Bacterial and fungal isolates from 107 cases of ulcerative keratitis in Japanese Thoroughbred racehorses (2017-2021).
Authors: Uchida-Fujii Eri, Kuroda Taisuke, Niwa Hidekazu, Kinoshita Yuta, Kano Rui, Tamura Takashi, Makimura Koichi, Ueno Takanori
Journal: Journal of equine veterinary science
Summary
# Editorial Summary: Antimicrobial resistance in equine ulcerative keratitis Infectious ulcerative keratitis remains a significant clinical challenge in racing Thoroughbreds, yet treatment decisions often rely on empirical antimicrobial protocols rather than culture-guided therapy. Between 2017 and 2021, researchers cultured corneal swabs from 107 Japanese racehorses with confirmed ulcerative keratitis, identifying bacterial and/or fungal pathogens in 85% of cases (91 horses), with Staphylococcus species predominating amongst bacteria and Aspergillus amongst filamentous fungi. Worryingly, complete ofloxacin resistance was documented in methicillin-resistant *Staphylococcus aureus* (MRSA) isolates, whilst 15.9% of non-MRSA *Staphylococcus* and 25% of *Streptococcus* isolates also showed quinolone resistance—a concerning upward trend over the preceding two decades. Although all *Aspergillus* isolates remained susceptible to voriconazole, other filamentous fungi including *Fusarium* demonstrated reduced susceptibility to this antifungal agent. For equine practitioners, these findings underscore the value of obtaining culture and sensitivity results before initiating topical antimicrobial therapy, particularly given the prevalence of quinolone-resistant bacteria and the variable antifungal responses in fungal keratitis cases; empirical fluoroquinolone monotherapy may be inadequate for Staphylococcal infections, and presumptive voriconazole use may fail against some fungal pathogens.
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Practical Takeaways
- •Corneal swab culture and antimicrobial susceptibility testing should guide treatment selection for equine ulcerative keratitis, as quinolone resistance is common in Staphylococcus isolates (15.9-100% depending on species)
- •Voriconazole is reliable for Aspergillus infections but alternative antifungal agents may be needed for Fusarium and other fungi; consider fungal culture before initiating antifungal therapy
- •MRSA represents a significant challenge in racehorses with ulcerative keratitis, with complete ofloxacin resistance; alternative antimicrobial classes should be selected based on susceptibility results
Key Findings
- •Bacterial and/or fungal isolates were obtained from 85.0% (91/107) cases of ulcerative keratitis in Japanese Thoroughbred racehorses
- •Staphylococcus was the primary bacterial isolate, with 100% ofloxacin resistance in MRSA (12/12 isolates)
- •Aspergillus was the primary filamentous fungal isolate with 100% voriconazole susceptibility, while other fungi including Fusarium showed reduced voriconazole susceptibility
- •Quinolone-resistant Staphylococci and Streptococci prevalence has increased over the past two decades