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.

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A method was developed to separate Toxoplasma gondii cysts from mouse brain tissue and to liberate intact bradyzoites from cysts. Brains were blended in a Waring blender with 20% dextran solution and the homogenate was centrifuged at 4,000 g for 10 min. Cysts present in the sediment were digested by adding an equal amount of a solution containing 1 g NaCl, 1.4 ml HCl, and 1 mg pepsin (1:60,000 assay activity) per 100 ml water, and incubated at 37 C for 1 min. This method is harmless for bradyzoites, as tested by bioassays in mice. It compares favorably with published methods for separating cysts that require 3 centrifugations to achieve similar results.

To investigate the oocyst-induced cycle with a 21+ day prepatent period, 32 cats were fed 5 x 10(5) to 2 x 10(7) sporulated oocysts of Toxoplasma gondii and necropsied between 4 hr and 41 days thereafter. The presence of the earliest stages in 7 cats was tested in mice. The tissues of 25 cats were studied histologically; 17 were bioassayed by feeding them to cats to determine, by the length of the prepatent period, whether bradyzoites were present. Based on previous studies, a short (3-10 days) prepatent period indicated that bradyzoites were present in an oral inoculum and a long (greater than 21 days) prepatent period indicated the presence of tachyzoites only. Tissues from 14 cats were also bioassayed in mice for the presence of bradyzoites, using their resistance to pepsin as indicator. Six were studied by both methods. Based on these criteria, tachyzoites predominated in extraintestinal organs during the first 14 days after infection. They were found as early as 4 hr in mesenteric lymph nodes where their number reached 10(4) after 6 and 9 days; they were present after 1 day in all levels of the small intestine and after 6 days in the liver, lung, and blood. Bradyzoites were first detected 10 days after oocyst feeding; they predominated by the third week of infection and were present up to 41 days. Enteroepithelial stages were found histologically only in 2 cats, 24 and 41 days after inoculation.(ABSTRACT TRUNCATED AT 250 WORDS)

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