Clinical Manifestations

The spongiform encephalopathies of humans include kuru, Creutzfeldt-Jakob disease, the Gerstemann-Straussler-Schenker syndrome and fatal familial insomnia. These subacute progressive degenerative diseases of the central nervous system are always fatal and are characterized by progressive dementia, myoclonus, ataxia, pyramidal and extrapyramidal signs, spiking slow waves on the electroencephalogram, absence of cerebrospinal fluid pleocytosis, and no changes in clinical chemistry or hematologic values.

Structure

In recent years we have come to recognize all these diseases as the transmissible cerebral amyloidoses, with their infectivity associated with the modification of the same host precursor protein into insoluble amyloid fibrils. These fibrils appear as rods by electron microscopy or as crystalline deposits in the form of extracellular amyloid plaques by light microscopy. The structure is mainly unknown but may be a normal host membrane sialoglycoprotein that crystallizes into rods and/or fibrils on extraction. There is no identifiable nucleic acid, no foreign protein, and no antigenicity. The agent is highly resistant to most organic and inorganic chemicals, ultraviolet light, and cobalt-60 radiation; is heat stable; is nonantigenic (no foreign protein); and has no virus-like structure by electron microscopy.

Classification

Classification is based on clinical signs, histopathologic lesions, and detection of a specific sialoglycoprotein of 27 to 30 kDa (PrP^27–30).

Multiplication

The replication process is presently unknown. Hypotheses include (1) autocatalytic alteration of a normal host precursor protein to a ß-pleated insoluble and protease-resistant form, and (2) induction of different post-translational modifications of a normal host protein.

Pathogenesis

The development of the natural disease is unknown. Infection can be caused by iatrogenic transmission via corneal transplant, implants of contaminated electrodes, injection of contaminated human pituitary gland-derived growth hormone, or gonadotropin, and transplantation of contaminated dura mater. Between 10 and 12 percent of Creutzfeldt-Jakob disease cases are of familial origin (autosomal dominant pattern); kuru is transmitted by ritualistic cannibalism. In general, the pathology is that of noninflammatory, transmissible, cerebral amyloidosis including slowly progressive vacuolation of neurons of the gray matter and to a lesser extent astrocytes (status spongiosis), astrocytic gliosis, neuronal dropout, and periodic acid-Schiff-positive amyloid plaques composed of precursor protein (PrP).

Host Defenses

None of the defense mechanisms known to control conventional viral diseases is effective against spongiform encephalopathy viruses. Inflammation does not occur and antibody is not produced during the infection, and the course of infection is not altered by suppression or potentiation of the host immune response. Also, interferon is undetectable and administration of exogenous interferon or interferon inducers is not protective. The diseases are always fatal.

Control

Medical practice has changed to eliminate the routes of transmission and to take into account the resistance to inactivation of these unconventional viruses. Cessation of the practice of ritualistic cannibalism has resulted in the disappearance of kuru.

Scrapie

Creutzfeldt-Jakob Disease

Clinical Manifestations

The unconventional viruses have also been called slow infections because the asymptomatic incubation period may extend over many years (Table 71-1). The criteria established for slow infections have four cardinal features: (1) a prolonged incubation period (several months to several years), (2) a progressive clinical course of disease always leading to death, (3) pathologic lesions limited to a single organ system, and (4) a limited natural host range.

Four diseases of humans (Table 71-2) and six diseases of animals are recognized as belonging to the subacute spongiform virus encephalopathies.

Table 71-2

Brain Lesions And Clinical Manifestations Of Subacute Spongiform Virus Encephalopathies In Humans.

Structure

The pursuit of the transmissibility and viral etiology of kuru and the presenile dementia of the Creutzfeldt-Jakob disease type has led to the definition of the unconventional viruses as a new group of microbes. Because of their very atypical physical, chemical, and biologic properties, this new definition has stimulated a worldwide quest to elucidate their structures, epidemiology, and clinical manifestations and to resolve the paradoxes involving the basic tenets of microbiology. We currently believe that these viruses are infectious proteins resulting from a modification of a host precursor protein (PrP) to an infectious form consisting of an insoluble cross-ß-pleated configuration by a process that may involve autonucleation and autopatterning.

