Bacterial Agents and Antimicrobial-Resistance Patterns in Canine Otitis Externa

Canine otitis externa is a common and recurrent ear infection in dogs, often caused by bacterial pathogens, and complicated by increasing antimicrobial resistance. The analysis of bacterial isolates from dogs with otitis externa revealed a predominance of Staphylococcus pseudintermedius, representing 41% of all cases, followed by Staphylococcus aureus (23%), and Pseudomonas aeruginosa (19%). Other less frequently isolated organisms included Escherichia coli, Streptococcus canis, and Proteus mirabilis. These results highlight the significant role of S. pseudintermedius in the pathogenesis of otitis externa in dogs, as well as the relevance of Gram-negative opportunistic pathogens like P. aeruginosa, which exhibits the highest recurrence rate, with 90% of the cases associated with highly resistant to β-lactams (93% for amoxicillin–clavulanic acid; >70% for third-generation cephalosporins). P. mirabilis showed complete resistance to tetracycline, partial resistance to doxycycline, and reduced susceptibility to carbapenems, nitrofurantoin, and polymyxin B. S. canis exhibited limited resistance to erythromycin and clindamycin, while S. epidermidis displayed extensive multidrug resistance, including β-lactams, fluoroquinolones, sulfonamides, and polymyxins. These findings highlight the high prevalence of multidrug-resistant pathogens in canine otitis externa, emphasizing the need for culture-guided therapy and raising concerns regarding One Health, due to potential zoonotic transmission and dissemination of genetic determinants of antimicrobial resistance.