II. Protozoa
III. Nematodes
IV. Cestodes
V. Pneumocystis carinii
A. Etiology: Myocoptes musculinus (A.), Myobia
musculi (B.), and Radfordia affinis (C.) are common fur mites.
Psorogates
simplex, the follicular mite, is rarely seen.
B. Transmission: Direct contact spreads the mite. Fur mites are usually host-specific. Prevalence of infestation is low in conventional colonies of mice.
C. Clinical Signs: Usually no clinical signs are observed. Black haired mice are thought to have an increased sensitivity to mites manifested by pruritus and alopecia progressing to excoriation, ulcerative dermatitis, and cicatricial disfigurement.
D. Diagnosis: On live mice, plucking hairs from the pelt
and examining under a dissecting microscope may reveal the eggs (laid on
the hair shaft) or the mites (A.). If the pelt from a recently killed
mouse is cooled to room temperature, the mites will crawl up to the tips
of the hairs, looking like white specks (B.). If an animal has been
dead for a while, the mites may have migrated to another host.
E. Treatment: Adults and weanlings can be dusted with silica dusts, pyrethrin dusts. Ivermectin solutions, using the horse preparations, have been made in concentrations providing 8 mg in every liter of drinking water. Treated water in bottles should be provided once a week for 3 weeks to eliminate the mites. Alternatively, 0.1% solutions in spray bottles can be used to deliver ivermectin. Care must be exercised as some mouse strains, such as the CF-1, will experience decreased production or death when treated with ivermectin. Treatment should be tailored to break the mite life cycle and to accommodate the research protocol or animal use.
F. Control: Regular examinations and treatments may eventually rid the colony of mites.
A. Spironucleus muris is a flagellated protozoan that dwells
on the small intestinal mucosa. 
1. Transmission: Ingestion of infective cysts is
the primary mode of transmission. A carrier stage occurs in adults.
The incidence of infection is low in conventional mouse colonies.
2. Clinical Signs: Weanling mice (3 to 6 weeks) appear to be most susceptible. Heavily infected mice are usually smaller in size, depressed and have abdominal distension. Dehydration and anorexia follow. A "sticky" fecal mass may be seen on the perineal area.
3. Pathology: The small intestines are rarely dilated and filled with gassy catarrhal fluid contents. Autolysis of the intestines appears to occur more rapidly. No gross lesions are observed in most juvenile and adult mice with this protozoal disease. Histologic exam of the pylorus and upper samll intestine reveal small oval protozoa in the crypts and ocasionally in the space between villi.
4. Diagnosis: Direct smears of small intestinal contents reveal fast darting protozoa.
5. Treatment: Administration of dimetridazole (1 gm/l drinking water) has been reported to reduce mortality, but does not effectively eliminate the parasites.
6. Control: Mice derived by cesarian section and maintained in a barrier environment are usually free of Spironucleus muris.
B. Tritrichomonas sp.(large intestinal flagellate) and Giardia sp. (small intestinal flagellate) are also common protozoal inhabitants of the mouse gut.
1. Transmission: The main route of infection is ingestion of the encysted stage of the organism which is passed in feces of an infected animal.
2. Clinical Signs: No clinical signs have been attributed directly to these organisms. Diarrhea is often exacerbated by the presence of these flagellates.
3. Pathology: No specific pathology accompanies these parasitic infections. Tritrichomonas is most commonly found in the lower small intestine and cecum while Giardia is found primarily in the jejunum.
4. Treatment: Antiflagellate therapy described for Spironucleus can be used, although success in eliminating the parasites is poor. In colony situations, no therapy is attempted.
5. Control: Same as for Spironucleus muris.
A. Pinworms: Syphacia obvelata (note round esophageal bulb in A.)and
Aspiculuris
tetraptera (note oval esophageal bulb in B.) are cecal pinworms commonly
found in mice and other rodents.
B. Transmission: Syphacia obvelata deposits eggs in the perianal region while Aspiculuris tetraptera releases eggs in the colon which then are passed in fecal pellets. Infection occurs via ova ingestion. The eggs of Syphacia obvelata are very light and have been shown to aerosolize, resulting in widespread exposure.
C. Clinical Signs: No signs are usually seen. It has been reported that heavy parasite loads may lead to rectal prolapse or perianal irritation.
D. Pathology: The pinworms are easily recognized as translucent to white, hairlike nematodes in the cecum or colon.
E. Diagnosis: Direct exam of cecal or colonic contents
will identify adults. fecal flotation (for both pinworms) and tape test
of the perianal region (for Syphacia only) will identify eggs. Syphacia
obvelata ova are asymmetrical with pointy ends (C.), while
Aspiculuris
ova are symmetrical and unembryonated (D.). Aspiculuris tetraptera
eggs will be missed by tape test alone.
