Immunization of cats with tissue cysts, bradyzoites, and tachyzoites of the T-263 strain of Toxoplasma gondii

Previous studies have demonstrated that oral administration to cats of tissue cysts of the oocyst-negative mutant strain of Toxoplasma gondii, T-263, induces immunity to oocyst shedding following challenge. Experiments were designed to compare the levels of protection induced by T. gondii T-263 when tissue cysts, bradyzoites released from tissue cysts, and tachyzoites were administered to cats. In 1 experiment, groups of cats received 2 oral doses of intact tissue cysts or released bradyzoites of T. gondii T-263 and were challenged 47 days later with the oocyst-producing strain of T. gondii T-265. All cats seroconverted following immunization and none of them shed oocysts following challenge. In a second experiment, groups of cats received tachyzoites of T. gondii T-263 as a single oral dose and either 1 or 2 intraduodenal doses; they were challenged 60 days after the last vaccination. All cats seroconverted following immunization. Following challenge, all cats shed oocysts except for 2 of 7 cats that received 2 intraduodenal doses of tachyzoites. Thus, orally administered bradyzoites of T. gondii T-263, either contained in intact tissue cysts or liberated from cysts, induced immunity to oocyst shedding. In contrast, tachyzoites did not completely protect against oocyst shedding, even when delivered directly to the duodenum and despite the development of high antibody titers.