How should I select antibiotics for urinary tract infections in dogs given current resistance guidelines?

Question

Accepted Answer

First-line empirical therapy for canine sporadic bacterial cystitis is amoxicillin or trimethoprim-sulfamethoxazole (TMS), selected before culture results are available. These are the agents recommended by the International Society for Companion Animal Infectious Diseases (ISCAID) as first-tier options, with fluoroquinolones and third-generation cephalosporins reserved for specific indications only.510 Culture and susceptibility testing is recommended for all cases of suspected bacterial cystitis before or concurrent with starting empirical therapy. Urine should be collected by cystocentesis to distinguish true bacteriuria from contamination, and subclinical bacteriuria — bacteriuria without lower urinary tract signs — does not require antimicrobial treatment.78 Standard culture and susceptibility testing takes 48–72 hours, during which empirical therapy proceeds.12 Empirical choice should be guided by local antibiogram data, not international guidelines alone. Regional resistance patterns vary substantially: amoxicillin susceptibility among urinary E. coli isolates from dogs in the Midwestern United States was only 53%, while amoxicillin-clavulanate susceptibility was 92% in the same population.14 In Saskatchewan, Canada, ampicillin resistance among urinary E. coli was approximately 15%, while fluoroquinolone susceptibility remained approximately 98%.5 Where amoxicillin resistance exceeds 20% locally, amoxicillin-clavulanate is a more reliable empirical choice for that region.14 The 80% susceptibility threshold — the point below which an empirical agent warrants reconsideration — is the accepted benchmark for first-line drug selection.3 Amoxicillin-clavulanate achieves high urinary concentrations and is effective against beta-lactamase-producing E. coli, Klebsiella, and staphylococci, but Pseudomonas and Enterobacter are intrinsically resistant. Plain amoxicillin reaches urinary concentrations that may exceed clinical breakpoints even when in vitro susceptibility t