F. Treatment: If treatment is desired, ivermectin solutions (from horse preparations) diluted to deliver 0.01 mg/ml of drinking water (0.4 mg/kg) can be supplied on days 1, 14, and 28 to break the life cycle. Piperazine (4 to 7 mg/ml water) for 3 to 10 days is also effective, but karo syrup may have to be added to the solution if the mice refuse to drink it. Feed fenbendazole sow cubes (Kent Feed) as sole food source for 3 days; repeat in 7 and 14 days. Have to consider the possible impact of treatment on use of mice before treatment begins.
G. Control: Fenbendazole treated feed has been successful in treating mice, but efforts must be made to remove ova from the environment to prevent re-infection. Disinfectants will not destroy pinworm ova, so they must be either physically removed or inactivated through heat sterilization (cages). Screen incoming animals for pinworms to prevent contamination of other animals. Cage bonnets will help prevent ova aerosolization during quarantine or treatment of infected mice. Regular parasite examinations with treatment of infected animals may control the parasitism. Cesarian derivation will also eliminate the infection.
Hymenolepid Tapeworms
A. Rodentolepis nana is the dwarf tapeworm and Hymenolepis diminuta is the rat tapeworm. Both tapeworms are capable of infecting mice. The incidence of parasitism is rare.
B. Transmission: Rodentolepis nana and Hymenolepis diminuta can be transmitted by an indirect mode with cockroaches, grain beetles, or fleas as intermediate hosts. Rodentolepis nana can also be transmitted by direct ingestion of hexacanth ova or by autoinfection in which the entire life cycle occurs in the host's small intestine (complete life cycle in 14 to 16 days).
C. Clinical Signs: Usually there are no external signs of infection. However, catarrhal enteritis, diarrhea, emaciation and chronic weight loss may occur with heavy infestations.
D. Pathology: Rodentolepis nana adults range from
25 to 40 mm long and less than l mm wide and have an armed rostellum (see
photo). Hymenolepis diminuta adults range from 20 to 60 mm in length
and 3 to 4 mm wide without hooks on the scolex. These tapeworms may
migrate up the pancreatic and biliary ducts. Since R. nana can complete
its life cycle without an intermediate host, strobelocerci may be observed
within the lamina propria of the small intestine.
E. Diagnosis: Visualization of the tapeworm in the small
intestine during necropsy, recovery of hexacanth ova by fecal flotation
or microscopic visualization of segmented parasites or encysted larvae
in histological sections of the small intestine villi (for R. nana) are methods of diagnosis.
F. Treatment: Niclosamide at 10 mg/100 gm body weight should be crushed and given in powdered feed for two treatments at 7 day intervals. Praziquantel, at the equivalent of the cat dose, has been used, but the efficacy of treatment has not been documented.
G. Control: Cockroaches should be eliminated and infected animals treated or eliminated.
Public Health Significance: Humans are susceptible to infections with R. nana; since autoinfection can occur, a heavy parasite load may quickly develop.
Encysted Tapeworms
Mice serve as intermediate hosts for the cat tapeworm Taenia
taeniaformis. The cysticercoid cyst (Cysticercus fasciolaris)
embeds in the liver and causes no clinical signs. Infection occurs
when mice ingest ova from food or bedding contaminated with cat feces.
No treatment is necessary, but feline fecal contamination should be prevented.
FUNGAL DISEASES
Pneumocystis carinii is an opportunistic pathogen of the
respiratory tract of mice, rats and probably all domestic mammals and man.
It is believed to be a fungus that is spread by the inhalation of infective
cysts. Mice that are immunosuppressed, either by treatment or by
virtue of a hereditary immunological defect, may develop a fatal
pneumonia. Affected mice display hunched posture, tachypnea, and
weight loss when the pneumonia has consumed a significant portion of the
lung. Affected lungs are diffusely red and fail to deflate when the
chest is opened (A.). Histologically, interstitial pneumonia is accompanied
by histiocytic alveolitis, with alveoli filled with eosinophilic foamy
material that may contain some degenerated macrophages or few neutrophils
(B.). PAS and Giemsa stains are needed to visualize the trophozoite
stages, and a silver stain is used to demonstrate the argyrophilic cysts
(C.). Sulfameraxine/trimethroprim antibiotics are used to treat or
prevent clinical disease in immunologically stressed mice (such as the
thymic deficient nude and severe combined immunodeficient mice).
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