lymphoma, pagetoid reticulosis type. Immunohistochemical staining with CD3 demonstrates that lymphocytes of T-cell type are primarily above the basement membrane in the lesion depicted in figure 51. Staining for B-cells demonstrated (more...)

Cutaneous nonepitheliotropic lymphoma, a disease primarily of dogs and cats characterized by malignant lymphoid proliferation largely occupying the deep dermis. Cutaneous nonepitheliotropic T-cell lymphomas occur as thickening of the skin that focally progresses to ulceration. The lesions are frequently associated with fading germinal centers, suggesting an origin in chronic benign lymphoid proliferation in the dermis. The disease is primarily seen in mature animals, and the tumor cells have small, irregularly indented hyperchromatic nuclei and relatively abundant, clear cytoplasm resembling that of epitheliotropic lymphoma [55]. (Fig. 17.76–17.78).

Fig. 17.76

Cutaneous nonepitheliotropic lymphoma. Intact skin and a narrow grenz zone of papillary dermis remained relatively free of infiltration above a dense lymphoid infiltrate that extends into the panniculus. The focal dark areas represent fading germinal (more...)

Fig. 17.77

Cutaneous nonepitheliotropic lymphoma Imunohistochemical staining for CD3 of the tumor in the figure 53 demonstrates strong positivity for the diffuse areas of cellular infiltration with sparing of the focal areas of dense cell accumulation that markCD79a (more...)

Fig. 17.78

Cutaneous nonepitheliotropic lymphoma. Cytologically, the invading T cells have small irregularly shaped nuclei with both compact and fine areas of chromatin, but not parachromatin clearing or nucleoli. The abundant clear cytoplasm surrounding each nuclei (more...)

Extranodal/peripheral T-cell lymphoma (PTCL)

It is a peripheral extranodal and often subcutaneous lymphoid proliferations of mixed lymphoid cells or with mixed inflammatory cells mimicking granulomatous inflammation that progress to aggressive multicentric T-cell lymphoma. They are infrequently recognized in dogs and likely occur in other species. The lesions may be markedly heterogeneous with small foci of necrosis surrounded by eosinophils, macrophages, and plasma cells. Malignant lymphocytes vary from small and compact with frequently deeply indented hyperchromatic nuclei and abundant, water-clear cytoplasm to large cells with vesicular nuclei and prominent nucleoli forming a relatively heterogeneous tumor (mixed lymphoid type). An unusual type of peripheral T-cell lymphoma consists of a mixture of the smaller cells with large cells with vesicular nuclei and prominent nucleoli, but includes a mixed array of inflammatory cells. These lesions are often felt to be the result of a puncture wound or foreign-body granulomas (mixed inflammatory type). Also, because of the large component of benign inflammatory cells, there tends to be an early response to steroid treatment, but ultimately the lesions spread widely to deeper organs. Malignant lymphocytes are variably positive with CD-3 and may, in focal areas, be a minority of the cells present. In human pathology, these tumors were identified as a mature or post-thymic T-cell type, and the term peripheral was applied to indicate that they arose outside of the thymus rather than outside of internal organs. All forms of peripheral T-cell lymphomas are characterized by prominent fine vascular proliferation and fine but dense sclerosis [55]. (Fig. 17.79–17.82).

Fig. 17.79

Extranodal/peripheral T-cell lymphoma, mixed lymphoid type. The lesion is a heterogeneous with a wide variation in cell size and shape and staining reaction. Larger cells have abundant cytoplasm that is strongly marked with cd3, identifying them as T (more...)

Fig. 17.80

Extranodal/peripheral T-cell lymphoma, mixed lymphoid type. Small to large lymphocytes vary in shape and in chromatin distribution. The cells with more dense cytoplasmic staining are necrotic and retracting from neighboring cells. Cellular boundaries (more...)

Fig. 17.81

Extranodal/peripheral T-cell lymphoma, mixed inflammatory type. Subcutaneous mass from the abdominal region; there is a solidly cellular mass with multiple foci of eosinophilic necrotic debris surrounded by pale areas of cellular reaction. Bar=0.1 mm. (more...)

Fig. 17.82

Extranodal/peripheral T-cell lymphoma, mixed inflammatory type. Predominant cell has a vesicular nucleus with peripheralized chromatin and one to two prominent nucleoli and abundant clear cytoplasm. In this area, there is a heavy background of mature benign (more...)

PTCL, mixed lymphoid type, a peripheral extranodal and often subcutaneous focal nonencapsulated proliferation of mixed lymphoid cells that are characterized by slow progression to a therapeutically intractable multicentric T-cell lymphoma. The mixed lymphoid type of PTCL forms a solid lesion that resembles other T-cell proliferations, eg, T-cell-rich B-cell lymphoma and angioimmunoblastic lymphoma, in having a fine, cohesive fibrovascular network. Cytologically, these tumors are remarkably heterogeneous containing plasma cells and atypical lymphocytes from small to very large, all with prominent nucleoli and cytoplasmic marking with CD3 [55].

PTCL, mixed inflammatory type, a peripheral extranodal and often focal subcutaneous proliferation of what resembles granulation tissue with neutrophils, eosinophils, macrophages, and a minor population of large malignant lymphocytes with prominent nucleoli. The mixed inflammatory type of PTCL forms a solid lesion like PTCL of the mixed lymphoid type, but also has foci of necrosis surrounded by a gradient of inflammatory and connective tissue cells that strongly suggests the presence of an infectious agent. The background proliferation of reticulin fibers also resembles the PTCL, mixed lymphoid type, but the presence of eosinophils and other normal reactive cells provides cytologic identity [55].

Adult T-cell-like lymphoma/leukemia, a syndrome of lymphoma/leukemia typically occurring in mature animals with a long-standing retroviral infection. The enzootic form of bovine lymphoma associated with the bovine leukemia virus (BLV) is B cell associated, but is similar in many respects to the adult T-cell leukemia/lymphoma of humans. BLV is transmitted by direct contact and is cell associated. Malignant transformation is a relatively rare event in infected animals and is generally seen in animals aged 6 or older. Cytologically, the malignant lymphocytes in cattle resemble the human disease in having variable nuclear shapes, even to the point of “floral” variants, as seen in humans, and also in chromatin distribution with residual dense chromocenters. The retroviral-associated lymphoid malignancies of the cat have many of the same characteristics, but tend to affect a wider age group. The feline leukemia virus (FeLV) infects both B and T lymphocytes, and viral infection is associated with both B- and T-cell lymphoma/leukemia [55]. (Fig. 17.83).

Fig. 17.83

Adult T-cell-like leukemia. Leukemic blood picture in a mature BLV-positive Holstein cow. Despite the rapid progress of the disease, the lymphocytes retain large chromocenters more like prolymphocytic than lymphoblastic leukemias. Cell metaphase upper (more...)

Angioimmunoblastic lymphoma (AILD), a lymphoma characterized by generalized marked lymphadenopathy with microvascular proliferation that is frequently accompanied by splenomegaly, skin disease, fever, and gammopathy; it is a rare lymphoma in dogs and likely occurs in other species [55]. (Fig. 17.84).

Fig. 17.84

Angioimunoblastic lymphoma. Vimentin stain to demonstrate te extent of fine vascular and reticulin arborization. Bar=50 microns.

Angiotropic lymphoma, a spectrum of diseases in which there is a strong homing of malignant lymphocytes to vessels, primarily small muscular arteries, resulting in multifocal ischemic infarction. Lesions characterized by angiocentric and angiodestructive patterns of growth were given several names in human pathology, including angiotropic lymphoma, intravascular lymphoma, and malignant angioendotheliomatosis. Several disease entities, including midline malignant reticulosis, polymorphic reticulosis, and lymphomatoid granulomatosis, that shared features of multifocal perivascular proliferations of mixed cellularity were subsequently found to be associated with malignant lymphocytes primarily of T-cell type. The nature of lymphomatoid granulomatosis is still in question, but there is now recognition of a series of human conditions characterized by malignant perivascular T-cell proliferation with varying levels of vascular invasion. These types of lymphomas centered on blood vessels are seen in dogs and cats and have occurred at sites of prior vaccinations. The lymphomas invading the vessel wall are primarily of the T-cell type in humans and apparently also in animals. In contrast, the intravascular lymphomas are primarily of B-cell type in humans and also in animals [55, 60].

Angiocentric lymphoma, an aggressive disease of malignant lymphoid proliferation forming dense cellular cuffs around small arteries and veins that compromise the vessel and occlude the lumen with minimal mural invasion [55]. (Fig. 17.85).

Fig. 17.85

Angiocentric lymphoma. Imunohistochemical reaction with cd3 demonstrating dense lymphoid infiltrates around small vessels marked strongly as T cells. Bar=0.15 mm.

Angioinvasive lymphoma, a focal and locally extensive lesion, usually of the skin in dogs and cats, characterized by mural invasion and luminal occlusion by malignant T cells, resulting in ischemic infarction [55]. (Fig. 17.86–17.87).

Fig. 17.86

Angioinvasive lymphoma. The dog had dependent edema with an area of skin on the neck oozing serosanguineous fluid. Intact skin with an irregular infiltration around the deep adnexa, below which there are large pale areas of ischemic necrosis with focal areas (more...)

Fig. 17.87

Angioinvasive lymphoma. Infarcted area with solid cellular infiltration into the lumen of dilated small muscular arteries. Bar=50 microns.

Intestinal T-cell lymphoma, a slowly progressive small cell lymphoma of the enteric tract that appears to arise in a background of chronic inflammatory bowel disease. Intestinal or enteropathy-associated T-cell lymphomas are seen primarily in the small intestine of mature cats and less frequently in dogs. The intestinal lesion consists of an intense infiltration of small, darkly stained T cells with irregular nuclear contours invading the epithelium, similar to epitheliotropic T-cell lymphoma. Small nests of tumor cells may occur within the epithelium, and focal ulceration is a frequent finding. There is irregular involvement of the lamina propria, and a feature of malignant transformation is the irregular involvement of villi, with some relatively normal villi adjacent to others that are heavily infiltrated [55].

Anaplastic large cell lymphoma (ALCL), a rapidly progressive large cell lymphoma characterized by large and markedly pleomorphic cells. Anaplastic large cell lymphoma occurs in dogs and cats and likely other species. The disease may arise in the dermis and rapidly spreads to regional nodes. The tumor cells are markedly anaplastic with peripheralized chromatin, irregular parachromatin clearing, prominent nucleoli, and relatively abundant cytoplasm. This disease corresponds to immunoblastic polymorphic type in the Working Formulation. A transmissible lymphoma of ferrets, likely viral associated, may be of ALCL type [55]. (Fig. 17.88, 17.89).

Fig. 17.88

Anaplastic large cell lymphoma. Dense cellular inphiltrate occupies the entire pappilary and reticular dermis and extends deeply into the subcutaneous fat. Bar=0.2 mm.

Fig. 17.89

Anaplastic large cell lymphoma. Cytologically, the tumor cells are extremely variable with irregular, sharply indented nuclear margins, peripheralized chromatin with irregular thickening of the nuclear membranes, irregular parachromatin clearing, prominent (more...)

17.1.3. Miscellaneous tumors

Mast cell tumors, single or multifocal proliferations of mast cells in which the clinical behavior varies with the species and level of cellular maturation. They are common in dogs and cats, less common in horses and ferrets, and rare in pigs and cattle. In dogs, the disease usually begins in the skin, but aggressive types recur after excision and spread locally. In cats, the tumors occur both as cutaneous lesions and in a visceral form that most frequently diffusely occupies the sinus area of the spleen. In all species, these lesions are usually accompanied by a variable number of eosinophils and frequently foci of collagen degeneration (see mesenchymal tumors) [55, 62, 63].

