Submitted to: Journal of Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 22, 1995
Publication Date: N/A
Interpretive Summary: Infection by the protozoan parasite, Toxoplasma gondii, is widespread in livestock and humans throughout the world. It causes abortion in livestock and mental retardation and blindness in children. Humans become infected by eating uncooked meat from infected animals or by ingesting food and water contaminated with infected cat feces containing oocysts (resistant form of T. gondii). Meat from domestic pigs is considered as one of the most important sources of T. gondii infection for humans. How pigs become infected with T. gondii in nature is not known. Scientists at the Beltsville Agricultural Research Center have found that pigs can be infected by feeding them one T. gondii oocyst. These studies are of epidemiologic significance because a cat can excrete millions of oocysts after eating infected animal tissues. The findings will be of interest to public health workers, veterinarians and epidemiologists.
In an attempt to define the infectivity of the VEG strain of Toxoplasma gondii, 42 pigs were fed estimated doses of 10, 1, or < 1 infective oocysts. Pigs were killed 38 to 99 days after inoculation and 50 g of tissue from their tongue, heart, and brain were individually homogenized in acidic pepsin solution and bioassayed in mice. Pools of brain, heart, tongue and skeletal muscle (total 500 g) were bioassayed in cats. Toxoplasma gondii was isolated by bioassays in mice and in cats from 13 of 14 pigs fed 10 oocysts, 13 of 14 pigs fed 1 oocyst and 4 of 14 pigs fed "less than 1 oocyst," indicating the high susceptibility of pigs to the VEG strain of T. gondii. All infected pigs developed modified agglutination test antibodies (> 1:50). Control pigs (n=6) remained seronegative (< 1:20) and T. gondii was not isolated from their tissues. Toxoplasma gondii was isolated from tongues of 27 (93%), brains of 21 (72%) and hearts of 13 (45%) of 29 pigs quantitatively tested in mice. The numbers of T. gondii-positive mice after inoculation of tongue, brain, and heart of infected pigs were 240 (80%), 84 (28%) and 36 (12%) of 300 mice inoculated with each organ, respectively. Thus, the VEG strain of T. gondii was localized more often and in higher numbers in the tongue than in the brain or the heart of pigs. The apparent muscle localization after infection with the low dose of the VEG strain of T. gondii is in agreement with other studies in livestock that suggest T. gondii is more neurotropic in mice than in livestock.