Persistence of tissue cysts in edible tissues of cattle fed Toxoplasma gondii oocysts.

Four 1-year-old steers were each inoculated orally with 10,000 Toxoplasma gondii oocysts of the GT-1 strain and euthanatized on postinoculation days (PID) 350, 539, 1191, and 1201. Samples (500 g) of tongue, heart, semimembranosus and semitendinosus muscles (roast), intercostal muscles (ribs), longismus muscles (tenderloin), brain, kidneys, liver, and small intestine were bioassayed for T gondii by feeding to cats and examination of cat feces for shedding of oocysts. Toxoplasma gondii was recovered by bioassays in cats for the 3 steers necropsied PID 350, 539, and 1191, but not from the steer euthanatized on PID 1201. Cats shed oocysts after ingesting tongue from 2 steers, heart from 3 steers, liver from 2 steers, and roast, ribs, brain, and intestines from 1 steer each. Toxoplasma gondii was not isolated from any of the other bovine tissues. In addition to tissues bioassayed in cats, homogenates of mesenteric lymph nodes, lungs, spinal cord, spleen, and eyes were bioassayed in mice for T gondii infection. Toxoplasma gondii was not recovered from the 135 mice inoculated with tissue from each of the 4 steers. All 4 inoculated steers developed high T gondii antibody titers (> or = 1:8,000) in the agglutination test, using formalin-fixed whole tachyzoites. In the steer euthanatized on PID 1201, agglutinating T gondii antibody titers decreased from 1:4,000 to 1:320 between 2 and 5 months after inoculation and to 1:20 by 19 months after inoculation.