Development of immunity to sarcocystosis in dairy goats.

To determine the virulence of a microscopic species of Sarcocystis, forty-six 39- to 57-day-old goats were inoculated with 10(1) to 10(6) sporocysts; 7 age-matched goats served as noninoculated controls. Four of 4 goats fed 10(6) sporocysts, 4 of 4 goats fed 10(5) sporocysts, 2 of 4 goats fed 5 x 10(4) sporocysts, and 1 of 4 goats fed 10(4) sporocysts died of sarcocystosis, whereas 30 goats fed 10(1) to 10(3) sporocysts and the 7 noninoculated controls generally remained asymptomatic. In 3 immunization trials, groups of 7 to 9 goats were challenge exposed with 10(5) or 10(6) (2 or 20 median lethal dose) sporocysts at 14, 30, and 60 days after vaccination. In the 1st trial (14 days after vaccination), 2 of 2 goats vaccinated with 10(3) sporocysts and 1 of 2 goats vaccinated with 10(2) sporocysts survived a challenge exposure with 10(5) sporocysts, whereas 2 of 2 goats fed 10(1) sporocysts and the control goat died of acute sarcocystosis. In the 2nd trial (30 days after vaccination), 2 of 2 goats vaccinated with 10(3) sporocysts and the 2 goats vaccinated with 10(2) sporocysts survived a challenge exposure with 10(5) sporocysts, whereas 2 goats vaccinated with 10(1) sporocysts and the control goat died of acute sarcocystosis. In the 3rd trial (60 days after vaccination), 2 goats vaccinated with 5 x 10(4) sporocysts, 2 goats vaccinated with 10(4) sporocysts, 2 goats vaccinated with 10(3) sporocysts, and the 2 goats vaccinated with 10(2) sporocysts survived after challenge exposure with 10(6) sporocysts, whereas 2 goats vaccinated with 10(1) sporocysts and the control goat died of acute sarcocystosis. The results indicate that goats vaccinated with 10(2) to 10(3) sporocysts develop protective immunity against lethal doses of Sarcocystis, whereas goats vaccinated with 10(1) sporocysts were not protected against a later infection with a lethal dose.