The structure of the unconventional viruses is implied by their spectrum of resistance and sensitivity to inactivating agents. These viruses are resistant, even when partially purified, to all nuclease, ß-propriolactone, EDTA, and sodium deoxycholate. They are moderately sensitive to high concentrations of phenol (>60 percent); chloroform; ether; urea (6 to 8 M); periodate (0.1 M); 2-chloroethanol; alcoholic iodine; chloroform-butanol; hypochlorite; alkali; chaotropic ions, such as thiocyanate, guanadinium, and tricholoracetate; proteinase K; and trypsin (when partially purified). However, such treatments inactivate only 99 to 99.9 percent of the infectious particle, leaving behind highly resistant infectivity. Sodium hydroxide and sodium hypochlorite cause medically acceptable inactivation. The viruses are relatively resistant to ultraviolet light frequencies that affect nucleic acids and aromatic amino acids. However, they are sensitive to frequencies that are thought to affect polypeptides. Moreover, they show remarkable resistance to ionizing radiation, which would indicate a target size of less than 10 kDa. These atypical properties have led to speculation that the infectious agents lack a nucleic acid and they may be self-replicating protein.

Classification

There are other persistent infections of the human central nervous system that are caused by classic viruses (see Ch 96). These viruses can be visualized as distinct morphologic structures by electron microscopy; they induce specific antigen-antibody reactions; they usually induce histopathologic lesions more generally associated with virus infections; and they have an RNA or DNA genome. Both conventional and unconventional viruses are capable of inducing subacute progressive degenerative diseases of the central nervous system many months to years after the initial infection. Examples of nervous system slow infections caused by conventional viruses include subacute sclerosing panencephalitis caused by measles virus; progressive multifocal leukoencephalopathy caused by papovaviruses (see Ch 66); rubella virus-induced encephalopathy (see Ch 55); cytomegalovirus encephalopathy (see Ch 68); adenovirus (see Ch 67); Russian spring-summer encephalitis virus (see Ch 54); HTLV-I, and human immunodeficiency virus (HIV) encephalitides (see Ch 62). Unlike these conventional viruses, the unconventional viruses, although not slow in replication, have a long progressive asymptomatic incubation period from months to years before onset of clinical signs. Oligomers or microfilaments of this amyloid subunit, PrP, may nucleate this subunit's own polymerization, crystallization, and precipitation as insoluble arrays of amyloid fibrils. Thus, a proteolytic cleavage and configurational change of PrP followed by oligomeric assembly produces an infectious fibril-amyloid-enhancing factor, which may be considered an “unconventional virus” or better, the “infectious agent.”

Pathogenesis

The basic histopathologic lesions in all of these diseases are progressive vacuolations in the dendritic and axonal processes, cell bodies of neurons, and, to a lesser extent, astrocytes and oligodendrocytes. Extensive astroglial hypertrophy and proliferation and spongiform change or status spongiosis of gray matter (Fig. 71-3) and extensive neuronal dropout and loss also occur.

Figure 71-3

Spongiform (intracellular vacuolation) change in the cortical gray matter of the brain.

These atypical infections differ from other diseases of the human brain subsequently demonstrated to be slow infections in that they do not evoke a virus-associated inflammatory response in the brain (i.e., no perivascular cuffing or leukocytic infiltration) or a pleocytosis or marked rise in the protein level in cerebrospinal fluid. Furthermore, there is no immune response to the causative virus and there are no recognizable virions. Instead, there are ultrastructural alterations in the plasma membrane lining the vacuoles, piled-up neurofilaments in swollen nerve cells, and strange arrays of tubules in postsynaptic processes that look like particles in cross-section.

