Streptococcus equi: a pathogen restricted to one host.

Strangles caused by the host adapted Lancefield group C Streptococcus equi subspecies equi (S. equi) is a frequently diagnosed infectious disease of horses worldwide. Critical to the global success of S. equi is its ability to establish persistent infections within the guttural pouches of recovered apparently healthy horses that can result in transmission to in-contact animals. Recent research has identified key events in the S. equi genome, which occurred during its evolution from an ancestral strain of S. equi subspecies zooepidemicus, that may enhance its ability to evade host innate immune responses and rapidly multiply in the tonsillar complex and draining lymph nodes. This review discusses the role of these genetic events on the evolution and emergence of this important host-restricted pathogen.