Hodgkin-like lymphoma, proliferations of small lymphocytes in which there are scattered large, often binucleate cells with prominent nucleoli. Most cases of Hodgkin-like lymphoma in animals are better grouped under other diagnoses, with T-cell-rich B-cell lymphoma as a primary rule-out. It would appear that the lymphocyte depletion and nodular sclerosis types of Hodgkin disease are not recognized in domestic animals, and it is unlikely they are being misdiagnosed. It is possible lesions of mixed cellularity or lymphocyte-predominant type might occur since veterinary pathologists do not routinely carefully screen diffuse lymphoid proliferations for the presence of Reed-Sternberg cells (large cells containing two tightly opposed nuclei, each with peripheralized chromatin and central very large nucleoli and abundant cytoplasm, often contracting from contact with other cells). As the Reed-Sternberg cell, itself, becomes better characterized on a phenotypic and molecular basis, it is possible that applying those techniques to animal lymphomas may identify previously unrecognized tumor entities [33, 36, 54, 55]. (Fig. 17.90–17.91).

Fig. 17.90

Hodgkin-like lymphoma. The liver lesion consists of a mixed population of small and medium sized lymphocytes with occasional very large cells with hyperdistended nucleoli occupying clear lacunae. Bar = 50 microns.

Fig. 17.91

Hodgkin-like lymphoma. Binucleated cell with peripheralized chromatin and prominent nucleoli resembling Reed-Sternberg cell. Bar=microns.

Thymoma, a benign usually encapsulated tumor of the mediastinum in which lymphocytes usually predominate over the neoplastic epithelial cells, with only mild atypia of either cell type. Thymomas occur relatively infrequently in cats and dogs and are more common in mature dairy goats. Epithelial tissue is always present in thymomas and may consist of Hassall corpuscles, or a diffuse cuboidal or spindle cell component may be the predominant cytologic feature [55]. (Fig. 17.92).

Fig. 17.92

Thymoma. Well-differentiated epithelial cells of a Hassal corpuscle in one of the residual medullary areas. Bar=50 microns.

Thymic carcinoma (malignant thymoma), a malignant tumor of thymic epithelial cells characterized by cytologic features of malignancy, nonencapsulation, and metastasis. Thymic carcinoma may occur in a number of forms, including squamous, nonkeratinizing, and syncytial carcinoma. Rare types include clear cell, basaloid, and mucoepidermoid carcinomas. In all of these lesions, the epithelial component can be identified by cytoplasmic filaments of epithelial type [55]. (Fig. 17.93–17.95).

Fig. 17.93

Thymic carcinoma. Area of compression of lung with a solidly cellular mass. Bar=0.2mm.

Fig. 17.94

Thymic carcinoma. Small vessel surrounded by epithelial cells with vesiculated nuclei and peripheralized chromatin with few small lymphocytes. Bar= 10 microns.

Fig. 17.95

Thymic carcinoma. Immunohistochemistry for cytokeratin to demonstrate that the predominant cell populations are spindle-shaped epithelial cells. Bar=10microns.

Myelolipoma, a benign nonencapsulated extramedullary proliferation of normal-appearing bone marrow, which is a nonclinical, incidental finding, usually in the liver, adrenal gland, or spleen. Myelolipomas consist of histologically normal-appearing bone marrow that has a variable, often high proportion, of fat cells. The hematopoietic cells are trilineage with synchronous maturation and appear normal in all respects. Myelolipoma is a rare type of proliferation found most commonly in mature dogs and cats. In the liver, the foci may be single or multiple and may protrude from the surface of the liver or be found deep within a liver lobule. Myelolipoma of the adrenal gland is rare, most commonly found in cattle and nonhuman primates, and may be accompanied by focal areas of metaplastic bone. Myelolipomas of the spleen are incidental findings and are occasionally seen in all domestic animals. In the spleen, myelolipomas replace normal sinus areas and histologically appear as darker, more cellular areas with sharply contrasting fat cells [55]. (Fig. 17.96).

Fig. 17.96

Myelolipoma. The cells are cytologically normal and largely or erythroid lineage. There are numerous hemosiderin-bearing macrophages (lower right). Bar= 10 microns.

Malignant fibrous histiocytoma (MFH), a stromal malignancy with a fibroblastic/myofibroblastic phenotype and characteristic histologic features, including spindle cells arranged in woven or storiform patterns admixed with histiocytoid and often multinucleated neoplastic cells. MFH, also known as giant cell tumor of soft parts, tends to occur as a focal or locally extensive proliferation in the dermis and subcutis without skin ulceration. Cytologically, the lesion consists of stromal cells that have a variety of woven or storiform patterns and may be multinucleated. Histiocytes infiltrate the stromal proliferation; they express CD18 and CD45, and less consistently contain lysozyme. These histiocytes are considered a benign accompaniment to the malignant stromal cells. The distinction from low-grade fibrosarcoma is made on the uniform intermixing of histiocytes and the irregular appearance of multinucleated cells. (Fig. 17.97).

Fig. 17.97

Malignant fibrous histiocytoma. The tumor cells are round to oval and irregularly vesiculated with thickened nuclear membranes and a coarsely branched chromatin pattern. The nuclei have moderate anisokaryosis with the largest one and a half red cells (more...)

17.1.4. Benign lymphoid proliferations

Follicular lymphoid hyperplasia, a benign response to persistent immune stimulation with proliferation of germinal centers into the deep cortex and medullary areas of the lymph node. Follicular hyperplasia is a B-cell response to persistent immune stimulation. To varying degrees, germinal centers may extend from their normal location in the outer cortex to involve medullary area. Benign germinal centers have a “polarity” due to the relationship of the germinal center to the source of antigen. The area of the germinal center closest to the source of antigen is composed of small lymphocytes, while the cells furthest from the source of antigen are much larger and may be accompanied by numerous tingible-body macrophages. These areas of the germinal center may be described as superficial and deep zones where their relationship to the peripheral sinus and source of antigen is apparent. Deep within the lymph node, the direction of lymph flow is less apparent and germinal center polarity is usually described as the dark and light poles, referring to the areas of small cell and large cell proliferation. These relationships hold whether the germinal center is in lymph node, tonsil, or Peyer patch. Benign follicles always have a polarity if the germinal center is sectioned in a plane that cuts the center from the dark to light zone areas. Since lymphoid tumors, including follicular lymphomas, tend to be composed of monotonous accumulations of cells of a single, or at most two-cell, type, the degree of cellular variation seen in reactive germinal centers is an important feature in distinguishing hyperplasias from lymphomas.

In follicular hyperplasia, the follicles may be tightly apposed or separated by residual areas of paracortex and medullary structures. In general, in the early stages of follicular hyperplasia, the paracortical area is only moderately reduced and the germinal centers are surrounded by a broad continuous rim of small, dark mantle cell lymphocytes. With persistence of the reaction, the germinal centers become more tightly opposed with further reduction in the paracortical area and the mantle cell cuffs become reduced in width or discontinuous. Early follicular hyperplasias are usually characterized by medullary cord plasma cell hyperplasia and some degree of sinus histiocytosis. With persistence of the reaction, medullary structures become compressed and reduced in proportion to the overall structure of the node. In follicular hyperplasias of long standing, there will usually be some accompanying increase in the collagenous superstructure of the lymph node, which may include thickening of the peripheral capsule as well as more prominent connective tissue raphe in medullary areas. Involvement of extranodal tissues may occur in benign hyperplasias induced by prolonged antigenic stimulation, as occurs in equine infectious anemia and bovine trypanosomiasis. When involvement of extranodular tissues occurs in benign hyperplasia, the peripheral sinus always remains intact [55].

Atypical follicular lymphoid hyperplasia, an exaggerated form of benign proliferation of germinal centers with large and irregularly shaped germinal centers distorting node architecture. Polarity remains apparent in the smaller germinal centers. Atypical follicular hyperplasia represents a gray area between follicular hyperplasia and follicular lymphoma. In atypical follicular hyperplasia, there is marked variation in germinal center size and shape with irregular attenuation of mantle cell cuffs. Cytologically, the cell types within the germinal centers remain heterogeneous with small, medium, and large lymphocytes present, indicating the benign nature of the lesion.

As in follicular hyperplasia, lymphocytic colonization of perinodal tissues may occur in atypical follicular hyperplasia, but again the peripheral sinus remains intact and is not obstructed by lymphoid infiltration in the capsule or the perinodal tissues. Changes of this type are typical of cats with FeLV/FIV infections [55]. (Fig. 17.98, 17.99).

Fig. 17.98

Atypical follicular lymphoid hyperplasia. Large and irregular follicular proliferations extend into the medulla of the node with marked atrophy of paracortical areas. The germinal centers vary markedly in size and shape, and the mantle cell cuffs are (more...)

Fig. 17.99

Atypical follicular lymphoid hyperplasia. Imunohistochemical reaction with CD79a for B cells clearly delineates the mantle cell cuffs and center cells of the larger follicles.

Paracortical lymphoid hyperplasia, focal and diffuse expansile areas of lymphoid proliferation beneath the subcapsular sinus characterized by cytologic heterogeneity. Paracortical hyperplasia is the characteristic early response to antigenic stimulation, which may persist as a diffuse but solidly cellular proliferation for 10 to 14 days before defined germinal centers appear. Diffuse paracortical hyperplasia is characterized cytologically by a predominantly small cell population with a minor population of large blastic-appearing lymphocytes and numerous tingible-body macrophages. Characteristically, there is a high level of cellular traffic within the walls of the high endothelial venules. The low-power appearance of this type of reaction has been described as “moth-eaten”.

Paracortical hyperplasia may occur as large focal areas beneath the peripheral sinus being devoid of germinal centers, often with a surrounding halo of lighter stained cells. This type of reaction is very frequently seen in the mesenteric nodes of mature rats. As in follicular hyperplasias, the level of cytologic heterogeneity identifies the reaction as benign [55]. (Fig. 17.100, 17.101).

Fig. 17.100

Paracortical lymphoid hyperplasia. There is a diffuse expansion of the superficial and deep paracortex. Bar = 0.2 mm.

Fig. 17.101

Paracortical lymphoid hyperplasia. The paracortex consists of cleaved nuclei with small intermediate nuclei plus larger lymphocytes with prominent nucleoli. Two tangible-body macrophages and large areas of clear cytoplasm are present. Bar = 10 microns. (more...)

17.1.5. Canine lymphoma

Lymphomas are among the most common neoplasms in dogs, with an incidence of 13–24 per 100 000 dogs, lymphoid and myeloid leukaemia being predominant. Incidence by age is estimated to be lowest in dogs under 1 year of age, remaining low between 1 and 4 years in approximately 10% of cases; incidence is highest between 5 and 11 years, and is diminished at advanced ages. The literature mentions a higher incidence in Scottish Terrier and Boxer breeds, without any sex related influence.

The mean life duration of dogs with multicentric malignant lymphoma is 10 weeks, after diagnosis. Survival over 6 months is extremely rare; a longer survival duration has been found in old dogs, aged 12 and over.

