Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) is considered a commensal of the upper airways in horses and an opportunistic pathogen causing respiratory disease. S. zooepidemicus, unlike S. equi subsp. equi (S. equi) that causes strangles, displays wide genetic variation. The importance of this genetic diversity to the pathogenicity of different strains is currently being investigated, as it appears that strains of S. zooepidemicus can differ in virulence. Whether certain strains of S. zooepidemicus can cause upper respiratory disease as a host-specific pathogen in horses, and if there are certain genogroups of S. zooepidemicus that are more virulent than others is of major clinical importance. Here, we describe an outbreak of upper respiratory disease that could be related to one strain of S. zooepidemicus.
An outbreak of upper respiratory disease occurred in a herd of 17 Icelandic horses. Twelve of the 17 horses were selected for bacterial sampling during the outbreak, of which ten displayed clinical signs of respiratory disease, while two horses were clinically healthy. Paired serum samples were collected from all 17 horses. Bacterial sampling was repeated twice after the outbreak had subsided. Samples were cultured, analysed by real-time PCR for S. zooepidemicus and S. equi, and genetically differentiated by sequencing of the SzP protein gene and MLST. Mitogenic activity and the presence of S. zooepidemicus superantigens SzeF, SzeN and SzeP were also investigated. Serum samples were analysed for antibodies against S. equi and common viral respiratory pathogens.
All horses were positive for S. zooepidemicus during the outbreak. All horses with clinical signs of disease carried the same strain, while the healthy horses carried other strains of S. zooepidemicus. None of the horses were positive for S. equi. Bacteriological, molecular and serological analyses support that the isolated S. zooepidemicus strain was likely to be responsible for the disease outbreak.