Attempted immunization of swine against acute sarcocystosis using cystozoite-derived vaccines

Low-dose Sarcocystis miescheriana infections have recently been shown to protect pigs against acute sarcocystosis. Because this protective immunity was short-lasting, an alternative immunization strategy was examined. Four experimental vaccines were prepared from S. miescheriana cystozoites and tested in 13 pigs. Two vaccines were prepared from intact organisms (live and formalin-fixed cystozoites) and 2 from subcellular cystozoite fractions (pellicle and protoplasm extracts). The live vaccine was injected intraperitoneally and the remainder were suspended in Freund's incomplete adjuvant and injected intramuscularly. An additional 5 pigs were injected with adjuvant or saline placebos and used as controls. Serum samples were collected regularly and tested in enzyme immunoassays for specific IgM and IgG antibodies. Low levels of IgM antibodies were detected after 8 days and elevated levels of IgG antibodies were detected after 22 days. The success of vaccination was tested 40 days after vaccination by lethal homologous challenge of each pig with 3 X 10(6) sporocysts. Despite the presence of specific antibodies at the time of challenge, all pigs died from acute sarcocystosis 12 days later. The cystozoite vaccines were therefore antigenic but not immunogenic and did not induce any protective immunity.