Vetnews | Desember 2025 10 « BACK TO CONTENTS Our results differ from those reported by Viegas et al. [11], who found MRSP strains in 17.6% of dogs with external otitis. According to the literature, the prevalence of MRSP among canine populations shows wide variation, with reported values spanning from 0% to 60% [11]. It is important to note that drawing direct comparisons across studies can be challenging, as MRSP prevalence is influenced by multiple factors, including the study population, sample type, geographic location, and methodological approach [11]. According to CLSI guidelines, staphylococci showing resistance to oxacillin are interpreted as resistant to each class of β-lactam antibiotics. In antimicrobial susceptibility testing, methicillin was substituted with oxacillin due to its greater stability [28]. Beyond their resistance to β-lactam antibiotics, MRSP strains commonly possess resistance to several other antimicrobial drug classes [13]. Regarding penicillin resistance among S. pseudintermedius strains isolated from the ears of healthy dogs, the prevalence observed in this study was 19.7% (Table 2). This value is lower than the 39.9% reported by Rubin and Chirino-Trejo [29], who analysed isolates from nasal, pharyngeal, and rectal sites. One possible explanation for this discrepancy is the anatomical site of sampling, as microbial populations and resistance profiles can vary significantly between different body regions due to differences in local microenvironments and antimicrobial exposure. In addition, geographic variation and differences in antimicrobial use practices between populations or regions may also contribute to this divergence. Similarly, the tetracycline resistance rate in our study was 25%, closely aligning with the 23.5% reported by Rubin and ChirinoTrejo [29]. This consistency might suggest a more stable pattern of tetracycline resistance across different body sites and potentially across different geographical regions. However, the similarity could also reflect the widespread and long-term use of tetracyclines in veterinary medicine, which may have exerted consistent selective pressure over time. Regarding MRSP, no strains were identified in our study. This is consistent with the findings of Rubin and Chirino-Trejo [29], who also reported no MRSP in clinically healthy dogs. The absence of MRSP in both studies could indicate a relatively low prevalence of methicillin resistance in S. pseudintermedius among healthy canine populations, at least in the sampled regions. It may also suggest that MRSP carriage is more closely associated with clinical infections [30] or prior antimicrobial exposure, which were not present in the healthy dogs studied. The current understanding of the epidemiology, zoonotic potential, and antimicrobial resistance patterns of S. pseudintermedius in healthy canine carriers remains limited. Given the implications for both veterinary and public health, further investigations are essential to better characterise and address these critical areas [29]. In this study, the S. pseudintermedius strains isolated from dogs with otitis externa exhibited a more diverse and extensive antimicrobial resistance profile compared to those isolated from clinically healthy dogs. Although the proportion of strains susceptible to all tested antimicrobials was relatively similar between the two groups (38.1% in dogs with otitis externa vs. 40.8% in healthy dogs), resistance to tetracycline, penicillin, and clindamycin was more frequently observed among pathogenic strains. Notably, MRSP strains were exclusively isolated from dogs with otitis externa. All isolates were susceptible to antibiotics reserved for human medicine—such as linezolid, teicoplanin, vancomycin, and tigecycline—which were included in the testing panel due to the zoonotic potential of S. pseudintermedius. No resistance was observed to fusidic acid either. These findings support the notion that S. pseudintermedius strains isolated from both healthy and diseased dogs may harbour antimicrobial resistance and pose potential risks for zoonotic transmission, although strains associated with otitis externa tend to exhibit more frequent and complex resistance patterns. Recent research has shown that S. pseudintermedius, particularly methicillin-resistant strains (MRSP), may pose a zoonotic risk. Sequence types such as ST45, ST71, and ST121 were detected in both companion animals and human clinical samples, and wholegenome comparisons revealed high similarity between isolates, suggesting possible recent interspecies transmission [31]. Genomic analysis revealed notable diversity, including 23 STs among animal-derived strains and ten novel variants. Most isolates carried common resistance genes like mecA and blaZ, along with others conferring resistance to aminoglycosides, macrolides, and tetracyclines. Virulence factors such as sps, ica, and leukotoxin genes were also present and appeared to vary by sequence type, suggesting host adaptation [31]. While no direct correlation between host and ST was found, the genetic overlap between isolates from animals and humans highlights the zoonotic potential of MRSP and supports the need for One Health surveillance strategies [31]. Mobile genetic elements, such as SCCmec types III, IVg, V, and VII, were identified across MRSP isolates and played a key role in the dissemination of resistance genes, particularly mecA. Specific point mutations associated with fluoroquinolone resistance (e.g., grlA S80I and gyrA S84L) were frequently observed, especially in ST71 and ST339. These genomic features, alongside the presence of biocide resistance and virulence-associated genes, underline the adaptability of S. pseudintermedius and its potential to persist in both veterinary and human environments [32]. Moreover, the recent literature provides additional context that complements our findings. Regional studies have highlighted the clinical relevance of S. pseudintermedius and other staphylococcal species isolated from cutaneous and mucosal infections in companion animals [33,34]. Comprehensive reviews have described major epidemiological shifts in MRSP prevalence and explored alternative therapeutic strategies to address emerging resistance patterns [35]. Large-scale surveillance investigations from European veterinary clinics have further reported substantial MDR prevalence and complex co-resistance profiles among S. pseudintermedius isolates [36]. In addition, meta-analyses of canine otitis externa and pyoderma cases confirm a wide range of prevalence values across geographical regions [37], while recent studies emphasise the influence of host-related and managementassociated factors on the development and spread of antimicrobial resistance in canine populations [38]. The statistical analysis further reinforces the robustness of our findings, demonstrating significant differences in prevalence and antimicrobial resistance between isolates from diseased and clinically healthy dogs. The inclusion of p-values, FDR-adjusted q-values, and relative risk estimates provides quantitative evidence of these differences, underscoring their clinical and epidemiological relevance. 3.1 Limitations. This study has some limitations. The sampling was restricted to a single urban area, which may affect the generalizability of the findings. Furthermore, genetic characterisation of resistance determinants was not performed. Leading Article
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