Clinical signs at onset are vague and non-specific. In multicentric forms, the first signs consist of an increase in volume of the lymph nodes; superficial or visceral lymph nodes are bilaterally and symmetrically enlarged, and they can be 3 to 10-fold larger than normal. They are easy to palpate, soft, well delimited, non-painful, elastic, mobile, rarely adherent to skin or adjacent tissues. During clinical evolution, a decrease in volume can be found, even in the absence of treatment, but always with a recurrence of growth. The spleen is enlarged and palpable, being well delimited and non-painful. The liver can also be palpable. When involved, tonsils are usually enlarged, pale, ulcerated and prominent. Subcutaneous edema with single or multiple locations in the posterior limbs, external genital organs, mandible and sternal region is characteristic. Mucosae are pale or icteric. Sometimes, the thymus is evidenced and palpated in the cervical region. In the case of digestive locations, occlusions may appear, and diarrhea with sanguinolent feces is present in 80% of cases. Rarer locations of lymphoma are mentioned in the orbital cavity, central nervous system, nerve substance, cranial cavity or medullary canal of the bone marrow. These locations cause nervous clinical signs, pareses, paralyses, etc. [10,20, 30, 42].

In dogs, two anatomical forms are predominant: multicentric and digestive, more rarely the thymic and cutaneous form.

The multicentric form is characterized by the bilateral and symmetric involvement of all superficial lymph nodes, of most deep lymph nodes, as well as of the liver and spleen. Other tissues and organs can also be affected: the digestive tract, kidneys, heart, pancreas and bone marrow. The eyes, skin, skeletal muscles and nervous system may be sporadically involved.

The affected lymph nodes are in the order of incidence: mandibular, cervical, prescapular, retropharyngeal, mediastinal, mesenteric, popliteal, sublumbar, tracheal, bronchial, iliac, axillary and inguinal. The thymus is more rarely affected [17] (Fig. 17.7).

Fig. 17.7

Malignant lymphoma, multicentric, early phase, lymph node.

The digestive tract form is differentiated from the multicentric form through the non-involvement of superficial lymph nodes, and the extremely sporadic involvement of the spleen. The gastrointestinal segment shows characteristic lesions throughout the whole tract and the attendant lymph nodes. Gastric mucosal ulcers are found in 50% of cases. Dogs with hyper-calcemia present polydipsia and polyuria, and finally collapse. Thoracic cavity organs, as well as other lymph nodes, are more rarely affected.

The macroscopic lesional picture is characterized by the increase in volume of the lymph nodes, which have diameters between 1 and 9 cm, are non-adherent, move freely, and mesenteric lymph nodes frequently become confluent, forming large compact masses. Capsules are under tension, in section the lymph nodes show liquefaction foci, and tissue is soft, with no delimitation between the cortex and the medulla, of red-gray or cream color.

The spleen is enlarged, friable, sometimes with hemorrhagic infarctions, and lymphoid follicles are found in section, as white-gray nodules or large, fleshy, dense nodular foci.

When involved, the liver is enlarged, with small whitish foci and well defined lobulation in section, or it may present large homogeneous gray tumors. The bone marrow can be changed, of red color, replacing the fat tissue. In kidneys, in both the cortex and the medulla, gray neoplastic nodules can appear or the lesion can be diffuse [38].

In the gastrointestinal tract, diffuse or nodular tumor proliferations occur especially in the Payer plaques, which thicken the stomach and intestinal walls (Fig. 17.9.).

Fig. 17.9

Alimentary malignant lymphoma, developing lesion in Peyer’s patch.

The microscopic lesional picture is dominated by neoplastic infiltrations and proliferations with prolymphocytes, histiocytes and lymphoblasts. Lymph nodes completely lose their normal structure due to the invasion of neoplastic cells in the architecture, capsule and the perinodal area. B lymphocytes can be identified in digestive forms.

In the spleen, tumor cells form periarteriolar nodules or are disposed as an arteriolar and trabecular envelope; more rarely, these cells accumulate in the red pulp, causing the generalized destruction of the architecture (Fig. 17.8.).

Fig. 17.8

Malignant multicentric lymphoma, spleen, with malignant cells extending in the white pulp.

In the liver, neoplastic cells accumulate in the portobiliary space and less around the centrolobular vein. Extramedullary hematopoietic foci are frequently found in both the liver and spleen.

Renal lesions have a perivascular onset in the cortical area, but the neoplastic infiltrate gradually extends to the medullary interstitium. The gradual interstitial development of the tumor causes the compression and atrophy of glomerules and uriniferous tubules, but also of blood vessels, followed by necrosis. Pulmonary lesions consist of perivascular and peribronchiolar infiltrations, more discretely in interalveolar septa or even in alveoli.

No relative or absolute increase in the number of lymphocytes or young forms is found in peripheral blood.

A diffuse invasion or a neoplastic cell focus is detected in the bone marrow.

Approximately 2/3 of the dogs with malignant lymphomas have anemia, usually the normocytic and normochromic type. These animals show a regeneration of erythrocytes, with the presence of nucleated cell elements and other young forms.

Plasmacytoid tumors are predominantly found in the intestine, skin and spleen. Lymphoplasmacytic enteritis is found in the intestine. Mediastinal lymphoma is associated with T-lymphocyte cells and pseudohyperparathyroidism with hypercalcemia (Fig. 17.14.).

Fig. 17.14

Solitary plasmocytoma.

Cutaneous lymphoma is found in dogs, cats, cattle and horses, but the highest frequency is reported in dogs. The cutaneous lesion can be of focal or diffuse type in dogs and horses, focal in cattle and diffuse in cats. Depending on the cutaneous invasion of neoplastic cells located at the dermoepidermal junction, dermotropic lymphoma and epidermotropic lymphoma can be differentiated [27].

Dermotropic lymphoma is characterized by plaque-like lesions, in 2-year-old cows. Microscopically, papillae are affected, with extension to the depth, to the papillary dermis, and epidermal foci under the form of microabscesses, described in humans as mycosis fungoides.

Epidermotropic lymphoma is characterized by a marked thickening of the epidermal layer, with tumor cells that infiltrate epidermal layers in a diffuse manner, known as the Woringer-Kolopp disease in humans. In animals, the disease has only rarely been reported in cats and dogs. Cutaneous lymphomas are predominantly formed by large cells and, in the mycosis fungoides lesion, nuclei are highly convoluted, probably of thymic origin. In humans, these lesions are not considered to derive from the bone marrow. In dogs, cattle and horses, some of these lesion forms, of mycosis fungoides type, progress slowly over several years, invading the organ parenchyma and bone marrow.

Large cell angiotropic lymphoma is common in humans and dogs, possibly in other species. Typical tumor cells are located under the intima, in the small muscular arteries of the lung; they can also be in the atrium and more rarely in other tissues. The manifestation is of lung congestion, hypoxia and death. Congestive pulmonary atelectasis is pathomorphologically found. Microscopically, typical large or cleaved tumor cells lie under the endothelium or on its luminal side, and erythrocytes are abundant in the center, forming drastic pulmonary congestion.

17.1.6. Feline lymphoma

Feline lymphoma is induced by the horizontal transmission of feline leukemia retrovirus (FeLV), being the most common neoplasm in cats. The disease affects in particular young subjects, under 5 years of age, without any sex-dependent sensitivity, although frequency is higher in males.

In affected cats, feline leukemia virus can be identified in lymphoid cell membranes from neoplastic lymph nodes, bone marrow, ileum and cecum, thymus, spleen, liver and kidneys. It is also found in megakaryocytes, blood platelets and other hematopoietic cells. Free viral particles have been identified in pulmonary alveoli, trachea, tonsils, renal tubules, urinary bladder and urine, salivary glands and intestinal lumen. More than 90% of the cats infected with FeLV are infected with feline immunodeficiency virus (FIV).

Feline leukemia virus has been identified in other diseases: microproliferative disorders, infectious peritonitis, transmissible fibrosarcoma, some forms of anemia, etc.

Due to the fatal acute evolution, the clinical picture is uncharacteristic, postmortem lesions being rather characteristic. The death of affected subjects occurs within 4–8 weeks, sometimes even 1–2 days after the detection of clinical signs. Digestive locations manifest by vomiting and diarrhea or constipation. In the thymic form accompanied by hydrothorax, cough, signs of suffocation, slow difficult abdominal breathing, cyanosis and occasional oral breathing occur. The general signs are lethargy, anorexia, weight loss, muscle emaciation, pale mucosae and recurrent fever. Sometimes, due to secondary tumor infiltrations in the kidney, uremia appears, and jaundice is the result of liver involvement. Hemorrhage occurs following liver failure and deficient platelet formation in the bone marrow. Paralyses due to the involvement of the spinal cord can also appear.

Depending on location, feline lymphomas can be classified into: thymic, multicentric, leukemic and solitary forms. The digestive form is the most frequent.

The digestive form manifests by the presence of a single tumor, but most frequently by multiple tumors along the digestive tract and in the mesenteric lymph nodes. Tumors are located in the pylorus and along the intestine up to the colon, but frequently in the jejunum and ileum. The kidneys and liver are frequently involved.

Macroscopically, lymph nodes are enlarged, and in section structure is homogenized, of cream color, with fleshy appearance. Diffuse thickenings or nodular proliferations are found in the intestinal walls. Tumor infiltration affects the mucosa, submucosa and the muscularis of the intestinal wall on extensive segments. The annular proliferations of the intestinal wall induce occlusions, followed by dilations in the anterior segment. The mucosal surface in tumor proliferation areas is ulcerated and hemorrhagic. Pleomorphic cells with diffuse and perivascular arrangement are histologically found. Cells have abundant eosinophilic cytoplasm, with granulations and/or vacuoles, large convoluted nucleus and prominent nucleolus [47].

The thymic form is characterized by the presence of large tumor masses in the cranioventral mediastinal region, occupying 2/3 of the thoracic cavity. Hydrothorax and pulmonary atelectasis, the involvement of sternal and mediastinal lymph nodes are concomitantly found.

The thymic tumor mass occupies the cranioventral thoracic region, with the possible deviation and compression of the trachea and esophagus, pulmonary and cardiac compressions. The tumor proliferation may also be found in the thoracic walls, pericardial sac and pleura.

Hydrothorax and pulmonary atelectasis are constant lesions.

The multicentric form appears with the involvement of the majority of lymph nodes, frequently the spleen and liver. Tumor foci can be identified in the digestive tract and kidneys.

Kidneys are frequently affected bilaterally. Prominent homogeneous consistent white-pink formations appear under the capsule. In section, the cortex is affected, but in longer evolutions the lesion also involves the medulla. Tumor infiltration may be diffuse, with the firm adherence of the capsule, which is difficult to detach.

The leukemic form is characterized by the involvement of the bone marrow, spleen and liver. Solitary lesions are also identified in the kidney.

The spleen can be slightly enlarged, with the evidencing of follicles, sometimes with obvious hypertrophy. The liver can be affected to various extents, sometimes being significantly enlarged, with increased lobulation through the tumor infiltrate, along with congestion and hemorrhage.

The microscopic picture is dominated by one of the four cell types: stellate cells, histiocytes, lymphoblasts and prolymphocytes. In the case of the thymic form, lymphoblasts and prolymphocytes are predominant. Intestinal lymphoid neoplasias of the digestive tract start in Peyer’s patches, then they gradually extend and become confluent. In the spleen, lymphoid cell proliferation starts either in the center or towards the periphery of lymphoid follicles. The involvement of the bone marrow in the case of lymphomas is inconstant.

Hematologic examination has demonstrated that in feline lymphoma, the peripheral blood picture is slightly changed and non-characteristic. With the advancement and aggravation of the disease, lymphopenia, normocytic and normochromic anemia occur.

17.1.7. Bovine lymphoma

Lymphoma in cattle predominantly manifests under the form of enzootic bovine leukosis, at adult age, and sporadic forms appear in young animals (the lymphonodular type of calves; the thymic form; the cutaneous form).

Enzootic bovine leukosis is considered as a retroviral infection, which is transmitted horizontally by direct contact, by natural breeding and by medical instruments.