Epidemiology

Medical practices have changed because of the routes of transmission of these unconventional viruses and their high resistance to physical and chemical inactivation. They are resistant to high concentrations of formaldehyde or glutaraldehyde, psoralens and most other antiviral and antiseptic substances, ultraviolet light, ionizing radiation, ultrasonication, and heat. In addition, iatrogenic transmission occurs through contaminated surgical electrodes, surgical instruments, corneal transplants, human growth hormone from pituitary glands, dura mater from cadavers, and possibly dentistry. This has led to changes in autopsy-room and operating-theater techniques throughout the world as well as to the precautions used in handling older and demented patients. Many of the gentle organic disinfectants, including detergents and the quaternary ammonium salts (often used for disinfection), and even hydrogen peroxide, formaldehyde, ether, chloroform, iodine, phenol, acetone, and ethylene oxide, are inadequate for sterilization of the unconventional viruses. Formaldehyde-fixed unconventional virus-infected brain tissue is much more resistant to inactivation by autoclaving than is unfixed infected brain tissue.

Clinical Manifestations

In the natural disease, onset is insidious, without any recognizable antecedent fever or other acute manifestations. Early signs include apprehension, restlessness, hyperexcitability, and aggressiveness, and some animals even manifest apparent dementia. Early in the disease, fine tremors of the head and neck are observed. As the disease progresses, the tremors become more generalized, involving the whole body and producing a shivering effect as disturbances in locomotion become evident. In advanced stages, the animals become stuporous and manifest visual impairment, excessive salivation, urinary and fecal incontinence, and wasting lassitude. The experimental host range includes sheep, goats, mice, hamsters and nonhuman primates. Field studies and experimental observations indicate a genetic influence of disease occurrence in sheep. In mice, there is genetic control of the length of the incubation period and of the distribution of pathologic lesions.

Pathology

Natural scrapie in sheep and goats is characterized by the presence of vacuolated neurons and extensive astrogliosis. In the experimentally transmitted disease, the neuronal vacuolation proceeds to a frank spongiform change. In sheep the presence of amyloid plaques in the brain is rare, whereas in mice the presence of amyloid plaques is dependent on the strain of scrapie virus being studied and the genetic line of the mice. Of interest is the alteration of the plasma membrane in vivo, resulting in the fusion of neurons with other neurons or astrocytes.

Clinical Manifestations

Kuru is characterized by cerebellar ataxia and a shivering-like tremor that progresses to complete motor incapacity with dysarthria and total loss of speech and death usually in less than one year from onset (Fig. 71-4). The clinical course of kuru is remarkably uniform. The disease first manifests as the insidious onset of ataxia, which becomes progressively more severe and is soon accompanied by a fine tremor involving the trunk, head, and extremities. Both involuntary tremor and ataxia increase and progress until the patient is unable to walk or stand unaided. The course of the disease is conveniently divided into three stages: ambulatory, sedentary, and terminal. Occasionally patients complain of headache and limb pains.

Figure 71-4

(A&B) Woman with stage 2 kuru. She had marked tremor and severe cerebellar ataxia. Although able to sit, she could not rise, stand, or walk without full assistance. Unlike patients with Cretzfeldt-Jakob disease, this patient did not manifest signs (more...)

The first stage is usually self-diagnosed before anyone else in the village, including doctors trained in the West, is aware that the patient is developing the disease. The early signs are subjective unsteadiness of gait and stance with postural instability and truncal tremor, titubation, and some degree of dysarthria. Speech deteriorates as the disease progresses, and eye movements become ataxic with some convergent strabismus.

The second stage of the disease is characterized primarily by the inability of the patient to walk without complete support. Tremors and ataxia become more severe, and rigidity of limbs frequently develops. This is associated with widespread clonus, coarse athetoid and choreiform movements, and an exaggerated startle response. The Babinski sign is negative, but ankle clonus is present. Emotional lability becomes apparent, as does mental slowing, but severe dementia is conspicuously absent.

In the third stage, the patient is unable to sit up without support and the ataxia, tremors, and dysarthria are severe. Some patients develop signs of extrapyramidal deficits of posture and movement, and urinary and fecal incontinence develops. Ultimately during these advanced stages, deep decubitus ulcers appear, hypostatic pneumonia develops, and the patient dies in a stage of terminal inanition.

