Experimental Isospora canis and Isospora felis Infection in Mice, Cats, and Dogs*

SYNOPSIS. Two 6‐month‐old gnotobiotic dogs and 4 five‐day‐old dogs became infected but did not shed oocysts within 15 days after ingesting the feline coccidian, Isospora felis . Infection of the dogs was evidenced by the shedding of I. felis oocysts by cats consuming extra‐intestinal organs of dogs fed I. felis. Likewise, cats became infected with the canine coccidian, Isospora canis , without producing oocysts. Dogs also became infected after ingesting mice previously fed I. canis oocysts. The prcpatent period for I. canis was slightly shorter in dogs fed infected mice (8–9 days) than in those fed oocysts (9–11 days).