Escherichia coli isolated from canine pyometra show decreased susceptibility to amoxicillin-clavulanic acid in dogs.

OBJECTIVE: To describe bacterial culture results and antimicrobial susceptibility patterns for dogs undergoing surgical treatment for pyometra. METHODS: This retrospective study included dogs that underwent ovariohysterectomy at referral hospitals and had a positive bacterial culture of uterine fluid or histopathologic changes consistent with pyometra between January 2020 and December 2024. Data collected from medical records included clinical signs on presentation, hematological and biochemical abnormalities, aerobic and anaerobic bacterial isolates, antimicrobial susceptibility results, and peri- and postoperative antimicrobial administration. Prevalence of antimicrobial susceptibility was compared across select geographic regions and between Clinical and Laboratory Standards Institute (CLSI) susceptibility interpretation guideline editions. RESULTS: 278 dogs were included. Neutrophilia was identified in 74.2% of dogs. Only 1 bacterium was isolated in 88.8% of dogs, while 10.4% of cases were polymicrobial. Escherichia coli was the most common isolate (73.1%). Amoxicillin-clavulanic acid and enrofloxacin were prescribed postoperatively to 77.6% and 51.1% of dogs, respectively. Using CLSI VET01S edition 4, E coli susceptibility to amoxicillin-clavulanic acid and enrofloxacin was present in 30.3% and 96.9% of isolates, respectively. Reclassification using CLSI VET01S edition 7 resulted in lower susceptibility classifications in 9 of 10 antimicrobials evaluated, including reductions of 46.9% for amoxicillin-clavulanic acid and 3.1% for enrofloxacin. CONCLUSIONS: Amoxicillin-clavulanic acid was frequently prescribed despite limited susceptibility among E coli isolates. Updated CLSI breakpoints reclassified susceptibility for most antimicrobials despite unchanged MICs. CLINICAL RELEVANCE: Regional antibiograms may show decreased susceptibility compared to historical recommendations. Judicious empiric antimicrobial selection should be guided by antimicrobial stewardship principles rather than routine reliance on any single antimicrobial.