Pathology

The neuropathologic lesions are widespread and consist mainly of marked proliferation and hypertrophy of astrocytes, mild status spongiosis of the gray matter, and diffuse neuronal degeneration, mostly in the cerebellum. The status spongiosis is true intracellular vacuolization and is caused by the coalescing of vacuoles in pre- and post-synaptic processes of the neurons and, to a lesser extent, in astrocytes and oligodendrocytes. Most of the human cases have periodic acid-Schiff-positive, doubly birefringent, amyloid plaques mainly in the molecular layer of the cerebellum.

Since the start of kuru investigations in 1956, more than 2,500 cases have been recorded. All of these have ended in death within less than two years, with few exceptions of somewhat more prolonged disease. Kuru mortality has declined continuously over the past 30 years, and the disease no longer appears in children, adolescents, or young adults. More than 200 patients died annually during the early years of investigation, but now only 5 to 10 patients, all older than 35 years of age, still die of the disease each year.

Etiologically, kuru was first thought to be an epidemic of an infectious disease. However, a degenerative rather than an infectious process was suggested by the absence of fever or antecedent acute disease, the absence of pleocytosis or elevated protein in the cerebrospinal fluid, and the lack of changes in clinical chemistry values or hematologic findings. Early isolation studies were negative. However, in 1959 the striking similarity in the neuropathology of scrapie in sheep and goats with that of the pathology in the brains of patients who had died of kuru suggested that since scrapie was a transmissible disease associated with an incubation period of three to five years, studies of the etiology of kuru should be reinitiated and should be based on the inoculation and long-term holding of a wide variety of species of animals, including chimpanzees and smaller nonhuman primates. In 1965, after one and one-half years of incubation, chimpanzees inoculated intracerebrally with suspensions of human brain from kuru patients developed the disease. This established for the first time that infection was the etiologic mechanism of subacute progressive degenerative diseases of the nervous system.

It has been demonstrated that the kuru, like scrapie, is caused by a virus-like agent that is filterable through 220-nm pore size membranes, is stable during storage at -70°C for many years, retains its infectivity following lyophilization, is highly thermostable, has the same high resistance as scrapie to UV, cobalt-60 radiation, and sonification, and resists inactivation with most organic and inorganic chemicals.

Clinical Manifestations

In most cases, Creutzfeldt-Jakob disease appears as a process primarily of the cortical gray matter of the brain. The disease usually presents after age 45 as a variable period of vague symptoms including nervousness, behavioral changes, visual problems, fatigue, weight loss, anxiety, sleeplessness, malaise, headache, and vague psychic disturbances, progressing in a few weeks or months to complications in higher cortical functions and a state of frank dementia including memory loss, intellectual function, and impaired judgment. There are cerebellar, extrapyramidal, or pyramidal symptoms followed by mutism, rigidity, and death. Most patients show intermittent myoclonus. The other major sign is the development of abnormalities in the electroencephalogram. In advanced cases, patients have periodic bursts of repetitive, high-voltage, tri- and poly-phasic sharp discharges. Thus, the diagnosis consists mainly in the detection of rapidly progressive dementia, myoclonus, and electroencephalogram abnormalities. As with kuru, there is no cerebrospinal fluid pleocytosis or consistent abnormality in clinical chemistry or hematologic findings.

During the early stages of Creutzfeldt-Jakob disease, the differential diagnosis may include Alzheimer's disease, cerebral vascular disease, pugilistic and dialysis dementia, brain tumors (glioblastomas and meningioma), brain abscess, progressive supranuclear palsy, stroke or senile dementia.

