The protective M proteins of the equine group C streptococci.

The group C streptococci are the most commonly isolated bacteria from disease states in the horse. Important virulence factors of S. equi and S. zooepidemicus are the hyaluronic acid capsule and the antiphagocytic fibrillar M protein located on the surface of the cell wall and extending into and through the capsule. The hyaluronic acid capsule is non-antigenic and so is not involved in protective immunity. The M protein, a superantigen, elicits very strong B and T cell responses that may result in protective immunity mediated by opsonic antibodies in plasma and by locally synthesized IgG and IgA on the pharyngeal mucosa. However, vaccines based on acid or mutanolysin extracted M protein do not confer a high level of protection against field exposure. Protective antibodies to S. equi or S. zooepidemicus can in part be assayed by the bactericidal test that measures opsonization for equine neutrophils. A mouse-challenge model has also been used to test immunizing potency of streptococcal extracts and in a passive protection test for protective antibody. There is as yet no means of distinguishing protective opsonic or mucosal antibodies from other antibodies produced against the many epitopes on the M molecule.