Submitted to: Journal of Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 25, 1996
Publication Date: N/A
Interpretive Summary: Infection by the protozoan (singled celled) parasite Toxoplasma gondii is widely prevalent in man and livestock. It causes abortion in livestock and mental retardation and blindness in children. Humans become infected by ingesting T. gondii encysted in undercooked meat or food and water contaminated with cat feces containing the environmentally resistant stage of the parasite (oocyst). Cats are the only hosts that can excrete oocysts in feces. Cats that have once shed oocysts usually became immune and do not shed oocysts again; the mechanism of this immunity is unknown. Scientists at the Beltsville Agricultural Research Center have found that T. gondii undergoes partial development in the intestine of immune cats challenged with T. gondii. These results provide a rational basis for the development of a vaccine in cats to reduce oocyst shedding.
Immunity to Toxoplasma gondii, as measured by oocyst shedding, was studied in cats. In 3 trials, 12 3-mo-old T. gondii-free cats were fed tissue cysts of the ME-49 strain of T. gondii. All cats shed T. gondii oocysts for approximately 1 wk starting 3 to 5 days after ingesting tissue cysts. One cat became ill because of toxoplasmic pneumonia and was euthanized 17 days after inoculation. The remaining cats remained clinically normal. Approximately 3 mo after primary infection these 11 cats (immune) and 11 age-matched or littermate uninfected cats (nonimmune) were challenged orally with tissue cysts of the ME-49 strain. In trials 1 and 3, 1 immune and 1 nonimmune cat was killed at 36 hr, 60 hr, 5 days, and 12 days after challenge and the development of T. gondii in intestines was studied histologically; in trial 2 cats were killed at 36 hr, 60 hr, and 5 days only. None of the "immune" cats shed oocysts after challenge. Asexual T. gondii types were found at 36 and 60 hrs, and at 5 days indicating partial development of T. gondii in the intestine of immune cats. There were no significant differences in lymphocyte CD4+:CD8+ from spleen, popliteal and mesenteric lymph nodes of immune cats compared to nonimmune cats.