Submitted to: Journal of Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 15, 1996
Publication Date: N/A
Interpretive Summary: Infection by the protozoan parasite Toxoplasma gondii is unduly prevalent in livestock and humans. It causes mental retardation and loss of vision in children and abortion in livestock. Cats are the only hosts of T. gondii that excrete in feces the environmentally resistant stage (oocyst). Oocysts can survive in the environment for months. How cats become infected with T. gondii is of epidemiologic importance. Oocysts are highly infective to pigs, humans, and their hosts of T. gondii. A scientist at the Beltsville Agricultural Research Center has found that oocysts are not very infective to cats. Cats fed 100-1000 oocysts did not acquire T. gondii infection. The results support the hypothesis that cats become infected in nature by preying on infected animals than by ingesting oocysts from the environment.
Toxoplasma gondii oocysts are highly infective to intermediate hosts including humans, pigs and mice, but are considered less infective for the definitive host, cats. To determine infectivity of T. gondii oocysts for cats, 20 2-3 mo-old T. gondii-free cats in groups of 4 were fed graded doses of oocysts estimated to have 1, 10, 100, 1,000, or 10,000 mouse infective oocysts of the VEG strain of T. gondii. Feces of cats were examined for at least 35 days after feeding oocysts. All cats were necropsied, their sera were tested for T. gondii antibodies, and tissues were bioassayed in mice. Three of the 4 cats each fed 10,000 oocysts, 3 of the 4 cats each fed 1,000 oocysts and 2 of the 4 cats each fed 100 oocysts shed 7.3 to 162 million T. gondii oocysts in their feces with a prepatent period of 18 to 44 days. Based on bioassay and antibody production, all 4 cats fed 10,000 oocysts, 3 of 4 cats fed 1,000 oocysts, 2 of 4 cats 100 oocysts, and 0 of 8 cats fed 1 or 10 oocysts acquired T. gondii infection. Antibodies to T. gondii were detected by the modified agglutination test in all 9 bioassay proven T. gondii- infected cats and none of the 11 cats without demonstrable T. gondii. In a series of other experiments, it was found that the age of the cat at the time of oocyst feeding and the administration of corticosteroids did not influence the prepatent periods after ingestion of oocysts. Review of published and unpublished data indicated that the minimum prepatent period to shedding of oocysts after the ingestion of oocysts by cats is 18 days.