The incidence of the disease varies from one country to another, but it is almost constantly diagnosed, sometimes even after some geographical areas have been declared unaffected. The mean age at which incidence is higher is estimated between 5 and 8 years, but all age groups are affected. Regarding breed sensitivity, this factor does not seem to significantly influence the disease percentage; however, the Danish Red breed has played an important role in the dissemination of enzootic leukosis.

Clinical signs do not appear immediately after infection, when an asymptomatic period occurs. The first changes consist of the hypertrophy of prescapular, precrural, retromammary and iliac lymph nodes. Other changes such as exophthalmia, tympanism, edema, splenic and hepatic hypertrophy may also appear.

Lesions have somewhat characteristic locations depending on the age of cattle. Thus, there are specific or marked intensity locations in unweaned calves, young cattle aged 6–30 months, and adult cattle.

In calves , lesions affect all lymph nodes, with bone marrow involvement, major liver and spleen involvement. In 50% of cases, other organs are affected: the thymus, kidneys, heart and uterus.

At this age, locations have a multicentric form, with the obvious development of hematopoietic tissues, which determines a high similarity to the leukemic form, without changes in the peripheral blood picture. Congenital forms with cutaneous [22] or lymph node and visceral locations [21] have also been diagnosed. Microscopically, they are characterized by: generalized hyperplasia of the lymphoid tissue; leukemia in 50% of cases and immunodepression; in other cases, severe leukopenia and hypogammaglobulinemia [4].

Young cattle , aged 6–30 months, constantly show changes of the thymus. This organ is highly hypertrophied in both the cervical and thoracic regions. Lymph nodes are involved to a smaller extent.

Adult cattle have a multicentric form, involving in decreasing order: the lymph nodes, heart, abomasum, kidneys and urethra, uterus, liver and spleen. Skin is affected sporadically, under the form of multiple nodules in the dermis, frequently in the dorsal and lateral region of the head, neck, body and perianal regions. Cutaneous leukosis is a sporadic form and is found in cattle aged 1–3 years, annual incidence being estimated at 1–3 cases per 1 million. The disease starts as urticaria-like lesions, with multiple cutaneous nodules that may regress and subsequently reappear. In this form of leukosis, the enzootic bovine leukosis virus has not been identified. Small perivascular lymphocytes are microscopically present. The presence of lymphocytes coincides with the regression and disappearance of neoplastic cells. A sustained immune activity is suggested by the activation of germinative centers from lymphoid formations [39].

The anatomopathological picture is dominated by changes in the lymphohematopoietic organs. Lymph nodes are 2–5-fold larger than normal, sometimes forming compact masses with sarcomatous appearance. Lymph nodes have a firm, fleshy or friable consistency, white-gray, light yellow or red-pink color. In section, structure is homogeneous, with the disappearance of the difference between the cortex and the medulla, and the presence of hemorrhagic or hemorrhagic necrotic foci. Hemonodules are sometimes enlarged, of red-cherry color, with neoplastic appearance.

Cardiac lesions start and are more marked in the myocardium of atria, especially the right atrium, under the form of large fleshy nodules and/or diffuse masses that can extend to the whole heart.

The abomasum and duodenum show an exaggerated wall thickness, with the diffuse extension of the tumor. The neoplastic lesion has a fleshy appearance, white or white-gray color. The mucosa of the abomasum and duodenum is atrophied, with numerous ulcerations.

The kidneys, uterus, liver and spleen may present proliferative nodular lesions or diffuse proliferations. The neoplastic formations are dense, lardaceous, yellow-gray, homogeneous in section, sometimes with hemorrhagic or hemorrhagic necrotic foci. In diffuse forms, the organs are significantly enlarged, and splenic follicles are obvious.

The thymus is significantly enlarged, either the whole gland or just the cervical or thoracic portion. The tumor development makes the lobular structure of the thymus disappear, having the aspect of neoplastic masses with scant poorly defined connective tissue, sometimes invading and infiltrating adjacent tissues. The blood vessels from the jugular groove, carotid arteries and jugular veins are incorporated in the tumor mass.

The bone marrow, affected in particular in calves, shows necrotic and hemorrhagic lesions. In long bones, the marrow appears with necrotic foci, with cheesy, friable, gray to yellow spots. The compact mass of the affected bones is rarefied (ribs, pelvis and skeletal bones), and neoplastic subperiosteal proliferations can sometimes appear.

The histopathological picture is characterized by the possibility of the presence, in decreasing order of incidence, of the following cell types: lymphocytes, prolymphocytes, lymphoblasts and histiocytes. Lesions can be produced by each of the mentioned cell types or by the mixed presence, in variable proportions, of these cells. The first changes in lymph nodes are detectable in medullary sinuses. The thymus can present nodular formations that are well delimited by fibrous connective tissue, with diffuse neoplastic infiltrations, with the formation of cell groups or bundle arrangement.

Electron microscopically, in both tumor formations and the peripheral blood of affected cattle, the lymphocyte nuclei have nuclear membrane projections, with digitiform appearance or undulating deformations. The presence of a type C oncornavirus in the blood of cattle with enzootic leukosis has been demonstrated. In thymic forms, the presence of the virus or antibodies has not been proved.

17.1.8. Ovine and caprine lymphoma

Ovine and caprine lymphoma has a lower incidence compared to bovine lymphoma, probably also due to the younger economic age of small ruminants. The bovine leukemia virus induces the disease, both naturally and experimentally, in ovines as well as caprines.

Lymphoma in small ruminants evolves under a multicentric form, in which there is a major involvement of lymph nodes, especially iliac, mediastinal and cervical ones. The affected organs are: the liver, kidneys and heart, with a lower incidence than in cattle; lesions may be identified in the abomasum, intestine and other abdominal viscera. The lesion appearance is similar to that of cattle.

Cytologically, the presence and frequency of blast cells of prolymphocytic, lymphocytic, lymphoblastic and histiocytic type can be found. Electron microscopy has detected the presence of oncornavirus-like particles. It seems that there is an etiological and pathogenetic similarity between ovine and bovine lymphoma.

17.1.9. Equine lymphoma

Lymphoproliferative diseases in horses are similar to bovine diseases. Malignant lymphoma in the horse has an incidence of over 50% at ages between 4 and 9 years, and under 10% in horses under 4 years of age [9, 36]. No breed or sex related predisposition has been reported. The disease has a sudden onset, with acute and short evolution. The clinical picture includes lethargy, weight loss and malnutrition. In the case of intestinal involvement, colic occurs. The topographic and cytological characteristics allow the differentiation of the leukemic form and lymphoma.

The leukemic form is usually found in thoroughbred horses that undergo training, manifesting by bone marrow involvement, with pancytopenia and severe thrombocytopenia, subcutaneous edema and petechiae of the serosae. Myelophthisis is found in the bone marrow, with the destruction of normal cells, of lymphocytes. Clinically, the disease evolves in the course of several weeks [24].

Cutaneous lesions are minimal at the limits with the mucosae, appearing as red spots, similar to mycosis fungoides in humans. Histologically, tumor cells are dermotropic, medium sized, with cerebriform nuclei.

Solid forms of lymphoid tumors in horses are characterized by subcutaneous, digestive, abdominal, splenic and thymic/multicentric locations.

Subcutaneous lymphoma has a benign evolution and is frequent in females. Subcutaneous lesions, with a diameter of 1–3 cm or larger, may regress. Cluster lesions are located in the thorax, flanks, perineum, limbs and they frequently develop on the face and nose.

Microscopically, the tumor develops subcutaneously, probably along the lymphatic areas. Tumor cells are particularly diverse, but large lymphocytic cells and small cleaved cells are characteristic, as well as a fine network of reticular fibers and collagen bands between cells. Anemia and hypoproteinemia gradually develop.

Subcutaneous lymphoma can also evolve under a diffuse dermal form, with the appearance of periorbital lesions 2–3 cm in diameter, with ulcerations. Microscopically, the lesion has a sclerotic character, with a remarkable tendency to local recurrence and frequent local dissection.

The digestive form presents clinical signs of decreased appetite, colic and sometimes diarrhea. Anemia can be of moderate intensity or extremely severe, normocytic, with hypoproteinemia. Neutrophilia usually appears with toxemia signs, due to intestinal mucosal ulceration, but without leukemia. Some animals with this tumor form show hemolytic anemia, which can dominate clinical signs. Peripheral and internal lymph nodes are normal, which makes difficult diagnosis by biopsy.

Tumor cell infiltration is obvious in the digestive tract: in the mucosa, muscularis mucosa and submucosa, but more discretely in the muscularis. Tumor cells have a uniform plasmacytoid appearance, with round nuclei, with the diameter of one or two erythrocytes, with abundant, intensely basophilic cytoplasm, and eccentric arrangement. Nuclear atypias are moderately present, nucleoli are not always obvious, and chromatin is hyperchromatic, with heterochromatin agglomerations and few mitoses.

The abdominal form of equine lymhoma is rare, and clinical diagnosis is made by the presence of large tumor masses in the caudal abdominal lymph nodes. Clinically, a gradual decrease of appetite occurs, with edema and recurrent diarrhea. Splenomegaly is morphologically found, as well as the development of hepatic lymph nodes, with the development of tumors in the small and large intestine, with mucosal ulcers. Microscopically, the tumor has a variable structure, with the predominance of diffuse large cell infiltration with numerous mitoses.

The splenic lymphoma is an isolated lesion with the exaggerated development of the organ, reaching 15 kg or even more. Animals show hemolytic anemia, which leads to the suspicion of infectious equine anemia. In the case of splenic lymphoma, lesions are almost exclusively located in the spleen. The tumor is formed by large immunoblastic cells.

Clinical diagnosis is extremely difficult, but in most cases, positive diagnosis is made by necropsy.

Multicentric lymphomas can be diagnosed at all ages, with a higher frequency in mature animals. The affected animals are in a poor condition but they keep eating, which results in a low protein level and the appearance of edemas. An exaggerated growth of peripheral mediastinal lymph nodes is found, and particularly of pharyngeal and mandibular lymph nodes. Leukemia is occasionally present, but the blood picture is frequently typical of cachexia. Morphologically, tumors are spread in the abdominal cavity and are frequently present in the mediastinum, developing in the heart and lung.

Cytologically, tumors have no typical cell component, but most cells are intermediate or with a high typization grade, immunoblastic cells or diffuse large cells. Most cells are T lymphocytes.

17.1.10. Swine lymphoma

Lymphoma in swine is a tumor with a high incidence, being considered the most frequent neoplasm in pigs, representing 23–41% of all neoplasms. Lymphoma usually affects pigs under 1 year of age, and it can be diagnosed even under 6 months.

In swine, the multicentric form is the most common, with the predominant involvement of visceral lymph nodes, and among organs, of the spleen, liver and kidneys, as well as the bone marrow. Less affected are the lungs, skin, serous membranes, mammary gland and other organs. The leukemic form is diagnosed occasionally.

In the etiology of swine lymphoma, the possible role of an autosomal recessive gene is mentioned, hence the aspect of familial disease, but also the possibility of the presence of a type C virus, detected in the neoplastic lymphoid cells, without the disease being transmissible [36, 51].

Histologically, the neoplasm develops as a lymphoma with centrofollicular cells, having diffuse local growth. Cells are immunoblastoid or plasmacytoid, containing Ig M and Ig G. These swine lymphomas resemble non-Hodgkin lymphoma in humans [26, 37].

17.1.11. Hodgkin-like lymphoma

In animals, a syndrome similar to human Hodgkin’s disease is rarely found. The majority of cases have been described in dogs, and extremely sporadic cases have been reported in horses and swine.

