Coccidiosis Of Cats And Dogs - Digestive System - MSD Veterinary ...

The most common coccidia of cats and dogs are Isospora spp.  Two species infect cats: I felis and I rivolta; both can be identified easily by oocyst size and shape.

Four species infect dogs: I burrowsi, I canis, I neorivolta, and I ohioensis. In dogs, only I canis can be identified by the oocyst structure; the other three Isospora spp overlap in dimensions and can be differentiated only by endogenous developmental characteristics.

When clinical coccidiosis is present it is often related to intercurrent infections, immunosuppression, or stress.

In dogs, the prepatent period is about 3 weeks. Most infection in dogs and cats is a direct life cycle involving the ingestion of oocysts in the feces once infective. Sporulation can occur in 6 hours in warm, moist, and oxygenated conditions but usually takes 7–10 days. Isospora spp have the potential for producing very large numbers of oocysts within a short period.

There is an indirect life cycle, especially with Isospora spp in cats and dogs, which can facultatively infect other mammals and also the definitive host’s own tissues and produce an encysted form (cystozoite) that can later become activated to infect the cat or dog. Thus, cystozoites of I felis are found in lymph nodes and abdominal viscera in paratenic hosts such as mice and rats and infective stages are released in the cat’s intestines. Cystozoite reactivation in cat tissues in stressed kittens or immunosuppressed cats may be important in causing disease in these specific hosts. However, this cycle is usually considered to maintain the life cycle but not sufficient to cause disease in most animals.  

There is some controversy over pathogenicity. Heavy infections leading to intestinal damage can produce decreased nutrient absorption, loss of fluid and electrolytes, and, with some Isospora spp, hemorrhage. High fecal shedding of oocysts is associated with clinical signs of disease, especially in kittens.