Monitoring trends in antimicrobial susceptibility patterns of bacterial isolates from horses with ulcerative keratitis

Abstract

SummaryBackgroundUlcerative bacterial keratitis can result in loss of vision or the eye. Antimicrobial susceptibility data are essential in selecting optimal therapy while pending culture results.ObjectivesTo document aerobic bacterial isolates and antimicrobial susceptibilities from horses with ulcerative keratitis and report susceptibility patterns compared to previously described data from the same hospital.Study designRetrospective clinical case series.MethodsMedical records from horses with positive bacterial cultures and a clinical diagnosis of infectious ulcerative keratitis at the University of Tennessee Veterinary Medical Center between March 2011 and December 2020 were reviewed and compared to previously published data from January 1993 to May 2004.ResultsThirty‐one bacterial isolates were cultured from 27 samples (26 horses). The most common bacterial genera were Streptococcus (35%), Staphylococcus (29%) and Pseudomonas (13%), with a shift towards more Staphylococcus and less Streptococcus compared to previous years. There were minimal temporal changes in susceptibilities for these three genera. Staphylococcus spp. were statistically more likely to be cultured than other bacteria after the use of topical neomycin‐polymyxinB‐dexamethasone (p = 0.008). In vitro antimicrobial susceptibility results suggest that targeted combination therapy covered a greater percentage of isolates, compared with monotherapy, but in vitro results may not mirror in vivo results.Main limitationsInconsistencies in available medical data, selection bias and a small sample size are inherent limitations in this retrospective study, along with the inability to test against all available ophthalmic medications, challenges of interpreting cultures with limited veterinary susceptibility data and no susceptibility guidelines for topical medications.ConclusionLimited changes have occurred over the past several decades in corneal bacterial isolates and susceptibilities within this equine ophthalmology referral population, with a shift in more Staphylococcus cultured over time. Monitoring temporal trends of bacterial isolates and antimicrobial susceptibility data is important in every hospital.