Toxoplasmosis in sheep. I. Effect of a killed vaccine on lambing losses caused by experimental challenge with Toxoplasma gondii.

A total of 50 two-tooth ewes were immunized pre-tupping using a killed vaccine of disintegrated Toxoplasma tachyzoites with Freunds incomplete adjuvant. At mid-pregnancy, the ewes were challenged intravenously with 1 x 10(4) live T. gondii tachyzoites. There was no difference in the lambing percentage between ewes vaccinated once and unvaccinated ewes. The lambing percentage for ewes vaccinated twice was significantly lower (p<0.05 normal score) than unvaccinated ewes. The serological response to vaccination and challenge was measured by the Toxoplasma dye test. The level of dye test antibodies at the time of challenge suggests that effective immunity to Toxoplasma n abortion is not dependent on dye test antibody titre. These results suggest that a killed vaccine would Toxoplasma on fertility and lambing performance in not be an effective tool to estimate the effects of commercial flocks.