Characterization of Toxoplasma gondii from the feces of naturally infected cats.

Feces of 1,000 cats from a humane shelter in Columbus, Ohio, were examined microscopically for oocysts of Toxoplasma gondii and by inoculation into mice. From the first 541 cats examined, oocysts of Toxoplasma were found in the feces of seven cats but in none of the remaining 459 cats. Results of the dye test in these seven cats showed titers of antibody of less than 1:2 in four cats, and of 1:8, 1:6, and 1:32 in the remaining three cats. The pathogenicity and infectivity of oocysts and cysts of all seven strains were compared in mice after oral and intraperitoneal inoculations. Oocysts and cysts were more pathogenic when administered by the oral route than by the intraperitoneal route. The cysts were less pathogenic than the oocysts. Excellent cross-immunity between six of these seven feline strains and the M-7741 strain was deomonstrated in cats by the fact that oocysts were not shed in feces of cats challenged with cysts of homologous or heterologous strains.