Submitted to: Journal of Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 20, 1997
Publication Date: N/A
Interpretive Summary: Infections by the single celled parasite, Toxoplasma gondii are widespread in animals and humans throughout the world. The ingestion of tissue cysts in undercooked meat and the ingestion of food or water contaminated with resistant Toxoplasma (oocysts) from cat feces are the 2 major modes of transmission. After ingestion of oocysts, sporozoites (stage inside oocysts) are liberated in the intestine and invade host tissues, eventuall becoming dormant. Oocyst-induced infections are considered to be more severe in humans and in animals than tissue cyst-induced infections. After ingestion of oocysts, hosts can die of severe enteritis before other organs are severely parasitized. However, little is known of the fate of sporozoites in host tissues. Scientists at the Beltsville Agricultural Research Center and the Montana State University studied migration pathway, and pathogenesis after feeding oocysts to mice. This paper documents the damage done to host tissues by sporozoites. The sequence of events described will be of interest of biologists, public health workers, and parasitologists in general.
The development of sporozoites to tachyzoites and bradyzoites was studied in mice after feeding 1 to 7.5 x 107 Toxoplasma gondii oocysts. Within 2 hr after inoculation (HAI) sporozoites had excysted and penetrated the small intestinal epithelium. At 2 HAI, most sporozoites were in surface epithelial cells and in the lamina propria of the ileum. At 8 HAI, T. gondii was also seen in mesenteric lymph nodes. At 12 HAI, sporozoites ha divided into 2 tachyzoites in the lamina propria of the small intestine. By 48 HAI, there was a profuse growth of tachyzoites in the intestine and mesenteric lymph nodes of mice fed 7.5 x 107 oocysts. Parasites had disseminated via the blood and lymph to other organs by 4 days after inoculation (DAI). Toxoplasma gondii was first isolated from peripheral blood at 4 HAI. Tissue cysts were visible histologically in the brain at 8 DAI. By using immunohistochemical staining with anti-bradyzoite-specific (BAG-5 antigen) serum, BAG-5 positive organisms were first seen at 5 DAI i the intestine and at 8 DAI in the brain. Using the bioassay in cats, bradyzoites were first detected in mouse tissues between 6 and 7 DAI and they were found in intestines before the brain; cats fed murine tissues containing 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.