Bacterial isolates and antimicrobial susceptibilities in equine bacterial ulcerative keratitis (1993--2004).
Authors: Keller R L, Hendrix D V H
Journal: Equine veterinary journal
Summary
# Bacterial ulcerative keratitis in horses: antimicrobial susceptibility patterns and implications for empirical therapy Between 1993 and 2004, researchers at the University of Tennessee cultured aerobic bacteria from 43 horses with ulcerative keratitis, identifying 51 isolates and testing their susceptibility to commonly used topical antibiotics. *Streptococcus equi* subspecies *zooepidemicus* dominated at 33.3% of cases, followed by *Pseudomonas aeruginosa* (11.8%), *Staphylococcus* species (11.8%), and Gram-negative non-fermenting rods (7.8%), with clinical history documenting prior antibiotic use in most positive cases. The critical finding was a notably high bacitracin resistance rate in *S. equi* ssp. *zooepidemicus* (36% of isolates) despite this being a component of the standard neomycin-polymyxin B-bacitracin combination, whilst both major pathogens remained universally susceptible to fluoroquinolones and aminoglycosides—though prior neomycin-polymyxin B-bacitracin therapy was documented in 11 of 17 horses harbouring *S. equi* and 4 of 6 with *P. aeruginosa*. Practitioners should avoid empirical use of topical corticosteroids (linked here to polymicrobial infection) and reserve fluoroquinolones for confirmed bacterial infection rather than prophylaxis, whilst recognising that previous antimicrobial exposure substantially influences susceptibility patterns and should factor into antibiotic selection when treating corneal ulcers.
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Practical Takeaways
- •Know the local bacterial prevalence in your region and tailor empirical therapy accordingly; S. equi ssp. zooepidemicus and P. aeruginosa dominate equine corneal infections
- •Reserve fluoroquinolones and aminoglycosides for culture-confirmed infections rather than prophylactic use, as resistance patterns are emerging with widespread antibiotic use
- •Always obtain a detailed antimicrobial and corticosteroid history before culturing corneal ulcers, as prior therapy significantly alters bacterial susceptibility profiles and risk of polymicrobial disease
Key Findings
- •Streptococcus equi subspecies zooepidemicus was the most common isolate (33.3%), followed by Pseudomonas aeruginosa (11.8%)
- •S. equi ssp. zooepidemicus showed 64% sensitivity to bacitracin despite common use, suggesting emerging resistance
- •No resistance detected in S. equi ssp. zooepidemicus to cephalothin, chloramphenicol, or ciprofloxacin; P. aeruginosa remained susceptible to gentamicin, tobramycin, and ciprofloxacin
- •Prior antibiotic therapy (neomycin-polymixin B-bacitracin) was documented in 11/17 horses with S. equi ssp. zooepidemicus and 4/6 horses with P. aeruginosa, indicating previous treatment influences susceptibility patterns