Microbial profiles and antimicrobial susceptibility in recurrent and non-recurrent canine otitis externa in Brazil

Canine otitis externa is a common inflammatory disorder in which bacterial and yeast proliferation may complicate treatment and contribute to recurrence. In Brazil, limited information is available comparing recurrent and non-recurrent cases while simultaneously characterizing microbial isolates and their antimicrobial susceptibility. This study compared clinical, otoscopic, and cytological findings between recurrent and non-recurrent otitis externa, identified bacterial and yeast isolates, evaluated their in vitro susceptibility to selected fluoroquinolones and antifungal agents, and investigated associations between clinical and microbiological variables and recurrence.

Fifty-seven dogs with otitis externa presented to a veterinary hospital in Brazil were evaluated by otoscopy and cytology. Ear secretions were cultured, and isolates were identified using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Antimicrobial susceptibility was evaluated by disk diffusion. Clinical and cytological findings were compared between recurrent and non-recurrent cases, and associations with disease status were assessed using Fisher's exact test and log-linear Poisson regression.

Cytology demonstrated bacterial proliferation in 54/57 (94.7%) and yeast proliferation in 25/57 (43.9%) samples. Microbial growth occurred in 46/57 (80.7%) samples, yielding 40 identified bacterial isolates and 16 Malassezia pachydermatis isolates. Staphylococcus was the predominant bacterial genus (21/40; 52.5%), and Staphylococcus pseudintermedius was the most frequent species (10/40; 25.0%). Resistance to both ciprofloxacin and enrofloxacin occurred in 37.5% of bacterial isolates. Among M. pachydermatis isolates, resistance occurred to fluconazole (43.8%), nystatin (31.3%), amphotericin B (12.5%), and ketoconazole (6.3%), whereas no isolate was resistant to miconazole. Recurrence was not significantly associated with the evaluated clinical or microbiological variables, although antifungal-resistant M. pachydermatis showed a nonsignificant trend toward association with recurrent disease. Cytological bacterial proliferation showed the strongest association with higher expected frequencies (RR = 18.0, p < 0.001).

S. pseudintermedius and M. pachydermatis were the most common microorganisms recovered from canine otitis externa. The substantial fluoroquinolone resistance among bacterial isolates supports culture and susceptibility testing rather than empirical therapy. Variable antifungal resistance among M. pachydermatis isolates, particularly in recurrent cases, warrants further investigation. These findings provide baseline data for antimicrobial surveillance and stewardship in canine otitis externa in Brazil.