Screening Equine Skin and Mucous Membranes for Carriage of Coagulase-Positive Staphylococci

Commensal populations of coagulase-positive Staphylococcus spp. (CoPS) in healthy horses are not well-defined.

To determine the point prevalence, anatomical distribution, antimicrobial susceptibility and genomic characteristics of CoPS in healthy horses.

One-hundred and fifty privately owned, systemically healthy horses from seven facilities were enrolled in this study.

Horses were sampled from the nares, buccal mucosa and three skin sites. Isolates were identified using matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry, and antimicrobial susceptibility testing was performed by gradient diffusion. Whole-genome sequencing with multilocus sequence typing (MLST) characterised Staphylococcus aureus lineages. In silico PCR differentiated Group A and Group B Staphylococcus delphini. Sampling site sensitivity was calculated using a multisite composite reference. Mixed-effects logistic regression evaluated associations between demographic factors and carriage.

Carriage of any staphylococcal species occurred in 111 of 150 (74%) horses: S. aureus 28 of 150 horses (18.7%) and S. delphini 23 of 150 horses (15.3%). Increasing age was associated with decreased odds of CoPS isolation (odds ratio 0.92/year; 95% confidence interval 0.86-0.98). The nares were the most sensitive single sampling site, whereas multisite sampling improved detection. Meticillin-resistance was absent in all CoPS isolates, and S. delphini isolates were fully susceptible to all antimicrobials tested. MLST revealed multiple S. aureus sequence types, some of which were shared across multiple horses at some boarding facilities. Both groups of S. delphini lineage were identified, although many isolates were nontypable.

Healthy horses harbour S. aureus and S. delphini, with minimal antimicrobial resistance. Shared S. aureus sequence types within facilities suggest possible lateral transmission.