INFECTIVITY OF LOW NUMBERS OF TOXOPLASMA-GONDII OOCYSTS TO PIGS

To define the infectiousness of the VEG strain of Toxoplasma gondii, 4 2 pigs were fed doses estimated at 10, 1, or <1 mouse infective oocyst s. They were killed 38-99 days after inoculation and 50 g tissues from their tongue, heart, and brain were individually homogenized in acidi c pepsin solution and bioassayed in mice. Pools of brain, heart, tongu e, and skeletal muscle (total 500 g) were bioassayed in cats. Toxoplas ma 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 high infectivity of VEG strain of T . gondii to pigs. All infected pigs developed modified agglutination t est antibodies (>1:50). Control pigs (n = 6) remained seronegative (<1 :20) and T. gondii was not isolated from their tissues. Toxoplasma gon dii was isolated from tongues of 27 (93%), brains of 21 (72%), and hea rts of 13 (45%) of 29 experimentally infected pigs by bioassay in mice . The number of T. gondii-positive mice after inoculation of tongue, b rain and heart from infected pigs was 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 and the heart of pigs. The apparent muscl e localization after infection with the low dose of the VEG strain of T. gondii agrees with other studies in livestock that suggest T. gondi i is more neurotropic in mice that in livestock.