The aim of this study was to compare the ability of a live incomplete strain (Strain 48) and a live complete strain (Strain 89) of Toxoplasma gondii to protect against abortion and congenital infection following an oral challenge of T. gondii oocysts. Sixty-nine two-tooth ewes were immunised pre-tupping with live Strain 48 of T. gondii tachyzoites and seventy ewes were immunised with Strain 89. Eighty-two serologically negative ewes served as controls. At mid-pregnancy half of the ewes were challenged orally with T. gondii oocysts (2x10(5)/ewe). The ewes vaccinated with Strain 48 were significantly (p<0.05) protected against the effects of experimental challenge and the rate of congenital infection was also significantly (p<0.15) reduced. The ewes vaccinated with Strain 89 were also significantly (p<0.05) protected. The serological response to challenge as measured by both the Dye test and the Indirect Haemagglutination test varied considerably between the two vaccinated groups.
Two-tooth ewes (n=48) were immunized pre-tupping with a live Toxoplasma gondii vaccine. At midpregnancy these ewes were challenged intravenously with 1 x 105 live T. gondii tachyzoites. The strain of T. gondii used for vaccination was an incomplete strain that did not produce oocysts. It was derived by continuous twice weekly passage in mice. The lambing percentage for ewes immunized with the live vaccine was significantly higher (P<0.001 normal score) than non-vaccinated control ewes. However, vaccination did not prevent foetal or placental infection. The serological response to vaccination and challenge was measured by both the Dye test and the Indirect Haemagglutination test. No significant relationship between titre of antibody and protection in the vaccinated ewes was observed.
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