Macroscopic lesions in dogs consist in an increase in the volume of neck, inguinal and pelvic lymph nodes, of mesenteric and anterior mediastinal lymph nodes. Lymph nodes are dense, fibrous, and in section they are white-gray. Tumor nodules are identified in the liver, spleen and lungs. Skin lesions have been more rarely reported [33, 54].

Histologically, the lesion is mainly characterized by lymphoproliferation, with the presence of characteristic Sternberg-Reed cells. In some cases, lymphohistiocytic, histiocytic proliferations are found, with the possible appearance of nodular sclerosis images. Characteristic of Hodgkin’s disease is the Sternberg-Reed cell, which has a markedly lobulated nucleus, creating the appearance of two or more separate nuclei, with large, prominent, acidophilic nucleoli. The Sternberg-Reed cell is derived from histiocytoid cells in which nucleus division occurs, without the separation of the cytoplasm. Histology shows the proliferation and predominance of histiocytoid cells or the predominance of lymphocytes; fibrosis, of variable intensities, can be diffuse or nodular. Hemorrhagic, necrotic foci and/or amyloid degenerations are found in the tumor mass [36].

17.2.1. MALIGNANT MYELOID PROLIFERATIONS

17.2.1.1. Acute myeloid leukemia (AML)

Precursor myeloid leukemia, a spectrum of diseases characterized by the presence of undifferentiated or poorly differentiated cells in the peripheral blood with a normal or only moderately elevated leukocyte count that rapidly progresses to marrow failure and death [23].

Acute leukemias with minimal maturation cannot be reliably diagnosed as of myeloid (granulocytic, megakaryocytic, or erythroid) or lymphoid lineage without special staining. In general, Sudan black B (SBB) and myeloperoxidase (MPO) are the principle reactions utilized for identifying primitive cells of myeloid origin on microscopic examination, or specific antibodies may be used in flow cytometry. These diseases are primarily seen in mature dogs and cats, but probably occur rarely in all species. The animal with acute myeloid leukemia presents with depression, epistaxis, anemia, and severe neutropenia. The marrow aspirate shows loss of erythroid precursors and of mature granulocytes with a predominance ( = or > 20%) of poorly differentiated blast cells. Residual promyelocytes and myelocytes of benign origin will be present and cannot be used to distinguish between acute leukemias of lymphoid and myeloid origin [55].

Acute myeloid leukemia in dogs and cats may be classified into subtypes depending on the morphological characteristics and the cellular types involved.

Acute myeloid leukemia/undifferentiated leukemia (AML M0), a peracute disease characterized by a proliferation of undifferentiated myeloid cells that generally occurs in young animals in good condition and progresses from apparent health to marrow failure with anemia and thrombocytopenia in less than a month [55].

This leukemia subtype was previously known as reticuloendotheliosis. Blast cells have an uncertain or lymphoid morphology and high phosphatase alkaline activity; they can be ranged in this category until the obvious specificity of a certain line is obtained. This category also includes cases in which the marrow has almost 100% blast cells, which cannot be classified based on morphological and cytochemical features. The myeloid identity of these blasts can be defined by electron microscopy, ultrastructural cytochemistry and/or immunophenotyping, in the way they are organized for acute undifferentiated leukemia in humans, which will allow to reconsider their categorization (Fig. 17.28. and 17.29.).

Fig. 17.28

Acute lymphoblastic leukemia (ALL). A mixture of small mononuclear cells with clumped chromatin typical of lymphocytes and blast cells considered to be lymphoblasts. Cytochemical reactions for peroxidase, nonspecific esterase, and alkaline phosphatase (more...)

Fig. 17.29

Acute undifferentiated leukemia (AUL). Bone marrow in this case was dominated by undifferentiated and poorly differentiated cells with deep blue cytoplasm, often forming broad pseudopodia, and densely stained nuclei with stippled chromatin and indistinct (more...)

Acute myeloid leukemia without maturation (AML M1) is characterized by the dominance of type I myeloblast in the bone marrow, while type II is rare. These cellular types represent >90% of all nucleated cells. Differentiated granulocytes represent <10% of non-erythroid cells and frequently include promyelocytes ready to segment into neutrophils and eosinophils. Erythroid megaloblast cells and mitotic figures can be more rarely present (Fig. 17.30. and 17.31). The blasts are large cells with round or oval nuclei, multiple nucleoli, and finely distributed, deeply staining chromatin, the cytoplasm is basophilic, and agranular.

Fig. 17.30

Myeloblastic leukemia without maturation (M1). Blast cells with fine chromatin and distinct or indistinct nucleoli and a thin rim of blue nongranular cytoplasm were predominant. Granulocytic differentiation was minimal. Cytochemical staining revealed (more...)

Fig. 17.31

Myeloblastic leukemia without maturation (M1). Myeloblasts without granulocytic differentiation were the chief cells. Myeloblasts had nuclei with smooth or stippled chromatin, distinct or indistinct nucleoli, and a small to fair amount of bluish cytoplasm (more...)

Acute myeloblastic leukemia is found in adult dogs and cats, with an increase tendency in males. The lesion has also been identified in swine and extremely rarely in other species.

Acute myeloid leukemia with maturation (AML M2) is characterized by myeloblasts >30% to <90% of all nucleated cells, with variable proportions of type II blasts. Differentiated granulocytes represent <10% of non-erythroid cells, with the predominance of promyelocytes or with a disproportionate number of promyelocytes readily segmented into neutrophils and occasionally eosinophils. Promyelocytes have more azurophilic granulations than type II myeloblasts. Granules are usually uniformly dispersed in the bluish cytoplasm, but sometimes they are concentrated in an area, especially in the proximity of the Golgi region. The amount of cytoplasm is generally higher than in myeloblasts, but it can be identical. The monocytic component is <20% of all non-erythroid cells. Delicate granules similar to Auer rods can be rarely identified. Approximately half of tumor cells give positive reaction to Sudan black and myeloperoxidase. Usually, the liver and spleen are non-palpable, and lymph nodes have normal characteristics. Acute myeloblastic leukemia results in death, caused by hemorrhage and infections. AML M2 is a peracute disease that generally occurs in young animals.

Anemia is moderate and is produced by myelophthisis or complications through thrombocytopenic hemorrhage. Anisocytosis is not severe and there are many poikilocytes, and the presence of Jolly bodies indicates a reduction in the splenic function. In dogs affected by this myeloblastic leukemia with maturation, marked thrombocytopenia is found, with rare immature or even absent platelets. Leukocyte count is high, within variable limits, frequently under 25×10⁹/liter, but with an increase of over 50 × 10⁹/liter in the proximity of death. In this case, absolute neutropenia and usual lymphopenia occurs, with myelocytes in which promyelocytes, metamyelocytes and myeloblasts are predominant.

Bone marrow aspirate presents a high cellularization and cellular density, with numerous lipid vacuoles. Normal medullary elements are present, few rubricytes, mature megakaryocytes and hemosiderin granules. Myeloblasts and promyeloblasts are predominant, while metamyelocytes are rare.

Necropsy shows the paleness of mucosae and tissues, lymph nodes have normal sizes, they are frequently edematous, the liver has normal sizes but is pale, the spleen is also within normal limits, with possible infarctions. The bone marrow does not show particular changes, with uniform vascularization, with hemorrhagic foci. The thymus is atrophied. No compact tumor foci have been noted in parenchymatous organs. Due to marked neutropenia, septic lesions and minimal cellular reaction may be found.

Histologically, neoplastic cells are detected, which are characterized by: round nuclei, peripheral chromatin, with prominent nucleoli and a moderately amphophilic cytoplasm. Mitoses are frequent and metamyelocytic cells are present. Mature cells from normal series are rare and these are of neutrophilic type, most segmented cells being normal eosinophils. Megakaryocytes and rubricytes are rare or absent.

The spleen shows lymphoid atrophy, more obvious in thymus-dependent areas. Few erythrocytes are present in the sinuses, and hypercellularity is conferred by tumor cells. Benign hematopoiesis is present and is characterized by paratrabecular thrombopoiesis. Degenerative ischemic images appear in the liver. The tumor has a sinusoidal distribution, and leukocytes are in a low number. The microscopic structure of lymph nodes is variable, with moderate lymphoid atrophy (Fig. 17.32–17.34.).

Fig. 17.32

Myeloblastic leukemia with maturation (M2). Myeloblasts and neutrophilic differentiation restricted to promyelocytic stage were evident in this case of AML. Myeloblasts had a small amount of nongranular blue cytoplasm, while promyelocytes had prominent cytoplasmic (more...)

Fig. 17.33

Myeloblastic leukemia with maturation (M2). Myeloblasts and neutrophilic differentiation from myielocytes to segmented neutrophils characterized bone marrow in this case. Large cells with many azurophilic granules, a clear area of Golgi apparatus, and nuclei (more...)

Fig. 17.34

Myeloblastic leukemia with maturation (M2). A slightly damaged promyelocyte containing many azurophilic granules along with delicate, rarely encountered, Auer rod - like structures in the cytoplasm. **)

Myeloblastic leukemia with maturation is different from acute lymphoblastic leukemia based on cytoplasmic granulations and positive reaction of tumor cells to chloracetate-esterase.

Acute promyelocytic leukemia (AML M3) is little defined, being considered comparatively to what is found in humans. Some promyelocytes contain many prominent cytoplasmic granules, but, unlike M₃ promyelocytes in humans, they have round nuclei, with a smooth nuclear outline, without folds. So, promyelocytic leukemia (M₃) has rather been included in M₂ than in M₃. (Fig. 17.102).

Fig. 17.102

Acute promyelocytic leukemia. Predominant peripheral blood leukocyte has a round or oval nucleus with a fine chromatin pattern, small nucleoli, and relatively abundant cytoplasm with a mixture of azurophilic and basophilic granules. Some larger cells (more...)

Promyelocytic leukemia has a rare incidence in mature animals, being reported in dogs, cats and swine. Clinical signs are similar to those of myeloblastic leukemia. Death usually occurs due to intracranial hemorrhage.

Acute myelomonocytic leukemia (AML M4), a form of acute myeloid leukemia, is recognized by the presence of granulocytic and monocytic lines. Myeloblasts and monoblasts represent together >30% of all nucleated cells, and differentiated granulocytes and monocytes represent each >20% of all nucleated cells. A higher differentiation of neutrophils can appear than in the case of M₂ (Fig. 17.35.).

Fig. 17.35

Myelomonocytic leukemia (M4). Blast cells with features of myeloblasts (nuclei with stippled chromatin) and monoblasts (nuclei with streaky nuclear chromatin) are present along with neutrophil granulocytes and monocytes. Blast cells have indistinct nucleoli (more...)

Myelomonocytic leukemia has rarely been reported in dogs, cats and horses. The disease frequently presents clones that act together in both tumors derived from monocytic and neutrophilic systems. This disease is defined by a percentage of at least 20% in both cell lines and by staining for the neutrophilic series (chloracetate esterase, Sudan black or myeloperoxidase activity) or for the monocytic series (fluoride sensitive esterase).

Affected animals show the paleness of mucosae and skin, anorexia, an uneven enlargement of cervical lymph nodes, slight splenomegaly and moderate fever. Thrombocytopenia is marked, and leukocytes are between 25 and 50 × 10⁹/liter. Morphologically, leukocytes are similar to myeloblastic leukemia with a certain maturation and acute monocytic leukemia in the same animals. Malignancy is determined by nuclear atypia and immaturity in peripheral blood, which are present in the whole system.

