Isospora Species of the Cat and Attempted Transmission of I. felis Wenyon, 1923 from the Cat to the Dog*

SYNOPSIS Fecal samples from 130 domestic cats from Illinois were examined for coccidia. Three species of Isospora were found: (1) I. felis Wenyon, 1923, with oocysts 38‐51 by 27‐39 μ with a mean of 41.6 by 30.5 μ and sporocysts 20‐26 by 17‐22 μ with a mean of 22.6 by 18.4 μ; it was found in 13% of the cats; (2) I. rivolta (Grassi, 1879) Wenyon, 1923, with oocysts 21‐28 by 18‐23 μ with a mean of 25.0 by 21.1 μ, and sporocysts 14‐16 by 10‐13 μ with a mean of 15.2 by 11.6 μ; it was found in 3% of the cats; and (3) I. bigemina (Stiles, 1891) Lühe, 1906, with oocysts 12‐15 by 10‐13 μ with a mean of 13.2 by 11.8 μ. and sporocysts 8‐10 by 6‐8 μ with a mean of 8.8 by 6.5 μ it was found in 1.5% of the cats. Four coccidia‐free puppies 1.5 months old were inoculated with 100,000 oocysts each of I. felis from the cat, but patent infections did not occur. Partial development of I. felis was not seen in tissue sections of the small intestine of a 5th pup killed 96 hours after inoculation with 150,000 I. felis oocysts. This coccidium is therefore presumably host‐specific.