OOCYST-INDUCED MURINE TOXOPLASMOSIS - LIFE-CYCLE, PATHOGENICITY, AND STAGE CONVERSION IN MICE FED TOXOPLASMA-GONDII OOCYSTS

The development of sporozoites to tachyzoites and bradyzoites was stud ied in mice after feeding 1-7.5 X 10(7) Toxoplasma gondii oocysts. Wit hin 2 hr after inoculation (HAI), sporozoites had excysted and penetra ted the small intestinal epithelium. At 2 HAI, most sporozoites were i n surface epithelial cells and in the lamina propria of the ileum, and by 8 HAI, T. gondii was also seen in mesenteric lymph nodes. At 12 HA I, sporozoites had divided into 2 tachyzoites in the lamina propria of the small intestine. By 45 HAI, there was a profuse growth of tachyzo ites in the intestine and mesenteric lymph nodes of mice fed 7.5 X 10( 7) oocysts. Parasites had disseminated via the blood and lymph to othe r organs by 4 days after inoculation (DAI). Toxoplasma gondii was firs t isolated from peripheral blood at 4 HAI. Tissue cysts were visible h istologically in the brain at 8 DAI. By using immunohistochemical stai ning with anti-bradyzoite-specific (BAG-5 antigen) serum, BAG-5-positi ve organisms were first seen at 5 DAI in the intestine and at 8 DAI in the brain. Using the bioassay in cats, bradyzoites were first detecte d in mouse tissues between 6 and 7 DAI, and they were found in intesti nes before they were found in the brain. Cats fed murine tissues conta ining bradyzoites shed oocysts in their feces with a short (< 10 days) prepatent period, whereas cats fed tissues containing tachyzoites did not shed oocysts within 3 wk. Using a pepsin-digestion procedure and mouse bioassay, bradyzoites were first detected in brain tissue at 7 D AI and in many organs of mice at 51 and 151 DAI. Individual bradyzoite s, small and large tissue cysts, and tachyzoites were seen in the brai ns of mice at 87 and 236 DAI.