Petechiae, infarction foci in the liver, kidneys, spleen and marrow are pathomorphologically found, as well as frequent oral ulcers. Microscopy shows tumor infiltration in the marrow, splenic sinuses, lymph nodes, in the portal area of the liver, and irregularly in other organs, including the lungs. Sometimes, in the case of benign extramedullary hematopoiesis, in adrenal glands and lymph nodes, myelomonocytic leukemia should be cytologically and cytochemically differentiated from monocytic leukemia, from which it cannot be histologically differentiated.

Acute monoblastic leukemia (AML M5A), an acute myeloid leukemia characterized by moderate leukocytosis with peripheral blood blasts that have nuclear indentations and fine perinuclear vacuolations in highly basophilic cytoplasm of moderate volume; it is a rare disease usually of young mature animals which presents with splenomegaly, hepatomegaly, and anemia. M5A is more likely to have extramedullary disease in the form of bleeding gums or focal skin involvement. The bone marrow is highly cellular with a high proportion (80%) of monocytic cells, 20% of which are blasts [55]. (Fig. 17.103).

Fig. 17.103

Acute monoblastic leukemia. Cow. A poorly differentiated blast cell in lower center with two promonocytes with cytoplasmic vacuolisations plus a basophilic rubricyte and a large granular lymphocyte.

Acute monocytic leukemia (AML M5B) is characterized by monocytic predominance. Monocytes and promonocytes are > 80% of non-erythroid cells in M5A, while in M5B they are between 30 and 80%, with a marked differentiation of monocytes. Blast cells in M5A may have clear areas in the Golgi space, which is not found in blasts in M5B. The granulocytic component is <20%. Occasionally, dyserythropoiesis appears under the form of megaloblastic erythroid cells.

Predominant cells in monocytic leukemia are monocytic precursors, which are defined by non-specific esterase activity (α-naphthyl butyrate). Monocytic myeloblastic leukemia can evolve under two variants. A first variant is characterized by the predominance of monoblasts with a low progression of monocyte forms in routine blood examinations; in the second variant, differentiated monocytes are obviously predominant in both the blood and bone marrow. Leukocytes tend to be decreased or normal in the majority of acute forms and increased in the differentiated variant (Fig. 17.22–17.24.).

Fig. 17.22

Monocytic leukemia, blood film.

Fig. 17.23

Monocytic leukemia, blood film.

Fig. 17.24

Malignant monocytosis, lymph node.

Monocytic leukemia occurs in young and adult subjects in both dogs and cats. In the acute form of the disease, the clinical manifestation is similar to myeloblastic leukemia. Affected animals present lethargy, weight loss, vomiturition, diarrhea, anorexia and, which is highly specific, recurrent epistaxis. Affected animals show marked anemia (<50 g/l) and are characterized by tumor cells in microvascularization. In general, evolution is of 2–4 weeks. In blood, tumor cytology is dominated by monomorphic cells, with the presence of monoblasts and promonocytes. Monoblasts are large, about three times the diameter of erythrocytes, with irregular nuclei and intensely basophilic cytoplasm. Promonocytes have small nuclei, with a diameter 3-fold higher than that of an erythrocyte, with convoluted or cerebriform outline, and chromatin is fine, having one or two nucleoli, of moderate sizes. The cytoplasm is intensely basophilic in benign young monocytes, characterized by a uniform density of the nuclei and cytoplasms. A clear area is detected around the lymphocyte nucleus. Azurophilic granulations appear in the Golgi region, as well as frequent vacuolizations, which penetrate the nucleus. Rubricytes can be present, and anemia is non-reactive (Fig. 17.36. and 17.37.).

Fig. 17.36

Monocytic leukemia (M5a). Monoblasts and promonocytes, cells with undulated nuclear outline and/or folded nuclear membrane and chromatin, predominate in this form of acute monocytic leukemia. Many of the cells stained positive for nonspecific esterase (fluoride (more...)

Fig. 17.37

Monocytic leukemia (M5b). Prominent monocytic differentiation from monoblasts to monocytes was the main feature of this form of acute monocytic leukemia. These cells showed a strong nonspecific esterase (fluoride inhibitable NAE) activity. **)

The marrow aspirate is intensely cellularized, with erythroid cells and megakaryocytes. Approximately 75% of the marrow cells are undifferentiated blasts, monoblasts and promonocytes, with hyperchromatic nuclei, prominent nucleoli and a moderate mitotic rate.

Lesions are similar to those of myeloblastic leukemia, with frequent local hemorrhage, and less common secondary infections. Lesions are distributed in particular in the urinary system and adrenal glands.

The diagnosis of certainty requires cytochemical investigations of the blood and bone marrow by aspirate or impression. (Fig. 17.104–17.106).

Fig. 17.104

Acute monocytic leukemia. There are three promonocytes, one of which contains an ingested red cell. Nuclei have irregular indentations and folding with a fine uniform chromatin pattern. Cytoplasm is basophilic and irregularly vacuolated. Bar = 10 microns. (more...)

Fig. 17.105

Acute monocytic leukemia, marrow aspiration. There are numerous primitive cells with finely aggregated chromatin, occasional large nucleolus and highly basophilic cytoplasm that is finely vacuolated and contains fine azurophilic granules.

Fig. 17.106

Acute monocytic leukemia. Within the liver of the dog, there is ischemic degeneration of Hepatocytes in central vein areas and dense cuffing of portal triads with tumor cells. Bar = 0.2 mm.

Erythroleukemia (AML M6A): in this case the erythroid component is >50%, and monoblasts and myeloblasts are <30% of all nucleated cells. Erythroleukemia is recognized when at least one of the following criteria is present:

• – myeloblasts and monoblasts in a proportion of >30% of all non-erythroid cells;
• – all blast cells, including rubriblasts, are >30% of all nucleated cells.

Erythroleukemia M₆ defines a predominance of rubriblasts in the erythroid component. In general, M₆ shows the significant value of the granulocytic and monocytic difference that can be present.

Erythroleukemia is found in parallel as an erythroid and myeloid (granulocytic) tumor. It is a rare disease, appearing as a primary form in dogs and cats, with sudden onset by weakness, depression and anorexia. Bone marrow lesions cause chronic areactive anemia, hemorrhage and septic lesions. Erythremic changes manifest after 1–2 months and result in the acuteness of myelogenic leukemia (Fig. 17.38. and 17.39.).

Fig. 17.38

Erythroleukemia (M6). Erythroid component was >50 % and blast cell count was >30 % of NEC. A few myeloblasts, megaloblastic rubricytes, and mitotic erytroid cells can be seen in this field. **)

Fig. 17.39

Erythroleukemia (M6Er). Bone marrow showed preponderance of rubriblasts having large, distinct nucleoli, patchy chromatin, and deep blue cytoplasm with a clear Golgi area. **)

The disease frequently continues with erythremic myelosis, but with a marked decrease in immature myeloids, the myeloblastic component finally becoming prominent. Dyserythropoiesis is marked and persistent, and anemia is severe and areactive. Thrombocytopenia is severe, and the total number of anucleated cells is 50 × 10⁹/liter, with 25–75 rubricytes. It is pathomorphologically similar to erythremic myelosis.

Erythremic myelosis (AML M6B) is defined by 50% or more nucleated erythroid cells in the bone marrow, with at least 10% dysplastic cells and at least 30% primitive cells. Erythremic myelosis that is associated with feline leukemia virus infections in cats has also been reported in mature Holstein cattle and extremely rarely in other species of domestic animals.

Clinical signs manifest by anorexia, occasional vomiting and diarrhea. Spleno-and hepatomegaly, unchanged lymph nodes, paleness of the mucosae and sometimes jaundice are morphologically found. The disease evolves in 1–3 months.

Microscopy shows intense cellularization in the bone marrow, with the absence of granulocytes. Approximately 90% of cells are blasts with small rests and apparent hemoglobin cytoplasm. Megakaryocytes are present in small nests. Primitive cells have fine chromatin granules, with a central prominent nucleolus. Nuclei are round or slightly irregular and the cytoplasm has a moderate volume and is intensely colored. The spleen shows the atrophy of thymus-dependent areas and a growth of the sinus stroma, with a loose arrangement and diffuse distribution of tumor cells. Lymph nodes generally have an atrophied cortex, with sinus histiocytosis and occasional focal invasion with neoplastic cells. Usually, the liver shows an evidencing of lobulation by perilobular ischemia and an increased hemosiderin amount in the Kupffer cells, which suggests an increase in erythrolysis. The tumor usually infiltrates in the sinusoids, with their dilation, and discrete tumor foci are found in the portobiliary space.

Erythremic myelosis should be differentiated from hemolytic anemia. In cats with immune hemolysis, the blood picture is sometimes marked by rubricytosis of 50–90 × 10⁹/l, but this precursor is accompanied by 300–600 × 10⁹ reticulocytes per liter. Circulating spherocytes and erythrocytes are present in immune hemolysis, erythroid immaturity is less severe in erythroid myelosis and nuclear atypia is absent [36]. (Fig. 17.107–17.108).

Fig. 17.107

Erythremic myelosis. Blood with a prorubricyte and many polychromatic rubricytes. Severe anemia is evident in the wide spacing and hypochromicity of red cells. There is marked anisocytosis, but little polychromasia. Bar = 10 microns.

Fig. 17.108

Erythremic myelosis. Poorly differentiated blast cells with round nuclei and multiple small nucleoli. The chromatin is increasingly aggregated into large chromocenters resembling prorubricytes and basophilic rubricytes. Bar = 10 microns.

Acute megakaryoblastic leukemia (AML M7) has been diagnosed in dogs. The differentiation of megakaryocytes in cats and dogs can be performed by positive reaction to acethylcholine-esterase and complemen-tary electron microscopy, when alpha granules and the membrane demarcation system can be noted. The diagnosis of myelodysplastic syndrome with megakaryocytic predominance or essential thrombocytopenia is made when the number of megakaryoblasts is <30%.

According to the French-American-British group, megakaryoblastic leukemia is diagnosed when the bone marrow aspirate contains >30% of all nucleated cells or non-erythroid cells, as well as megakaryoblasts. Mega-karyoblasts can be detected in the blood, but they are frequently in low numbers. They can be identified by peroxidase reaction, immunological reactions for glycoproteins IIb/IIIa or electron microscopy. In megakaryoblastic leukemia, many patients show myelofibrosis and reticular growth, with the occasional increase in the number of mature megakaryocytes.

In human medicine, recent evidence estimates that this form has an incidence of about 8% of all acute leukemias. Megakaryoblastic leukemia affects humans and animals over an extremely wide age range.

Clinical signs are characteristic of acute leukemia, frequently with fever, subcutaneous hematomas and gastrointestinal hemorrhage. Animals have a good maintenance state, which shows a rapid evolution of 2–4 weeks, and hepato- and splenomegaly are characteristic.

Moderate to severe pancytopenia, areactive anemia and thrombocytopenia characterizes the disease, and the total leukocyte content is about 5 × 10⁹/l with neutrophils and peripheral blast cells. A characteristic of nuclei is their diameter 2 to 3-fold larger than the erythrocyte diameter, and these are round or irregular and sometimes binucleated. Chromatin is dense and cribriform, and nucleoli are not prominent. Clear vacuoles traverse the nuclei and the cytoplasm of tumor cells. Malignant myeloblasts with oval cytoplasm and indistinct outline can appear, having 1–2 slightly rounded pseudopodia. Basophilia is marked, there are poorly evidenced azurophilic granulations, with a variable presence of cytoplasmic protrusions.

