Outbreak of upper respiratory disease in horses caused by Streptococcus equi subsp. zooepidemicus ST-24

Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) is generally considered a commensal and an opportunistic pathogen of the upper airways in horses. Establishing whether certain strains of S. zooepidemicus can cause upper respiratory disease as a host-specific pathogen of horses, and if there are certain genogroups of S. zooepidemicus that are more virulent than others is of major clinical importance. In this study, we describe an outbreak of upper respiratory disease in horses that was associated with S. zooepidemicus. Upper respiratory samples were cultured, analyzed by real-time PCR for S. zooepidemicus and S. equi, and genetically differentiated by sequencing of the SzP protein gene and multi-locus sequence typing (MLST). Serum samples were analyzed for antibodies against S. equi and common viral respiratory pathogens. The ST-24 strain of S. zooepidemicus was isolated from all horses with clinical signs of disease, while the healthy horses carried other strains of S. zooepidemicus. Bacteriological, molecular and serological analyses strongly suggest that a single strain (ST-24) was responsible for the disease outbreak, and that certain strains of this presumed commensal may be more virulent than others.