Placebo is non-inferior to postoperative antimicrobial treatment in uncomplicated canine pyometra – A double-blinded randomized controlled trial

Postoperative antimicrobial therapy is frequently administered after surgical treatment for pyometra in dogs, despite limited evidence supporting its necessity in uncomplicated cases. Antimicrobial resistance concerns call for re-evaluation of such practices. This randomized, double-blinded, placebo-controlled, non-inferiority clinical trial was conducted on client-owned dogs at the Veterinary Teaching Hospital of the University of Helsinki. All dogs received perioperative sulfadoxine-trimethoprim prophylaxis and were then randomized to receive either placebo or a five-day course of oral sulfadiazine-trimethoprim as active control. The primary outcome was the incidence of surgical site infections (SSI) within 30 days; the secondary outcome was the incidence of clinical urinary tract infections (UTI) within 12 days. The non-inferiority margins were set at 7 % (SSI) and 15 % (UTI). Infection outcomes were assessed in both the intention-to-treat (ITT) and per-protocol populations (PP). A total of 152 dogs were included. In the ITT population, SSI incidence was 2.7 % in the placebo group and 7.8 % in the sulfadiazine-trimethoprim group, with an absolute difference of -5.1 % (CI_{95 %}: -13.6-2.8 %). UTI incidence was 0 % in the placebo group and 3.9 % in the sulfadiazine-trimethoprim group (difference -7.7 %, CI_{95 %}: -24.1-8.2 %). Results in the PP were similar. All findings supported the non-inferiority of placebo. A single perioperative dose of sulfadoxine-trimethoprim appears sufficient for preventing postoperative infections in this low-risk surgical population.