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.