Bacterial isolates, antimicrobial susceptibility, and clinical characteristics of bacterial keratitis in dogs presenting to referral practice in Australia

Objective: To identify clinical characteristics, bacterial isolates, and their antimicrobial susceptibility patterns in a prospective study of dogs presenting with suspected bacterial keratitis to aid selection of appropriate therapy.

Animals: Seventy-one client-owned dogs presenting to two referral hospitals in Australia.

Procedure: Corneal swabs and clinical information were collected from dogs presenting with suspect bacterial keratitis from April 2012 to December 2014. Cytology and bacteriologic culture were performed on samples with in vitro antimicrobial susceptibility performed on bacterial isolates.

Results: Positive cultures were obtained from 42 of the 71 ulcers sampled (59%), with 45 bacteria isolated. The most commonly isolated bacteria were β-hemolytic Streptococcus (14/45, 31%), Pseudomonas (14/45, 31%), and Staphylococcus species (8/45, 18%). β-Hemolytic Streptococcus spp. were resistant to neomycin, polymyxin B, gentamicin, framycetin and fusidic acid and >80% were resistant to ciprofloxacin but remained susceptible to chloramphenicol and cephalexin. Pseudomonas aeruginosa isolates were resistant to chloramphenicol, cephalexin, and fusidic acid; however, >90% were susceptible to ciprofloxacin, polymyxin B, and gentamicin. Brachycephalic breed (34/42, 81%), previous ocular surgery (15/42, 36%), prior use of a topical corticosteroid (13/42, 31%), and concurrent ocular surface disease (10/42, 24%) were common in dogs with bacterial keratitis.

Conclusion: Our study identified β-hemolytic Streptococcus and Pseudomonas spp. as the most common bacterial pathogens in canine bacterial keratitis presenting for referral. Many cases exhibited clinical factors known to influence corneal integrity that may predispose them to ulceration and infection. Based on in vitro antimicrobial susceptibility patterns and clinical outcomes, monotherapy with a fluoroquinolone may be ineffective in ulcers caused by β-hemolytic Streptococcus spp.