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 gondi i, T-263, induces immunity to oocYst shedding following challenge. Exp eriments were designed to compare the levels of protection induced by T. gondii T-263 when tissue cysts, bradyzoites released from tissue cy sts, and tachyzoites were administered to cats. In 1 experiment, group s of cats received 2 oral doses of intact tissue cysts or released bra dyzoites of T. gondii T-263 and were challenged 47 days later with the oocyst-producing strain of T. gondii T-265. All cats seroconverted fo llowing immunization and none of them shed oocysts following challenge . In a second experiment, groups of cats received tachyzoites of T. go ndii 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 se roconverted 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, ind uced immunity to oocyst shedding. In contrast, tachyzoites did not com pletely protect against oocyst shedding, even when delivered directly to the duodenum and despite the development of high antibody titers.