II. Flagellated Protozoa
III. Malocclusion
IV. Miscellaneous Bacterial Diseases
V. Nematodes
VI. Tyzzer's Disease
A. Rodentolepis nana (dwarf tapeworm) and Hymenolepis diminuta (the rat tapeworm) can infect rats. However, the incidence of infection is rare. For information about these parasites, please see the Hamster Parasitic Disease section.
B. Taenia taeniaformis - Rats serve as intermediate
hosts for the cat tapeworm Taenia taeniaformis. The cysticercoid
cyst (Cysticercus fasciolaris) embeds in the liver and may cause
a palpably enlarged liver if cysts are numerous. Infection occurs
when rats ingest ova in rat food or bedding contaminated with cat feces.
No treatment is necessary but prevention of feline fecal contamination
of rat food or bedding is required. This infection uncommonly occurs
in research facilities since bedding and food sources and storage practices
prevent cat fecal contamination.
Spironucleus muris and Giardia muris are protozoa that colonize the upper small intestine; Spironucelus muris dwells in the mucosal crypts of the small intestine and in the pylorus, and Giardia muris resides along the villous enterocytes in the anterior small intestine. Trichomonads can be found in the lower small intestine and the large intestine. Ingestion of infective cysts is the primary mode of transmission. Infected adult rats are clinically unaffected. Direct smears of the small intestinal contents can be performed to look for the fast darting motility of Spironucleus and "tumbling leaf" motility of Giardia. Trichomonads have a wobbly motility and the action of the undulating membrane can be easily observed. Cysts can be detected in the feces. Dimetridazole (1mg/ml) or metranidazole (2.5 mg/ml) in drinking water has been reported to decrease the parasite load, but does not eliminate the infection.
Malocclusion is a condition caused by overgrown incisors which result from trauma, bacterial infection, or genetic factors. Hypersalivation may be observed. Treatment consists of clipping overgrown teeth with guillotine-type nail clippers or a Dremmel-type dental disc every 2 to 3 months.
IV. Miscellaneous Bacterial Diseases
Rats are reservoirs for bacterial diseases that are pathogenic to man. Salmonellosis, especially Salmonella enterica serovars Enteritidis and Typhimurium have been diagnosed in research and pet rats. Salmonellae are Gram-negative toxin-producing enteric bacteria that are transmitted through contact with infected feces. Rats are frequently asymptomatically infected, and rarely exhibit systemic infection. Definitive diagnosis of infection is provided by culture on selective media such as selenite, brilliant green and MacConkey agars, with subsequent serotyping of the isolate. Another zoonotic pathogen harbored by rats is the agent of rat bite or Haverhill fever. The most common etiologic agent is Streptobacillus moniliformis, and rats carry S. moniliformis asymptomatically in the nasopharynx. The bacterium is a Gram-negative pleomorphic rod which requires media supplemented with 20% serum or ascites fluid for cultivation. The incidence of infection of either of these agents in laboratory rats is rare. There is no information about the incidence of infections in pet rats.
Public Health Significance: Humans ingesting Salmonella - contaminated food or water experience a transient diarrhea. Children or immunosuppressed adults are susceptible to more serious clinical disease from Salmonella infection. People infected with S. moniliformis develop irregular recurrent fever with lymphadenopathy, petechial hemorrhages, polyarthritis and endocarditis. Bacterial infections respond well to tetracycline therapy.
A. Syphacia muris (Rat Pinworm)
1. Transmission: Syphacia muris deposits eggs in the perianal region. Transmission of infection occurs via ova ingestion. The eggs are very light and have been shown to aerosolize, resulting in widespread exposure. The incidence of infection is high.
2. Clinical Signs: No signs are usually seen. It has been reported that heavy parasite loads may lead to catarrhal enteritis, or perianal irritation causing hair to be chewed off the tail base.
3. Diagnosis: Direct exam of cecal contents, fecal flotation,
and tape test of the perianal region will identify adults and eggs, respectively.
The eggs are slightly smaller and more symmetrical than those of Syphacia
obvelata.
5. Treatment: If treatment is desired, piperazine (4 to 7 mg/ml water) for 3 to 10 days is effective. Karo syrup can be added to the solution if the rats refuse to drink it. Fenbendazole medicated feed provided every other week for 5 weeks has been an effective treatment method. Other successful treatments include ivermectin at 0.2 mg/kg subQ weekly for 2 to 3 weeks or in drinking water (25 mg/liter) for 4 to 5 daily treatments 3 days apart.
6. Control: Prevention and control of infection are often difficult. Rigid sanitary procedures, use of filter hoods to prevent aerosol transmission, and regular ova examinations with treatment may control the parasitism.
B. Syphacia obvelata and Aspiculuris tetraptera pinworms may transiently infect rats. The comments made about Syphacia muris also apply to Syphacia obvelata, with which it is frequently confused. Aspiculuris tetraptera inhabits the colon and deposits its eggs in the feces; thus, it cannot be diagnosed by use of the anal tape test. Treatment is the same as for Syphacia muris.
A. Etiology: Clostridium piliforme is a gram-negative, obligate intracellular rod that produces spores.
B. Transmission: Infection is established via the fecal-oral route by ingestion of spores. Predisposing factors to disease include age (commonly 3 to 7 weeks) and physiological stresses such as concurrent infections, experimental manipulations, or poor housing conditions. Incidence of disease is low in the rat.
C. Clinical Signs: Subclinical infection is the most common
form of infection in the rat. Clinical manifestations of Tyzzer's disease
include anorexia, hunched posture, distended abdomen, rough hair coat and
death. Diarrhea has not been reported in the rat. 
D. Gross Pathology: A "fat rat" syndrome with a flaccid
segmental dilatation of the intestine (especially of the ileum) has been
described. The liver typically has multiple pale foci throughout.
Circumscribed, greyish foci may sometimes be observed in the myocardium.
Multiple necrotic foci in sections of liver and myocardium are often
surrounded by a pyogranulomatous cellular infiltrate. Silver stains
(Steiner or Warthin-Starry) will help deliniate the bacterium (arrow),
which can be observed within the cytoplasm of viable cells bordering the
necrotic foci.
E. Diagnosis: Since the organism cannot be propagated on
artificial media, histopathologic diagnoses are made by demonstration of
the bacillus in the enterocytes, hepatocytes or cardiac myocytes
bordering necrotic foci in tissues stained with silver stains. A
serological assay and a PCR assay have been developed to identify C.
piliforme infected rats.
F. Treatment: Oxytetracycline at 0.1 mg/ml water for 30 days was reported to abate mortality of an epizootic in mice. Treatment is usually not warranted.
G. Control: Avoidance of stress and strict sanitation help prevent outbreaks. Spores are very resistant to deterioration in the environment and are resistent to most disinfectants except bleach. Cages and bedding should be decontaminated by autoclaving prior to routine washing.
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