Neospora caninum: oocyst challenge of pregnant cows.

Three pregnant cows were each orally challenged at 10 weeks of gestation with 600 sporulated oocysts of Neospora caninum. The number of oocysts was limited by those available. In concurrent bioassays, one oocyst per os infected each of two gerbils. Challenged cattle developed Neospora-specific antibody, cell proliferation and gamma-interferon responses. N. caninum specific PCR demonstrated persisting infection in the brains of cows 4 months after calving. Abortion was not induced and there was no evidence of transplacental infection in the healthy calves born at full-term. This experiment suggests that the dose threshold for induction of abortion exceeds 600 oocysts.