The bone marrow aspirate can be cellularized, but irregular and rapid sclerosis makes useless the obtaining of biopsy for the evaluation of bone marrow. This causes an early reduction in erythro- and granulopoiesis and a decrease of the granulocyte reserve. Medullary cells, hyperchromatic blasts with variable sizes, shapes and number of nuclei in the cells vary between 70 and 90%. A characteristic of megakaryoblastic leukemia is the presence of a high number of nuclear fusions.

Histology shows lymphoid atrophy in the spleen and an obvious expansion of blast cells in the sinus, irregular nests of megakaryocyte-like cells, with marked atypia and sometimes with extremely large nucleoli. An increase of splenic stroma is found, and extramedullary hematopoiesis is absent or minimal. The tumor is present in hepatic sinusoids and irregularly in lymph nodes. Emboli in pulmonary arterioles are found, with enlarged tumor cells under the form of syncytia. Fibrin is detected in emboli foci, which shows a tendency of this tumor to produce disseminated coagulations. The presence of a benign stroma colonized by malignant megakaryoblasts is found in the bone marrow. (Fig. 17.109–17.111).

Fig. 17.109

Acute megakaryoblastic leukemia. Marrow histology demonstrating early sclerosis and irregular sinusoidal dilatation.

Fig. 17.110

Acute megakaryoblastic leukemia. Marrow histology with intrasinusoidal a typical megakaryocytes with incomplete nuclear fusion. Bar = 10 microns.

Fig. 17.111

Acute megakaryoblastic leukemia. Marrow histology indicates myelophthisis of normal marrow cells by dense fibrosis. A typical megakaryocytes persist in upper right. Bar = 50 microns.

Myelodysplastic syndrome (MDS) is characterized by a percentage of <30% blast cells of all nucleated cells or non-erythroid cells of which the erythroid component is more or less than 50%. The bone marrow is usually normal or hypercellular, but it can also be hypocellular. Dysplastic changes are more prominent in one or more cellular lines, erythroid, myeloid or megakaryocytic. Peripheral blood frequently indicates leukopenia with or without deviations to the right. Typically, the myelodysplastic syndrome is recognized when blast cells are <30% of all nucleated cells from the marrow and with <50% erythroid cells. When the erythroid component is >50%, the myelodysplastic syndrome is recognized if blast cells are <30% of all nucleated cells or non-erythroid cells including rubriblasts. In this case, the term myelodysplastic-erythroid syndrome (MDS-Er) can be used to indicate the predominance of erythroid cells in the bone marrow. Erythremic myelosis can remain below M₆Er or MDS-Er, since the erythroid component is >50% of all nucleated cells, and blast cells including rubriblasts can be more or less than 30% (Fig. 17.40.).

Fig. 17.40

Myelodysplastic syndrome (MDS). Myeloblasts were <30 % of NEC. Neutrophilic differentiation was prominent and approximately 1 % of basophils were present in the bone marrow. Two cells with large, purplish basophil granules are present in this (more...)

17.2.2. Lymphoproliferative disorders

Lymphocytic leukemia is one of the most frequent leukemia forms found in dogs and cats. Sarcomatous masses are not present in true leukemia, but lymphosarcoma may be expected. This is an important differentiating feature in approximately 11–28% of dogs and 27% of cats, which present a leukemia of the blood picture.

Acute lymphoblastic anemia is characterized by immature cells in the blood, and the identification of cellular lines requires cytochemical examinations, alkaline phosphatase activity, but this cytochemical procedure is not specific for myeloid leukemia. In chronic lymphocytic leukemia, lymphocytes are small and well differentiated. This leukemia form requires a differentiation from physiological lymphocytosis, especially in cats, and chronic lymphocytosis induced by autogenic stimulation in canine ehrlichiosis [29].

Plasmacytic leukemia: its diagnosing requires the identification of four factors: a monoclonal gammopathy, the presence of more than 20% plasmacytes in the bone marrow aspirate, Bence Jones proteinuria and radiographic evidence of osteolysis. Plasmacytic myeloma is sporadic in dogs and cats, and plasmacytic leukemia is rare.

Plasmacytic myelomas are infrequent. EDWARDS et al. (1993) analyzed 10 cases of plasmacytic leukemia in horses. The authors found a mean age of 11 years (3 months – 22 years), and an equal proportion between females and males. The main clinical signs identified in affected horses were: anorexia, fever, limb edema, pneumonia, ataxia, epistaxis, lymphadenopathies and bone pain. The blood picture was characterized by leukopenia, thrombocytopenia and circulating plasmacytes, in variable forms. Biochemistry showed an abnormal protein concentration and hyponatremia and less constant hypocholesterolemia, hypercalcemia and azotemia. Some subjects manifested a decrease in the normal immunoglobulin concentration. The Bence-Jones protein was found in one subject, and the radiography of three horses showed lytic bone lesions. Typical plasmacytes or an increase in the number of plasmacytes in bone marrow sections were found in seven of the eight horses examined. Extraosseous lesions, plasmacytic infiltrations, were located in the lymph nodes, kidney, spleen, liver, lung, skin, digestive tract mucosae, nervous system and orbit [1, 16, 35, 40, 43]. Cutaneous plasmacytomas in dogs can be associated with the presence of primary amyloid, of immunoglobulin origin (IgG) [43].

In cats, vomiting and splenomegaly, gastric and duodenal ulcers are found, with the presence of mastocytoid neoplasms. Splenomegaly can be massive, consistency high, with the fibrous thickening of the capsule. Microscopy shows diffuse infiltration in the spleen, which destroys normal architecture through mast cells. The cytoplasm is abundant, with specific granules. Hepatomegaly is present, with diffuse infiltration or under the form of irregular tumor cell foci, visible on the liver surface. In other organs, such as the kidney and bone marrow, infiltrations are much more discrete. Sometimes, extramedullary plasmacytoma can be associated with primary amyloidosis of immunoglobulin origin [7].

17.2.3. Chronic myelogenous leukemias (CML)

Neoplastic diseases characterized by the overproliferation of mature or well-differentiated leukocytes, erythrocytes, or platelets and accompanied by blasts in the peripheral blood.

Chronic myelogenetic leukemia, chronic monocytic leukemia (CMoL) and chronic myelomonocytic leukemia (CMMoL) can be considered under the incidence of MDS because blast cells are <30% of all nucleated cells and the erythroid component is <50%. These leukemias are distinguished from classic MDS by the absence of dysplastic changes in the bone marrow and the presence of marked leukocytosis in the blood. The latter is frequently accompanied by deviations to the left in chronic myeloid and absolute monocytic leukemia, in chronic monocytic leukemia (CMoL) and in chronic myelomonocytic leukemia (CM MoL). Some dysplastic granulocytes and megakaryocytes have been found in CM MoL in humans.

Chronic myelogenetic leukemia, also known as chronic granulocytic leukemia, is most frequently found in dogs and adult cats, and probably in other species as well. Male cats are more frequently affected.

Paleness of mucosae, glossless hair, weight loss, splenomegaly and decreased cardiac rate are clinically found. Lymph nodes can be unchanged or significantly enlarged.

The presence of segmented granulocytes can be predominant in peripheral blood. Malignancy images can be present in all three granulocytic lines, but neutrophils are the most frequently and intensely affected. Eosinophilic line leukemia is more common in cats, and basophilic tumors are more frequent in swine. Hypersegmentation and irregular segmentation is characteristic. The cytoplasm is slightly basophilic and granular in promyelocytes and myeloblasts.

The bone marrow aspirate is hypercellular, with a 50 or higher percentage of myeloids-erythroids, and a marked increase in metamyelocytes and the neutrophil group, which occupy with many segmented cells more than half of the bone marrow volume. Erythropoiesis is synchronous and is not deficient like in acute leukemia. Hemosiderosis is normal or reduced, and megakaryocytes have a normal level for a long period, but it becomes deficient with the progression of the disease and the increase of aggressiveness.

Macroscopy shows a paleness of tissues, muscle emaciation, hemorrhagic foci, blood in the intestine and chronic cutaneous and oral mucosal ulcers. The spleen is enlarged, dry in section and fleshy. The liver is enlarged to various extents, pale, with hemorrhagic foci, infarctions or infiltrations. Lymph nodes are unevenly enlarged in volume, and all the bone marrow is intensely red.

Microscopically, the bone marrow is highly cellularized, with sinus reduction. Metamyelocytes and groups of numerous giant cells are predominant. Erythropoiesis is relatively significantly reduced, with the presence of numerous cavities in the marrow. Megakaryocytes can be numerous and hyperlobulated, except for the case in which the nucleus appears as a condensed mass, and the cytoplasm is reduced in volume in relation to the nucleolus. These proliferative cells have a round or oval nucleus, with poor chromatin granules and a single central, pink nucleolus. The cytoplasm of these cells is abundant and moderately amphophilic.

Lymph nodes have hyperplastic follicles due to secondary infections. Primitive tumor cells colonize the medullary area, and mature cells are predominant in sinuses. These tumors are obviously eosinophilic or basophilic, colonization is extensive and is frequently associated with medullary sclerosis.

The spleen has a thin capsule and foci, follicles are prominent, with few cells. Sinuses are hypercellularized and contain foci of primitive cells, with basophilic cytoplasm and several mature cells, less than in the marrow. Muscle vessels have the endothelium colonized by tumor cells.

The liver has sinusoidal regression foci, leukemic infiltrates in the portal space and around the large veins. Invasive foci can be identified in the adrenal medulla, around renal cortical arterioles and less frequently in the myocardium, in the reproductive system organs, lung and intestine.

Chronic myeloid leukemia should be differentiated from leukemoid reactions and the hypereosinophilic syndrome with the presence of blasts in peripheral blood and tissue damage caused by infiltrated tumor cells [51].

17.2.4. Myelodysplastic syndromes

Idiopathic myelofibrosis/myeloid metaplasia (MMM), a chronic disease characterized by leukocytosis with a moderate left shift usually accompanied by moderate splenomegaly and poorly responsive anemia. Myeloid metaplasia with myelofibrosis is most often seen in dogs and cats and rarely in other species. The disease, also known as agnogenic myeloid metaplasia, is characterized by apparently benign myeloid hyperplasia without an apparent target in the peripheral blood. These diseases tend to progress to marrow failure with multiple cytopenias or pancytopenia. A characteristic finding is the presence of oval or teardrop red cells in the peripheral blood, indicating the presence of developing myelofibrosis. (Fig. 17.112).

Fig. 17.112

Idiopathic myelofibrosis/myeloid metaplasia. Histology of marrow demonstrates a complete loss of fat cells and an increased area of hematopoiesis, but reduced cellular density. There is abundant coarsely granular hemosiderin and widespread coarse and (more...)

Chronic myelomonocytic leukemia (CMML), a chronic disease, seen primarily in mature dogs, consisting of mild to moderate pancytopenia that may continue for a number of months to a year. It was previously known as the “preleukemic syndrome”. In dogs, the disease is characterized by a total leukocyte count in the 4,000/μl range, with neutropenia and atypical monocytoid cells as the predominant leukocyte. There is nonresponsive anemia and moderate thrombocytopenia. Paradoxically, the marrow is hyperplastic, and on initial examination, it may appear that the marrow is about to rebound from a period of depression, but an increase in the peripheral blood cell counts never materializes [55]. (Fig. 17.113, 17.114).

Fig. 17.113

Chronic myelomonocytic leukemia. In this photomicrograph of the blood, there is a promonocyte with irregular-shaped nucleus, high cytoplasmic basophilia and vacuolation. Bar = 10 microns.

Fig. 17.114

Chronic myelomonocytic leukemia. Marrow cytology. A cluster of cells including blasts and a number a promonocytes with no erythroid precursors. Bar =10 microns.

