Using minimum inhibitory concentration values of common topical antibiotics to investigate emerging antibiotic resistance: A retrospective study of 134 dogs and 20 horses with ulcerative keratitis.

Objectives To identify the minimum inhibitory concentration (MIC) distribution for commonly used topical antibiotics from isolates of dogs and horses with ulcerative bacterial keratitis, and to investigate changes in MIC values over time and following treatment with topical fluoroquinolones. Animals studied One hundred thirty-four client-owned dogs and 20 client-owned horses with bacterial ulcerative keratitis. Procedure Minimum inhibitory concentration values for 14 topical antibiotics were reported for canine and equine cases of bacterial ulcerative keratitis between 2013 and 2018. Changes in MIC values over time and after treatment with topical fluoroquinolones were reported. Results The three most common bacterial genera isolated wereStaphylococcus,Streptococcus,andPseudomonas. Together, these represented 79.4% of canine cases and 77.4% of equine cases. Overall, isolates from horses tended to have lower MIC values, as didPseudomonasisolates from both dogs and horses, compared to other bacterial genera, especiallyStaphylococcusspp. The MIC values of erythromycin and trimethoprim sulfa forStaphylococcusspp., and the MIC value of moxifloxacin forPseudomonassignificantly increased over time. Previous topical fluoroquinolone use was associated with a significant increase in the MIC value of ofloxacin in canine and equineStaphylococcusisolates and current topical fluoroquinolone use was associated with significant increases in the MIC values of ciprofloxacin, moxifloxacin, and ofloxacin in canineStaphylococcusisolates. Conclusion Patients previously or currently treated with topical fluoroquinolones, particularly inStaphylococcusinfections, may require alternative antibiotics or additional antibiotic classes other than fluoroquinolones. Bacterial culture with MIC susceptibility testing should be highly recommended when a Staphylococcal infection is suspected.