Immunization but not natural infection of horses results in antibody activity against the S protein of Streptococcus equi subsp equi.

Abstract

Evaluate the immunogenicity of a vaccine targeting the S protein (Ssee) of Streptococcus equi subsp equi and determine antibody activity against Ssee in horses with strangles. The study was designed as a prospective experiment using 20 university-owned Quarter Horses and a cross-sectional serosurvey of 78 privately owned horses with strangles. Horses were immunized IM with 0 (n = 4), 200 (n = 8), or 400 (n = 8) μg of recombinant Ssee at weeks 0, 4, and 12. Serum and nasal secretions were collected at weeks 0, 4, 6, 12, 16, and 28 and tested by ELISA for immunoglobulin (Ig)-G against Ssee; nasal secretions were also tested for anti-Ssee IgA. The function of anti-Ssee IgG in serum was tested for complement deposition onto Ssee and opsonophagocytic killing of S equi subsp equi. Serum from horses with strangles was tested by ELISA for anti-Ssee IgG activity. Immunization with Ssee significantly (P < .05) increased serum and nasal IgG (but not nasal IgA) against Ssee for up to 12 weeks after the third immunization, and serum from vaccinated horses mediated significantly (P < .001) greater complement deposition onto Ssee, but not opsonophagocytic killing (P > .05), than controls. Horses with strangles did not develop high levels of serum IgG activity against Ssee. Immunizing horses with Ssee resulted in increased activity of functional IgG in serum and nasal secretions, and horses with strangles had very low levels of serum IgG activity against Ssee. S protein has potential as a vaccine to reduce the severity of strangles and differentiate between infected and vaccinated horses.