Influence of Serum Antibody on Occurrence of Streptococcus zooepidemicus in the Nasopharynx of Weanling Foals

Streptococcus zooepidemicus (Sz) is a tonsillar commensal of horses that opportunistically invades the respiratory tract of weanling foals causing purulent rhinitis and tracheobronchitis. Ten weanling foals in an isolated herd with a history of Sz-associated respiratory disease in recent years were immunized with recombinant proteins SzPNC78, endopeptidase, and hyaluronidase prior to weaning to determine the effect of serum antibodies on populations of nasopharyngeal Sz and mucopurulent nasal discharges. Levels of serum antibodies to all three proteins were significantly (P ≤ .01) increased compared to levels in 10 nonimmunized control foals at weaning (day 0). Numbers of Sz in nasopharyngeal washes of control foals were initially higher on day 35 than in washes of immunized foals. However, this difference was not maintained, and counts in washes of both groups were similar over the following 7 weeks. By day 104 postweaning, Sz was no longer detected in washes of six foals in each group. Streptococcus zooepidemicus (ST-45) expressing a specific protective protein (SzP) phenotype dominated from days 35 to 104 in both groups. A second clone, ST-10, was detected intermittently in five immunized and two control foals. Mucopurulent nasal discharges of seven immunized and eight control foals on days 78 and 85 were associated with counts of Sz that ranged as high as 106 colony-forming units/mL and with significantly increased levels of SzP-specific IgG in washes. Increase in specific IgG but not IgA was temporary suggesting transudation through inflamed mucosa. This increase was generally greater in washes of immunized (mean enzyme-linked immunosorbent assay optical density [ELISA OD] = 1.73) than control (mean ELISA OD = 1.08) foals suggesting a benefit of systemic immunization may be therapeutic by enhancing clearance of Sz following inflammation provoked by local invasion.