Sixty-four ewes were vaccinated with tachyzoites of an incomplete strain (S48) of Toxoplasma gondii grown either in the peritoneal cavity of mice (group 1) or vero cell culture (group 2) and 30 ewes (group 3) were not vaccinated. All the ewes were mated 77 days later and challenged orally with 2000 sporulated oocysts at 89 to 90 days of gestation. Ten additional unvaccinated (group 4) and 10 vaccinated (group 5) control ewes were not challenged. The unvaccinated ewes developed a characteristic febrile response to challenge while in the vaccinated ewes the fever commenced earlier but was less severe and of shorter duration. After challenge, the antibody titres against T gondii rose rapidly to high values in the vaccinated ewes while the ewes in group 3 responded more slowly. Only eight of the 45 fetuses/lambs (17.8 per cent) from group 3 were viable compared with 72.3 per cent of those in group 1 and 80.8 per cent of those in group 2. Gestation in the unvaccinated challenged ewes was shortened and the mean birthweight of their single, viable offspring was significantly lower than the weight of single lambs from the vaccinated (groups 1 and 2) and control ewes (groups 4 and 5). Examination of precolostral sera showed that almost two-thirds of the lambs from the vaccinated ewes were infected in utero. The 20 control ewes appeared clinically normal at all times and lambed normally. The two vaccine preparations were equally effective.
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