Antibiotic Resistance Patterns in Staphylococcus Species Isolated from Equine Dermatological Samples: A 10-Year Retrospective Study

Cutaneous bacterial infections are a frequent and widespread challenge in equine medicine, with allergic horses at increased risk for recurrent Staphylococcal infections. Empiric and prophylactic antibiotic use is common, but overuse threatens therapeutic efficacy. Because underlying causes of infection can be difficult to determine or control, antibiotics often remain the mainstay of treatment, underscoring the need to monitor resistance patterns over time to guide therapy.

Prior studies and clinical observations cited indicate: (1) increasing resistance trends in veterinary settings, including equine hospitals, over time; (2) differing resistance patterns between small animals (e.g., rising oxacillin resistance in dogs) and livestock/horses (lower oxacillin resistance), potentially linked to regulatory differences and antibiotic availability; (3) variable trends in trimethoprim/sulfonamide (TMS) resistance across regions (increases in Canada; stationary in France), suggesting geographic and practice-related influences; (4) decreased antibiotic use associated with lower methicillin resistance in livestock; (5) minimal fluoroquinolone use in horses compared to small animals; and (6) coagulase-negative staphylococci historically exhibiting higher resistance than coagulase-positive species.

Retrospective analysis of equine aerobic and dermatology cultures collected at a tertiary referral institution from 1 January 2009 to 1 October 2019. From 2300 cultures, 77 Staphylococcus-positive samples met inclusion criteria. Isolates were identified to species where available and grouped for analysis into: Staphylococcus aureus, Staphylococcus pseudintermedius, non-hemolytic Staphylococcus, beta-hemolytic Staphylococcus, and a miscellaneous group (MG: S. hyicus, S. xylosus, S. epidermidis, S. schleiferi, and Staphylococcus spp. not otherwise specified). Antimicrobial susceptibility testing included: erythromycin, gentamicin, imipenem, oxacillin, rifampin (all 77); and amikacin, ampicillin, azithromycin, cefazolin, chloramphenicol, clarithromycin, doxycycline, penicillin, tetracycline, and trimethoprim/sulfonamide (tested in subsets). Sample sites included pyoderma lesions, superficial wounds, abscesses, surgical incision sites, nose, and foot. Statistical analyses: Kruskal–Wallis rank-sum tests to compare susceptibility by sample collection site and by isolate group; Steel–Dwass post hoc tests for pairwise comparisons; Spearman’s rank correlation to assess temporal (year) trends in resistance; and Kendall’s tau to evaluate inter-antibiotic susceptibility correlations. Prevalence of resistance and intermediate susceptibility was summarized for selected antibiotics by organism group.

