To the Editor: Staphylococcus aureus is a well-known pathogen of horses (1), but the role of other coagulase-positive staphylococcal species in these animals is unclear. S. pseudintermedius and S. delphini, members of the S. intermedius group (SIG), cause infections in some companion animals and equids (2), can be multidrug resistant, and could be a concern in horses. Members of SIG are difficult to differentiate by using biochemical methods and require molecular techniques for accurate species-level identification (3); therefore, misidentification of these pathogens could occur.

Methicillin-resistant or unusual staphylococci that are isolated at the Ontario Veterinary College Health Sciences Centre by the University of Guelph Animal Health Laboratory (AHL) routinely undergo further characterization. During 2011, the laboratory tested 5 isolates from different horses that were coagulase-positive staphylococci other than methicillin-resistant S. aureus (MRSA). Isolates were identified by using matrix-assisted laser desorption/ionization–time of flight (MALDI-TOF) mass spectrometry, S. pseudintermedius or S. delphini PCR (4), and sodA sequence analysis (3). Isolates were further characterized, as indicated, by direct repeat unit typing (5), pulsed-field gel electrophoresis (PFGE) (6), mecA PCR (7), penicillin-binding protein 2a latex agglutination test, and antimicrobial drug susceptibility testing by broth microdilution and/or disk diffusion. A search of AHL’s database was performed to identify other S. pseudintermedius and S. delphini isolates for all submissions of samples from equids during January 2011–August 2012.

Of the 5 isolates from the horses, 1 was identified as methicillin-resistant S. pseudintermedius (MRSP) and 4 as methicillin-susceptible S. delphini (Table). The MRSP isolate was classified by direct repeat unit typing as dt11a, a predominant MRSP clone in dogs in North America (8). In addition to β-lactams, the MRSP isolate was resistant to chloramphenicol, clindamycin, erythromycin, gentamicin, tetracycline, and trimethoprim/sulfamethoxazole and susceptible to nitrofurantoin, rifampin, streptomycin, and vancomycin.

The 4 S. delphini isolates were initially identified biochemically as S. pseudintermedius but subsequently classified as group A (n = 1) and group B (n = 3) S. delphini by molecular methods (Table). One isolate (SD-4) was resistant to only erythromycin; the remaining isolates were susceptible to all tested antimicrobial drugs. PFGE showed that 2 of the S. delphini isolates (SD-1 and SD-2) were possibly related, with a 4-band difference. The remaining isolates were unrelated to each other and the 2 related isolates. Two of the horses (sources of isolates SD-2 and SD-3) had been recently acquired at the same auction and were sampled on the same day; however, PFGE showed that these samples were not related and came from different groups (A, B). No common epidemiologic links were identified for any of the horses.

The AHL database search identified 8 additional horses from which S. pseudintermedius was biochemically identified; on the basis of drug-resistance patterns, 6 (75%) of these isolates were determined to be MRSP (Table). One additional S. delphini isolate was identified by using MALDI-TOF. No common epidemiologic links were identified for these infections.

MRSP is an emerging pathogen in dogs and cats (1) but has been rarely identified in horses (2). The role of these bacteria in disease in horses is unclear, but given their ability to cause opportunistic infections in other species, these pathogens should not be dismissed. S. pseudintermedius rarely causes disease in humans (9), and transmission normally occurs from infected or colonized animals. Although rarely reported, infection with MRSP might be overlooked in horses; misidentification as S. aureus is possible if laboratories assume that coagulase-positive staphylococci from horses are S. aureus, and misidentification as methicillin susceptible is possible because the use of cefoxitin susceptibility and S. aureus breakpoints is ineffective for determination of methicillin resistance in S. pseudintermedius (10). Additionally, S. pseudintermedius generates coagulase-positive results by tube testing but coagulase-negative results by slide testing, which creates the potential for misidentification as coagulase-negative staphylococci. Given the rapid expansion of S. pseudintermedius infections among dogs, the potential for zoonotic transmission, and the highly resistant nature of this pathogen, ongoing surveillance is indicated in the equine population.

Recently, S. delphini has been divided into groups A and B (3). The typical hosts for group A are believed to be mustelidae (i.e., mink, ferret, badger), whereas hosts for group B remain unknown. S. delphini has rarely been identified in horses, but, as we observed, it may be misidentified by conventional methods. Although colonization or contamination appeared most likely in the instances we describe, these findings suggest that this opportunistic pathogen can be found in horses and might be pathogenic in certain situations.

Our findings highlight the importance of using additional identification methods (e.g., MALDI-TOF, Staphylococcus species–specific PCR) for differentiation of SIG members (notably S. delphini and S. pseudintermedius) to effectively document the emergence of these species in horses. In addition, these findings indicate the need to ensure proper differentiation of S. aureus from SIG in equine isolates, despite the historical predominance of S. aureus, because of the differences in methods for determination of methicillin resistance. Future studies are needed to determine prevalence trends and disease roles for these species in equids.

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