Tritrichomonas spp. — The most common endoparasite in rodents is the flagellate Tritrichomonas muris. It is nearly the rule to find it in the caecum and colon of conventional rodents, but it may be found in barrier-protected rodents as well.132 Transmission occurs by ingestion of pseudocysts. It seems to be apathogenic.

Encephalitozoon cuniculi — Encephalitozoon cuniculi, a protozoan parasite belonging to the sub-phylum Microspora, is the etiological agent of a spontaneous disease in rabbits, which should be called encephalitozoonosis but often is called nosematosis, as the agent previously belonged to the genus Nosema. The most common way of transmission seems to be the oral via infectious feces and urine. It is unclear whether vertical transmission may also occur.170 The sporoplasm is extruded from a spore, which enters the host cells to multiply and mature into spores, which after cell rupture, restart the cycle. Infection has been described in many species, e.g., rabbits,171 rats,172 mice,172 and guinea pigs,170 but it is only common in rabbits and guinea pigs, while infection in other species is rare and probably the result of contact with contaminated rabbit colonies.172 Infection is usually latent, but occasionally, rabbits exhibit various neurological signs, such as convulsions, tremors, torticollis, paresis, and coma. Lesions in the kidneys of infected rabbits are frequent, grossly manifested as multiple, pinpoint areas, randomly scattered over the surface, or more usual, as 2 to 4 mm indented gray areas on the cortical surface. In histopathology, granulomatous nephritis173 and granulomatous encephalitis174 are observed. Guinea pigs do not seem to develop disease. Infection is diagnosed by serology.175 The disease is usually diagnosed by recognition of typical lesions and sometimes also the agent by histopathology of kidneys.

Eimeria spp. — Various Eimeria species are known to cause intestinal coccidiosis in rabbits. Infection is fairly common, but if observed, clinical symptoms are mostly found around weaning in breeding facilities and are rare in experimental facilities, although heavily infected rabbits may also develop clinical symptoms. Such rabbits show diarrhea in varying degrees, thirst, and dehydration. In subclinical cases, weight loss may be observed. Also, peracute cases with deaths prior to the presence of oocysts in the feces may be observed. The small and large intestines of heavily infected rabbits may show multiple white spots on the mucosa with a mixed mononuclear and polymorphnuclear exudate. Infection occurs orally with coccidial oocysts, which burst in the intestines, releasing sporozoites to invade the intestinal mucosa cells. Here they multiply into schizonts, which break, extruding a huge number of merozoites into the lumen, from which they invade new cells to repeat the process. After an unknown number of such asexual generations designated schizogony, the merozoites differentiate into female macrogametocytes or male microgametocytes. The microgametocytes leave the cells and unite with the macrogametocytes to form the oocysts. The most common enteric Eimeria species in the rabbit are E. irresidua, E. magna, E. media, and E. perforans.

E. stiedae produces hepatic coccidiosis with multiplication in the bile ducts, but this infection is uncommon in laboratory rabbits. Clinical symptoms of this infection are mostly related to chronic debilitation of the liver, i.e., weight loss and chronic intestinal symptoms in rare cases leading to death.1

The diagnosis of Eimeria infection is made by the observation of oocysts in the feces in the flotation test. The diagnosis coccidiosis is made by observation of the clinical and pathological symptoms combined with the observation of huge numbers of oocysts in the feces. Outbreaks of coccidiosis may be treated with sulfonamides, e.g., sulfadimidine as a 0.02% solution in the drinking water.

Pneumocystis carinii — The eucaryote P carinii, occasionally referred to as a fungus, infects a wide range of mammal species, but it is difficult to diagnose and fully asymptomatic in immune-competent animals. However, in SCID mice and other immune-deficient animals, it may cause fatal pneumonia.176,177 It is easily diagnosed in immune-deficient animals, e.g., by immunofluorescence staining of lung washings, and has shown, and it is quite widespread, even in barrier-protected colonies. Transmission seems to be airborne.178

Ectoparasites — In conventional rats and mice, ectoparasites are a common finding.35 Mites such as Myobia musculi, Radfordia affinis, and Myocoptes musculinus in mice and R. ensifera in rats are the most common. This may cause no symptoms or a variety of skin lesions in the range from mild pruritus to serious pyodermia. Other ectoparasites are more rare, although lice infestations with Polyplax spp. have been observed in conventional rodent colonies. In barrier-protected rodent colonies, ectoparasites are generally absent.

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