Gerstmann-Straussler-Schenker Syndrome

Certain cases of Creutzfeldt-Jakob disease present clinically as progressive cerebellar ataxia with amyloid plaques in the brain. In these cases the disease usually lasts longer than in the classic cases. In clinical symptomatology they more closely resemble kuru, mainly because of the severe ataxia; yet they are considered a subgroup of Creutzfeldt-Jakob disease and are identified as Gerstmann-Straussler-Schenker syndrome (GSS). A higher percentage of these cases are of the genetically determined familial type. In such families, several mutations resulting in several different amino acid substitutions have been found in the human homolog of scrapie precursor protein PrP^33-35C. These cases exhibit an autosomal dominant pattern with nearly complete penetrance. A proline-leucine substitution at PrP codon 102 is linked to the disease. These results clearly support the single autosomal dominant gene pattern of occurrence in familial Creutzfeldt-Jakob disease and Gerstmann-Straussler-Schenker syndrome, even though the disease is transmissible to experimental animals. Creutzfeldt-Jakob disease became the first human infectious disease in which a single gene was demonstrated to control the susceptibility and occurrence of the disease.

Etiology

Creutzfeldt-Jakob disease and Gerstmann-Straussler-Schenker syndrome are transmissible to chimpanzees and Old and New World monkeys and occasionally to domestic cats, guinea pigs, golden Syrian hamsters, and mice. Incubation periods from the time of inoculation to the onset of clinical disease vary with the strain of the virus and the experimental host. Pathology in experimental animals is indistinguishable from that induced by other members of the subacute spongiform virus encephalopathies group (Fig. 71-5).

Figure 71-5

Female chimpanzee that developed Creutzfeldt-Jakob disease 12.5 months after inoculation with a brain suspension from a chimpanzee that had died of the disease. The chimpanzee showed tremor, incoordination, myoclonic jerking, fasiculation, right-sided (more...)

The spongiform encephalopathy agents are highly resistant to complete inactivation by exposure to heat, ultraviolet and ionizing radiations, or a variety of chemicals that disinfect conventional viruses. They elicit no detectable antibodies or cell-mediated immunity, either in susceptible hosts or in resistant animals repeatedly injected with infected tissues with or without adjuvants. This constellation of properties is unique among infectious agents.

One theory is that the unconventional properties of spongiform encephalopathy agents result from their having a structure and replicative strategy without parallel among infectious pathogens—that they consist of post-translationally modified forms of a normal host protein (thereby generating prions or infectious amyloids) without any nucleic acid genomes. Intriguing hypotheses have been proposed for possible mechanisms by which abnormal proteins might catalyze their own synthesis by protein-based coding mechanisms or by some kind of modulation of an infectious protein by host-encoded nucleic acids.

The leading contender as the putative prion or infectious amyloid is an abnormal protein that accumulates primarily in the brain of patients with spongiform encephalopathies and was first demonstrated as abnormal filamentous structures called “scrapie-associated filaments” (SAF) and later as an abnormal protein band (called prion protein 27–30 or PrP^27–30"). Antisera prepared against PrP reacted with SAF as well, establishing that they are essentially the same. The PrP^27–30 was found to be the truncated cleavage product of a larger protein, called “PrP^33–35” or later PrP^Sc. That protein has the same primary structure as a normal “control” protein (PrP^C) expressed in all subjects. PrP^C is secreted at the cell surface, and is apparently not essential for life (at least in experimental animals), and its normal function remains unknown.

The SAF/PrP protein plays an important role in the pathogenesis of the spongiform encephalopathies as well as in susceptibility to disease. PrP accumulates in the brains of most, if not all, subjects with spongiform encephalopathies, forming an important component—probably the major part—of the amyloid plaques. In animals the most important gene controlling the incubation period of spongiform encephalopathies is closely linked if not identical to that encoding PrP, located on the short arm of chromosome 20 in humans. Animals not expressing PrP^C (so-called “PrP-knock-out” mice, with a disruption engineered into the PrP gene) developed and behaved normally, but they have thus far been resistant to scrapie disease.

The PrP^Sc/PrP^27–30 proteins have usually been found in partially purified preparations containing infectivity, and the amounts of PrP detected have often correlated with estimated titers of infectivity after a variety of treatments. Increased expression in mice of an engineered PrP gene with a mutation similar to one frequently found in patients with GSS caused a disease resembling scrapie, although apparently no replicating infectious scrapie agent has been convincingly demonstrated in those mice.

