Submitted to: Journal of Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 24, 1998
Publication Date: N/A
Interpretive Summary: Infections by the protozoan parasite, Toxoplasma gondii are unduly prevalent in livestock and humans. It causes mental retardation and loss of vision in children and abortion in livestock. Humans become infected by ingesting food and water contaminated with oocysts (resistant stage of T. gondii) from feces or tissue cysts in uncooked meat. Infected pigs are considered an important source of infections in humans in the U.S. At present there is no vaccine to prevent T. gondii infection in animals or man. Scientists at the Beltsville Agricultural Research Center have found that feeding T. gondii irradiated oocysts partially protected pigs against toxoplasmosis. These results will be useful for vaccine development and will be of interest to public health workers, veterinarians, and parasitologists.
Immunity to toxoplasmosis was investigated in outbred pigs after feeding Toxoplasma gondii oocysts inactivated by irradiation. In 2 experiments 23, 2-3-mo-old pigs were fed (12 pigs only once and 11 pigs twice) 100,000 oocysts irradiated at 0.3 or 0.4 kGy of Cesium-137 and 11 pigs served as unvaccinated controls. Eleven to 13 wk later, 15 vaccinated pigs and 5 non-vaccinated pigs were challenged orally with a high dose (100,000 or 1,000,000) of oocysts, and 7 vaccinated and 6 non- vaccinated pigs were challenged orally with a low dose (100 or 1,000) of oocysts; 1 vaccinated pig was not challenged. All non-vaccinated pigs challenged with a high dose became ill starting 4-5 days post challenge (p.c.). One pig died day 9 p.c., 1 died day 16 p.c., and 1 was killed day 10 p.c. because of weakness. All vaccinated pigs, non-vaccinated pigs challenged with a low dose, and 1 vaccinated unchallenged pig remained clinically normal. Bioassay of pig tissues (tongue, diaphragm, brain) in mice indicated fewer tissue cysts in tissues of vaccinated pigs compared with unvaccinated pigs following challenge with live oocysts. However, vaccination with 1 or 2 doses of irradiated oocysts did not prevent formation of tissue cysts even in pigs challenged with 100 oocysts.