Persistence of acquired immunity to Sarcocystis miescheriana infection in growing pigs

Sixteen 8-week-old pigs were each experimentally immunized by subclinical infections with 10(3) Sarcocystis miescheriana (syn. S. suicanis) sporocysts and 8 other pigs served as non-immunized controls. Four groups of pigs (each consisting of 4 immunized plus 2 control pigs) were then challenged by infection with 3 X 10(6) sporocysts at either 40, 80, 120 or 160 days post-immunization (dpi) to determine the persistence of the protective immunity against acute sarcocystosis. Pigs challenged 40 dpi demonstrated a solid immunity to lethal challenge and disease. They survived challenge following a mild fever phase whereas both controls died from acute disease. This immunity however, did not prevent the further establishment of parasitic cysts within the host musculature following challenge. The protective immunity against acute disease persisted to 80 dpi, but was not evident thereafter. At necropsy, the clinical and pathological findings in all pigs which had been subjected to challenge were consistent with anamnestic responses of sensitized hosts to re-infection.