• Species distribution among 77 included Staphylococcus isolates: S. aureus 62.3% (48/77), S. pseudintermedius 9.1% (7/77), S. hyicus 2.6% (2/77), S. xylosus 2.6% (2/77), S. epidermidis 2.6% (2/77), S. schleiferi 1.3% (1/77); non-speciated: non-hemolytic 10.4% (8/77), beta-hemolytic 7.8% (6/77), Staphylococcus spp. 1.3% (1/77).
• Sample sources: pyoderma lesions 46.8% (36/77), abscesses 19.5% (15/77), superficial wounds 13% (10/77), surgical incision sites 6.5% (5/77), nose 10.4% (8/77), foot 3.9% (3/77).
• Site-specific susceptibility: Gentamicin was the only antibiotic with significant susceptibility differences by collection site (Kruskal–Wallis; Steel–Dwass). All abscess isolates were susceptible (15/15, 100%), whereas resistance was observed in 70% of surgical incision isolates, 33.3% foot, 30.5% pyoderma, 30% superficial wounds, and 12.5% nose. Surgical incision vs abscesses showed statistical significance (p = 0.003); pyoderma vs abscesses was not significant (p = 0.1225).
• Temporal trends (Spearman): Significant upward resistance trends over 10 years for ampicillin (ρ = 0.2649, p = 0.0290), cefazolin (ρ = 0.3012, p = 0.0086), and penicillin (ρ = 0.2541, p = 0.0380). Other antibiotics did not show significant trends; TMS showed a non-significant upward trend (ρ = 0.1697, p = 0.1428).
• Differences by isolate group (Kruskal–Wallis with Steel–Dwass): Eight antibiotics showed significant group-based differences (ampicillin, azithromycin, cefazolin, clarithromycin, erythromycin, imipenem, oxacillin, penicillin). The MG was more likely than S. aureus to be resistant to ampicillin, azithromycin, cefazolin, erythromycin, imipenem, oxacillin, and penicillin. Non-hemolytic Staphylococcus were more likely than S. aureus to be resistant to azithromycin, clarithromycin, erythromycin, and oxacillin. S. pseudintermedius showed similar likelihood of resistance to ampicillin and penicillin as S. aureus. MG was more likely to be resistant to cefazolin than beta-hemolytic Staphylococcus.
• Correlations between antibiotics (Kendall’s tau): Many statistically significant positive correlations across antibiotics, often within the same class or usage tier; one significant negative correlation observed between cefazolin and amikacin.
• Selected resistance prevalence over the study period (from tables): Oxacillin resistance: S. aureus 10.4% (5/48), non-hemolytic 80% (4/5), MG 85.7% (6/7), S. pseudintermedius 50% (2/4), beta-hemolytic 0% (0/5). Penicillin resistance: S. aureus 28.6% (12/42), non-hemolytic 83.3% (5/6), MG 100% (7/7), S. pseudintermedius 28.6% (2/7), beta-hemolytic 20% (1/5). Ampicillin resistance: S. aureus 28.6% (12/42), non-hemolytic 57.1% (4/7), MG 85.7% (6/7), S. pseudintermedius 28.6% (2/7), beta-hemolytic 20% (1/5). Intermediate susceptibility to cefazolin was common in some groups (e.g., S. aureus 38.3% [18/47]).

Upward resistance trends for beta-lactam antibiotics (ampicillin, cefazolin, penicillin) likely reflect overall use patterns in equine medicine, including non-dermatologic indications (e.g., surgical prophylaxis, respiratory infections, neonatal sepsis). Skin infections often require higher doses and longer durations due to limited skin perfusion and tissue distribution challenges, potentially promoting resistance. TMS showed a non-significant upward trend but remains clinically relevant given frequent first-line use; literature shows geographic variability in TMS resistance, warranting further study. Oxacillin resistance remained low in equine isolates compared to rising rates in companion animals, aligning more with livestock data and possibly attributable to stricter antibiotic regulations and fewer available drugs in large animals. Some beta-lactam resistance may be driven by beta-lactamase production rather than mecA-mediated methicillin resistance. Fluoroquinolone use in horses appears low relative to dogs, which may mitigate selection pressure for related resistances. The only site-specific difference identified was for gentamicin (abscesses uniformly susceptible vs high resistance in surgical incision isolates), though small sample sizes caution interpretation. Positive inter-antibiotic correlations largely tracked drug classes and usage tiers, consistent with shared mechanisms and selection pressures. Consistent with historical data, coagulase-negative staphylococci exhibited higher resistance than coagulase-positive species; clinically, most equine pyodermas are caused by coagulase-positive staphylococci, informing empiric therapy choices.

Resistance to beta-lactam antibiotics among equine Staphylococcus isolates appears to be increasing at the studied tertiary referral institution. The specific Staphylococcus species isolated influences susceptibility patterns, and antibiotics within similar classes tend to show parallel resistance behavior. Ongoing surveillance and larger-scale studies are needed to validate these trends and optimize empiric therapy in equine dermatology.

Key limitations include small sample size relative to the total patient population and the retrospective design with limited searchability of records. Prior antimicrobial use was unknown in most cases (49/77) and present in many others (24/77), introducing selection bias since cultures are often submitted after failed empiric therapy. As a tertiary referral center, the case mix likely overrepresents chronic, refractory infections, potentially inflating observed resistance rates. The significant gentamicin difference by sample type may be influenced by small subsample sizes (e.g., n=5 for surgical incision, n=15 for abscesses).