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Otitis externa (OE) in dogs is one of the most common and increasingly difficult to treat diseases in small animal medicine. Several predisposing factors, as well as primary and secondary factors, play a role in its development, secondary infection – typically with the facultative pathogen Pseudomonas aeruginosa – often aggravates the clinical picture. A significant percentage of affected animals develop chronic or recurrent ear infections that can last for months, and treatment is often challenging due to the potential multiresistance of the bacteria. Without proper treatment, irreversible damage can occur, which may result in total ear canal ablation. P. aeruginosa can also cause disease in humans, primarily in the form of nosocomial infections. Its zoonotic and reverse-zoonotic potential – i.e., transmission between animals and humans – has been documented.

In our study, we performed genotypic characterization on Pseudomonas aeruginosa isolates obtained from 70 ear swab samples, with isolation successful in 67 cases. The bacterial strains were collected by the Department of Pharmacology and Toxicology in 2023–2024 from the external ear canals of dogs in Hungary that had previously been diagnosed with otitis externa caused by P. aeruginosa. Using whole genome sequencing, we mapped resistance genes – their number, location within the genome, transferability, frequency and differences between strains.

The number of resistance genes per isolate was most commonly 46–47, by drug class relevant to OE therapy, we typically identified 21–22 fluoroquinolone-, 10 aminoglycoside-, 12 penam-, 13 cephalosporin-, 11 carbapenem-, and 7 peptide-resistance genes. The differences were mainly due to variants of the OXA and PDC genes and occasional gene duplications.

Among the 67 sequenced isolates, mobile genetic elements (MGEs) were detected in 64. In 43 isolates (~64% of the sample set), at least one resistance gene was MGE-associated. We found plasmid-encoded resistance genes in three samples (isolates 4., 25., and 67.), in all three cases, the CrpP gene encoding a ciprofloxacin-modifying enzyme associated with fluoroquinolone resistance was present. Compared to the total number of resistance genes per isolate, the proportion of MGE-associated genes was distributed as summarized below: 0% in 23 samples, 0–5% in 21 samples, 5–10% in 8 samples, 10–15% in 7 samples, and >15% in 5 samples.

This research is part of one of the largest genomic studies of P. aeruginosa conducted in the Hungarian animal health sector, contributing to the understanding of antimicrobial resistance (AMR) and the development of new, targeted therapeutic strategies.