Protective immunity against clinical toxoplasmosis in dairy goats vaccinated with Hammondia hammondi and Hammondia heydorni.

To determine the virulence of Toxoplasma gondii, twelve 2- to 3-month-old goats were inoculated orally with 1,000 to 100,000 infective oocysts of the GT-1 strain of T gondii. Four of 4 goats inoculated with 100,000 oocysts, 4 of 4 goats inoculated with 10,000 oocysts, and 2 of 4 inoculated with 1,000 oocysts died (or became moribund) of acute toxoplasmosis 7 to 26 days after inoculation. Goats vaccinated with Hammondia hammondi or H heydorni were challenge exposed with 10,000 or 100,000 oocysts (10 or 100 median lethal doses). Four of 4 goats vaccinated with H hammondi survived after challenge exposure with 10,000 T gondii oocysts, whereas only 2 of 4 goats survived challenge exposure with 100,000 T gondii oocysts. Of 4 goats vaccinated with H heydorni, 3 died or were euthanatized 13, 14, and 26 days after challenge exposure with 10,000 T gondii oocysts. Goats vaccinated with H hammondi developed low levels of Sabin-Feldman dye test antibodies (less than or equal to 1:64) to T gondii antigen, whereas H heydorni-vaccinated goats generally remained seronegative. The results indicate that goats may be a model to investigate immune protection after vaccination with H hammondi against clinical toxoplasmosis in animals.