Reservations about the “all-protein” or prion hypothesis include: (1) reinterpretation of irradiation-inactivation kinetic studies suggested that the presence of a small nucleic acid genome was not excluded; (2) no confirmed studies have demonstrated the physical size of the infectivity-bearing agent to be less than that of a small virus; (3) no confirmed studies have demonstrated that a purified protein, uncontaminated with some nucleic acid from an infected host, replicated the scrapie agent; (4) in several experimental systems, the kinetics of PrP formation failed to correlate with kinetics of infectivity; (5) tissues of the “PrP-knock-out” mice, although not susceptible to overt scrapie disease, nonetheless appear to have supported replication of the infectious scrapie agent; and (6) strains of the scrapie agent have properties that “breed” true on passage in inbred mice and show sudden changes resembling the mutations that occur in genomes of conventional pathogens. Thus, at this time it seems premature to accept any hypothetical structure for the transmissible agents of spongiform encephalopathy as having been rigorously proven.

Genetics

(Including the Gerstmann-Straussler-Schenker and Fatal Familial Insomnia Syndromes)

CJD has long been recognized in families; in many series 5 or 10 percent of CJD patients have a family history of presenile dementia. Most pedigrees of FCJD suggest an autosomal dominant mode of inheritance: the disease occurs without skipping generations and often affects approximately half the siblings of propositi, both males and females in equal numbers. GSS also shows an autosomal dominant pattern of inheritance.

The genetic basis for FCJD/GSS appears to reside in a series of mutations in the gene coding for the SAF/PrP precursor protein, currently designated the PRNP gene, on the short arm of human chromosome 20. PRNP has an open reading frame of 759 nucleotides (253 codons), in which 10 point mutations and a variety of insertions have already been linked to FCJD or GSS.

Abnormal PRNP genes as well as different point mutations (some are summarized in Table 71-3) have also been demonstrated in association with FCJD and GSS; in addition to the most common ²⁰⁰Lys mutation, FCJD and GSS in other kindreds have been linked to other point mutations. Other mutations in the PRNP gene associated with FCJD and GSS probably await discovery.

Table 71-3

Point mutations in the Prion-Protein Gene Associated with Familial Spongiform Encephalopathies.

Fatal Familial Insomnia

Recently, “fatal familial insomnia” (FFI), an inherited syndrome with an autosomal dominant pattern of occurrence characterized by progressive severe insomnia and dysautonomia with selective atrophy of two thalamic nuclei, was described, first in a large northern Italian kindred and later in several others. Patients with FFI have ataxia, myoclonus, and other signs resembling those of CJD/GSS, and a few affected patients had spongiform changes in the cerebral cortex. Those findings prompted the hypothesis that FFI might be a new “prion” disease; indeed, protease-resistant PrP was detected in brains of patients with FFI, although it apparently differed somewhat from the PRP found in CJD/GSS patients. In patients with FFI, a mutation was found in the PRNP gene at codon 178 (¹⁷⁸Asn) identical to that found in some kindreds with FCJD. However, the two groups of patients differed in PRNP sequences at another codon of the abnormal allele, 129. It remains unknown how the same point mutation in one codon of a gene might interact with different, otherwise normal polymorphic nucleotides in another codon of the same gene to produce different clinical illnesses.

Until recently it was thought that FFI also differed from FCJD in another important way in that, no agent transmitting encephalopathy had been demonstrated in brain tissues of FFI patients, although the number of attempts has been small. Recently, however, there have been two reports describing the transmission of FFI to transgenic mice and to a wild strain of mice. This is an area of active research. It is not yet known if these polymorphisms play any role in neurological diseases. Finally, it must be noted that not all subjects with mutations in the PRNP gene have expressed disease, even in affected families. It is not known if unaffected family members bearing those mutations in the PRNP gene have “inapparent” infections.