Refractory anemia with excess blasts (RAEB), a myelodysplastic syndrome characterized by solidly cellular bone marrow occupied by an excess of poorly differentiated blast cells, usually of myeloid type, accompanied by fine and progressive reticulin sclerosis and ineffective hematopoiesis. Refractory anemia with excess blasts is primarily seen in cats. The disease tends to occur in young mature animals and is characterized by persistent pancytopenia, often terminating in bacteremia. Cytologically, the marrow is characterized by an increase in erythroid blasts that may constitute up to 20% of marrow nucleated cells, with a low level of blast cells in the peripheral blood. The erythroid lineage characteristically has cytologic abnormalities consisting of binucleated late-stage rubricytes and small satellite nuclei, as well as nuclear budding in young rubricytes. Pathologically, the marrow is solidly cellular and cohesive, indicating a considerable degree of fine sclerosis [55].

17.2.5. Solid myeloid proliferations

Cutaneous histiocytoma, a relatively common benign tumor of epidermal Langerhans cells, usually occurring as a simple lesion, often on the head or ear of young dogs. The epidermis is usually hyperplastic and often ulcerated; epidermal invasion by histiocytoma cells occurs frequently, and intraepidermal aggregates of histiocytes may resemble Pautrier aggregates that are found in epitheliotropic lymphoma, mycosis fungoides type. The nuclei of histiocytoma cells are of moderate size and generally round to oval or slightly indented with a vesicular appearance. There are usually numerous mitoses. Nucleoli tend to be small or inconspicuous. Histiocytomas must be distinguished from mast cell tumors and from lymphoma, especially epitheliotropic lymphoma [55]. (Fig. 17.115–17.116).

Fig. 17.115

Cutaneous histiocytoma. Lesion is composed of a dense proliferation of large cells with round to oval nuclei, one and a half to two red cells in diameter. The chromatin is hyperchromatic, coarsely granular, and peripheralized. There are one to two small (more...)

Fig. 17.116

Cutaneous histiocytoma. Skin mass with intra-epidermal nests of histiocytes, and perivascular to diffuse lymphoid infiltrates indicative of early regression. Bar = 100 microns.

Cutaneous histiocytosis (cutaneous reactive histiocytosis), a rare condition occurring in young mature to aged dogs of many breeds and characterized by multiple focal dermal plaques or nodules of interstitial (dermal) dendritic cells and T cells. Cutaneous histiocytosis (CH) is a disease caused by multifocal proliferation of activated dermal dendritic cells, a type of interstitial dendritic cell. These particular interstitial dendritic cells express CD1, CD11c, and MHC class II, similar to histiocytoma, but also express CD90 (Thy-1) and CD4, which, in this instance, are markers of dermal dendritic cells and activation, respectively. The lesions of cutaneous histiocytosis usually involve the deep dermis and subcutis and, occasionally, the draining lymph node(s). In contrast to histiocytoma, involvement of the superficial dermis is inconsistent and epitheliotropism of the histiocytes is not observed. The lesions of cutaneous histiocytosis develop around dermal vessels (angiocentric) and consist of perivascular histiocytic infiltrates containing lymphocytes and other inflammatory cells in variable proportions. The histiocytic cells in CH have oval, twisted, or folded vesicular nuclei with inapparent or small nucleoli and abundant cytoplasm with indistinct cell borders. Lesions of similar histology that involve skin and also mucosal sites (conjunctiva, nasal) and other tissues are considered to be systemic histiocytosis (SH). Cutaneous histiocytosis is not a neoplastic disease but an immunoregulatory disorder. Response of lesions to corticosteroids is inconsistent. More profound immunosuppression with either cyclosporin A or leflunomide is highly effective [55]. (Fig. 17.117).

Fig. 17.117

Cutaneous histiocytosis, canine skin. Bar = 80 microns.

Systemic histiocytosis (systemic reactive histiocytosis), a familial disease of Bernese mountain dogs and sporadic in other breeds in which large, dense non-neoplastic proliferations of interstitial dendritic cells occur in a number of tissues, especially skin, peripheral lymph nodes, and ocular and nasal mucosa (Fig. 17.118–17.119).

Fig. 17.118

Systemic histiocytosis in a Bernese mountain dog. Lymph node. Perivascular histiocytic infiltrates expand the paracortex (arrows). The capsule and trabeculae are also infiltrated. HE stain; bar = 80 microns.

Fig. 17.119

Systemic histiocytosis in a Bernese mountain dog. Pleocellular infiltrates surround and infiltrate a small interstitial pulmonary vessel. Histiocytes and lymphocytes predominate. These lesions can expand and coalesce, but destructive mass formation as (more...)

Systemic histiocytosis (SH) is a disease characterized by multifocal proliferation of activated interstitial dendritic cells. As such, the histiocytic cells in SH express a constellation of identical antigens to those expressed in cutaneous histiocytosis; namely CD1, CD11c, MHC class II, CD90 (Thy1) and CD4. The skin is almost always involved in systemic histiocytosis, and the lesions are histologically identical to those of cutaneous histiocytosis with a characteristic angiocentric and angioinvasive polymorphous infiltrate dominated by histiocytes and lymphocytes. The mixed infiltrate may suggest granulomatous or pyogranulomatous inflammation, again necessitating special stains and cultures to exclude the possibility of an infectious etiology. Lesions in the skin may be multiple, and additional organ involvement can be extensive, but the lesions are not neoplastic. Immunosuppressive therapy with cyclosporin A or leflunomide is highly effective, and corticosteroids are rarely effective [55].

Histiocytic sarcoma/malignant histiocytosis, a disease primarily of dogs and less commonly of cats characterized by a proliferation of malignant histiocytes that are occasionally avidly hemophagocytic.

Histiocytic sarcomas (HS) are either solitary or multiple lesions that rapidly disseminate in most instances and are therefore, often clinically indistinguishable from malignant histiocytosis, which, by definition, implies a multicentric origin. Primary sites for disseminated HS and malignant histiocytosis are spleen, lung lymph node, bone marrow, skin, and subcutis. Solitary HS can occur in any of the above sites, although solitary lesions closely associated with subsynovium of large appendicular joints and in brain deserve special mention. Liver involvement is consistently secondary to primary splenic disease. The majority of histiocytic sarcomas and malignant histiocytoses are tumors of dendritic antigen-presenting cells (APC). In lymphoid tissues, these malignancies appear to arise from the interdigitating dendritic cell population, but the exact cellular origin is more obscure at other sites (Fig. 17.120).

Fig. 17.120

Histiocytic sarcoma, canine spleen. The red pulp is infiltrated by mononuclear cells and multinucleated giant cells. Nuclei are complex and frequently hyperchromatic. The cytoplasm exhibits fine vacuolation. Bar = 40 microns.

A less common variant of histiocytic sarcoma or malignant histiocytosis displays prominent hemophagocytosis, and the cells of these malignancies express markers characteristic of resident macrophages (and not dendritic cells) in the splenic red pulp and bone marrow, with secondary liver involvement being typical. A dog with the hemophagocytic form of malignant histiocytosis often presents with splenomegaly, hepatomegaly, and severe, nonresponsive or mildly responsive anemia. The anemia is due in part to massive destruction of erythroid cells within the malignant histiocytes, although an associated concurrent erythroid suppression cannot be excluded.

Histiocytic malignancies can, therefore, be functionally divided into dendritic cell tumors and macrophage tumors, the latter being associated with the hemophagocytic syndrome. Tumor cells exhibit marked pleomorphism, including large mononuclear and multinucleate round-to-polygonal cells and also spindle cell forms. The tumor cells may have huge nuclei, up to 10 red cells in diameter, that are variably hyperchromatic with nucleoli larger than red cells [55, 56].

Granulocytic sarcoma, an extramedullary proliferation of granulocytic cells that may be of neutrophil or eosinophil type. The disease occurs in dogs, where the lesions tend to involve the lung, gut, and skin; in cattle, the skeletal muscle is usually involved. In some cases, the tumors may progress to a leukemic phase, but this is less common than local extension [55].

17.2.6. Benign myeloid proliferations

Leukemoid reaction consists of a persistent elevation in total leukocyte count, consisting primarily of cells of the neutrophil series with immaturity that may extend to myelocytes or promyelocytes, but not to blast cells. A typical hematologic response of the bitch to pyometra has been called a leukemoid reaction, but, in this condition, the level of cellular maturity and the presence of toxic changes as well as systemic disease requires that malignancy is seldom a serious rule-out. Leukemoid reactions may occur in the dog as part of the myeloid metaplasia and myelofibrosis syndrome, which is chronic and a myelodysplastic syndrome, or as an early and fleeting response to estrogen toxicity [55, 57].

Post-toxic response. Animals may present with blood dyscrasias, suggesting there are toxic injuries to bone marrow. Dogs, usually female, given toxic levels of diethylstilbestrol at a moderate level will respond with a leukemoid-type reaction that may persist for 1 to 2 weeks; profound pancytopenia, often terminating in death due to marrow failure. In lead poisoning, the main changes consist of the presence of rubricytes or nucleated red cells in the peripheral blood of a nonanemic animal associated with an unusual level of basophilic stippling in younger red cells. Occasionally, both companion and larger domestic animals may be inadvertently exposed to warfarinlike compounds; this intoxication usually presents an anemia due to enteric bleeding. Where animals such as ruminants and swine are exposed to high levels of copper in the feed, as may occur from pasture dressing with chicken manure, a variety of syndromes may result, including acute hemolytic crisis [55].

Loeffler-like syndrome, a disease of marked persistent eosinophilia with mild left shift usually associated with diffuse lung infiltration. It is occasionally seen in domestic animals, primarily in dogs and cats. Persistent eosinophilia is usually associated with inflammatory reaction of body surfaces, including the skin, gut, and lung. Fleeting eosinophilia may occur in pigs and calves exposed to high levels of ascarid egg contamination [55].

The investigations and recommendations published by the group for the study of leukemias in animals [23] offer criteria for the identification and definition of different leukemia forms. At the same time, the authors emphasize the fact that there are still cases in which positive diagnosis requires additional, highly specific investigations. Thus, techniques for the demonstration of enzymatic markers, ultrastructural examination and/or immunophenotyping can be used. The recommendations of the cited authors mention expressely the need for the examination of at least 200 blast cells from carefully selected areas in order to establish the number of differentiated leukocytes in the blood and to calculate the number of blast cells, as well as the percentage from the total number of nucleated cells and the total number of non-erythroid cells from the marrow. “Border” cases require the examination of 500 or even 1000 cells. The number of blast cells is the sum of myeloblasts, monoblasts and megakaryoblasts.

Cytomorphological diagnosis in acute myeloid leukemia and myeloblastic syndrome uses bone marrow examination, while blood examination is not a routine procedure. The immunophenotypic aspect and the method of the specific binding of antibodies to the cytoplasm and by membrane determinants is frequently used in human medicine, for the diagnosing of uncertain cases. Cytogenetic and molecular analyses play an important role in the diagnosis of animal and human leukemias.

Fig. 17.5

Malignant lymphoma, histiocytic, lymph node. ^*)

Fig. 17.10

Lymphoblastic leukemia, blood film. ^*)

Fig. 17.11

Lymphoblastic leukemia, bone marrow. ^*)

Fig. 17.12

Lymphoblastic leukemia, spleen. ^*)

Fig. 17.13

Lymphoblastic leukemia, limph node with leukemic cells in medullary cords. ^*)

Fig. 17.17

Myeloid leukemia, blood film. ^*)

Fig. 17.20

Myeloid eosinophilic leukemia, lymph node. ^*)

Fig. 17.21

Acute erythraemia, bone marrow. ^*)