Prior antimicrobial use as a risk factor for resistance in selected Staphylococcus pseudintermedius isolates from the skin and ears of dogs.

BACKGROUND: Antimicrobial resistance within bacteria continues to present therapeutic challenges. One presumed risk factor for increased rates of resistance is prior exposure to antimicrobial drugs.
OBJECTIVES: To examine the impact of time since most recent exposure, the number of prior antimicrobial exposures and duration of use on antimicrobial resistance rates in Staphylococcus pseudintermedius isolates.
METHODS: Inclusion of a case in the study required laboratory isolation of S. pseudintermedius from a clinical specimen. Antibiograms and information regarding prior antimicrobial exposures were extracted from the medical records of dogs diagnosed with pyoderma or otitis externa.
RESULTS: Meticillin resistance (MR) was identified in 48.1% of isolates. Recent use of beta-lactam antimicrobials was associated with increased odds of resistance to meticillin (P < 0.001) and fluoroquinolones (P < 0.001). Antimicrobial therapy within 1 month prior to sampling was also associated with MR (60.7%; P = 0.009) and multidrug resistance (61.9%; P = 0.029). The number of prior exposures to beta-lactams or fluoroquinolones were associated with resistance to these same classes (P = 0.001 and 0.02, respectively) and to other antimicrobial classes (P = 0.016 for resistance to fluoroquinolones following treatment with beta-lactams and P = 0.015 for MR following treatment with fluoroquinolones). Longer treatment duration with beta-lactam drugs was associated with higher proportion of MR (P = 0.004). CONCLUSIONS AND CLINICAL IMPORTANCE: Treatment based upon culture and susceptibility testing is highly recommended for dogs that have received multiple antimicrobial drug exposures or that were treated within the preceding month. This may be especially important when the prior therapeutic regimen included a drug from the beta-lactam or fluoroquinolone classes.