Unusual patterns of serum antibodies to Streptococcus equi in two horses with purpura hemorrhagica.

An enzyme-linked immunosorbent assay (ELISA) was developed for use in horses to determine serum titers of antibodies of the immunoglobulin classes IgA, IgG, and IgM to Streptococcus equi M-like protein and culture supernatant protein antigens. Serum antibodies were determined in 28 adult horses, including 9 horses with recent S. equi infections, 17 horses without known exposure to S. equi, but without a history of respiratory disease in the preceding 4 months, and 2 horses with clinical purpura hemorrhagica. Serum IgA titers to culture supernatant protein antigen were highest in recently infected horses (P less than 0.001). Serial determinations of antibody titers in the horses with purpura showed that IgG antibodies to both S. equi M-like protein and culture supernatant protein antigens were undetectable initially, but later rose coincidental with clinical recovery from the disease. Possible mechanisms for these findings are discussed.