Myobia musculi

Agent. Fur mite, order Acarina.

Life Cycle. Myobia musculi has egg, larval, nymphal, and adult stages. Eggs are oval, about 200 rtm long, and usually found either attached to the base of hairs or inside mature females. Eggs hatch in about 7 days, and completion of the entire life cycle requires about 23 days.

Animals Affected. Mice and rarely rats and other laboratory rodents.

Epizootiology. Mites can be seen anywhere on the body but are most numerous alongside the hair bases in the more densely furred parts of the body (i.e., over the head and back). Transmission is by direct contact. The dynamics of mite populations on a host are very complex and are influenced by factors that include grooming, strain susceptibility, and host immune responses. Athymic (nu/nu) and other furless mice are not susceptible to infestation.

Clinical. The general appearance of infested mice is not directly related to the size of the mite population. Infestations are commonly subclinical. Clinical signs include scruffiness, pruritis, patchy alopecia, self-trauma, ulceration of the skin, and pyoderma. Close inspection often reveals bran-like hyperkeratotic debris and mites on the skin around the base of the hairs.

Pathology. Mice of the C57BL strains and their congeneic sublines are particularly susceptible to severe M. musculi-related skin disease. Lesions vary from mild to severe. Initially there is mild hyperkeratosis, but this often progresses to severe hyperkeratosis with fine bran-like material on the skin over virtually all of the body but particularly abundant over the dorsum, head, and shoulders. In more advanced cases, there is patchy alopecia and chronic ulcerative dermatitis most frequently distributed asymmetrically in the shoulder and neck regions. Secondary bacterial infection commonly leads to suppurative and granulomatous inflammation. Hyperplasia of regional lymph nodes, splenic lymphoid hyperplasia, and increased serum immunoglobulins are common.

Diagnosis. Diagnosis requires demonstration and identification of mites, while excluding other causes of dermatitis such as fungi (ringworm) or Staphylococcus aureus. Mites can be demonstrated by using a stereoscopic microscope or hand lens to examine the pelage, particularly over the back and head. Alternatively, mice can be killed and placed either on black paper and left at room temperature or in tapesealed Petri dishes and refrigerated for an hour. As the body cools, the mites leave it and can be collected from the paper or Petri dish. The mites are mounted under a coverslip on glass slides with immersion oil and identified microscopically on the basis of anatomic features.

Control. Cesarean derivation and barrier maintenance are the most effective methods for eradication of mite infestations. Insecticides can be used, but they may alter experimental results.

Interference with Research. Behavioral patterns are likely to be altered by hypersensitivity to these mites. Secondary amyloidosis caused by chronic infestation can interfere with research results.

Suggested Reading

• Flynn, R. J.1973. Parasites of Laboratory Animals. Ames, Iowa: Iowa State University Press. 884 pp.
• Friedman, S., and S. H. Weisbroth. 1977. The parasitic ecology of the rodent mite, Myobia musculi. IV. Life cycle. Lab. Anim. Sci.27:34-37. [PubMed: 850386]
• Galton, M.1963. Myobic mange in the mouse leading to skin ulceration and amyloidosis. Am. J. Pathol.43:855-865. [PMC free article: PMC1949769] [PubMed: 14075019]
• Weisbroth, S. H.1982. Arthropods. Pp. 385-402 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press.
• Weisbroth, S. H., S. Friedman, and S. Scher. 1976. The parasitic ecology of the rodent mite Myobia musculi. III. Lesions in certain host strains. Lab. Anim. Sci.26:725-735. [PubMed: 979139]

Myocoptes musculinus and Radfordia affinis

Agent. Mites, order Acarina.

Pathology. M. musculinus causes lesions similar to, but usually milder than, those caused by Myobia musculi. R. affinis is not a significant pathogen.

Interference with Research. Mite infestations due to M. musculinus have been reported to reduce the contact sensitivity of mice to oxazolone.

Suggested Reading

• Flynn, R. J.1973. Parasites of Laboratory Animals. Ames, Iowa: Iowa State University Press. 884 pp.
• Laltoo, H., and L. S. Kind. 1979. Reduction in contact sensitivity reactions to oxazolone in mite-infested mice. Infect. Immun.26:30-35. [PMC free article: PMC414570] [PubMed: 500208]
• Weisbroth, S. H.1982. Arthropods. Pp. 385-402 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press.

Other Ectoparasites

For more in-depth coverage or information on less common ectoparasites of mice and rats, consult comprehensive reference works on the subject.

Suggested Reading

• Flynn, R. J.1973. Parasites of Laboratory Animals. Ames, Iowa: Iowa State University Press. 884 pp.
• Hsu, C.-K.1979. Parasitic diseases. Pp. 307-331 in The Laboratory Rat . Vol. I: Biology and Diseases, H. J. Baker, J. R. Lindsey. and S. H. Weisbroth, eds. New York: Academic Press.
• Owen, D.1972. Common Parasites of Laboratory Rodents and Lagomorphs. MRC Laboratory Animals Centre Handbook No. 1. London: Medical Research Council. 64 pp.
• Pratt, H. D., and J. S. Wiseman. 1962. Fleas of Public Health Importance and Their Control. Insect Control Series: Part VIII. PHS Publ. No. 772. Washington, D.C.: U.S. Department of Health, Education, and Welfare.
• Weisbroth, S. H.1982. Arthropods. Pp. 385-402 in The Mouse in Biomedical Research. Vol. II: Diseases, H. L. Foster, J. D. Small, and J. G. Fox, eds. New York: Academic Press.