Observations on a feline coccidium with some characteristics of Toxoplasma and Sarcocystis.

Two morphologically different cysts were found in skeletal muscles of mice inoculated with fecal material from a stray cat containing Isopora-type oocysts. The most common cyst contained bradyzoites resembling those of Toxoplasma and resulted from an oocyst measuring 11 times 13 mum which appeared to be identical to that of Toxoplasma. The other cyst, observed in only a few mice, contained bradyzoites resembling those of Sarcocystis, but the oocyst or sporocyst that gave rise to it was overlooked and apparently lost. Two more strains of the parasite resembling Toxoplasma were found in feces of stray cats. When inoculated into mice, the oocyst of this parasite routinely produced chronic infection and formed cysts similar to Toxoplasma in skeletal muscles and occasionally in the central nervous system. The majority of infected mice developed Toxoplasma antibody, but only to low titers. Cats fed carcasses of infected mice remained healthy and shed nonsporulated oocysts following a prepatent period of about 5 days. Cats did not develop Toxoplasma antibody. There was little or no cross immunity between the parasite and T. gondii in cats or mice. Transmission of the parasite between mice by the cyst stage normally was not possible; however, mice inoculated with cortisone acetate did become infected when inoculated with cysts. In other laboratory animals inoculated orally with the oocyst asymptomatic infection was detected in 3 species of rats, in guinea pigs and in dogs, but not in monkeys, pigeons or Japanese qualis. Fluorescent antibody tests on human sera failed to provide evidence of natural human infection with the parasite.