Today, antimicrobial resistance is a significant global health concern, one that should encourage each of us to analyze how and when we turn to antimicrobial therapies. In a randomized, double-blinded, placebo-controlled clinical trial at University of Helsinki, researchers evaluated the effects of postoperative antimicrobial therapy for 152 client-owned dogs following surgery for uncomplicated pyometra. All dogs received prophylactic perioperative sulfadiazine-trimethoprim, then were randomly sorted to either receive a placebo (75 dogs) or 5-day course of oral sulfadiazine-trimethoprim (77 dogs). Researchers evaluated the incidence of surgical site infection within 30 days and urinary tract infection within 12 days.
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Anna Ylhäinen, DVM, works in the emergency and intensive care unit at University of Helsinki small animal veterinary teaching hospital. She is writing her doctoral dissertation on the treatment of pyometra in female dogs, specifically investigating the effect of postoperative antimicrobials on surgical site and urinary tract infections as well as the characteristics of the bacteria causing those infections.
Q: This clinical trial found no statistically significant association between the 2 treatment groups—placebo or antimicrobial therapy—and infection rates. This highlights the importance of evidence-based prescribing practices rather than an argument of habit or precaution for antimicrobial therapy. Would you consider that to be the primary takeaway of this study—antimicrobial stewardship?
Absolutely. The study shows that routine postoperative antimicrobial therapy is not necessary for uncomplicated pyometra when appropriate perioperative prophylaxis is used. For many of us in clinical practice, the challenge has been uncertainty—knowing when it’s truly safe to do less, particularly when postoperative infections carry real consequences. These results help provide evidence to support those decisions and reinforce antimicrobial stewardship by reducing unnecessary selection pressure for multidrug-resistant bacteria.
Q: The overall occurrence of surgical site infections and urinary tract infections was low (8 dogs and 3 dogs, respectively) and consistently lower in the placebo group. These findings indicate that a single perioperative dose of sulfadiazine-trimethoprim sufficiently prevents postoperative infections in dogs with uncomplicated pyometra. Did this finding change your clinical treatment of patients with uncomplicated pyometra?
In many ways, it reinforced my existing approach; but it also helped clarify how broadly that approach can be applied. Like many clinicians in my area of northern Europe, I had already moved away from prescribing antimicrobials in the most straightforward, uncomplicated pyometra cases, which was part of the rationale behind the study hypothesis. What had been less clear was where to draw the line between uncomplicated and complicated cases.
Prior to this study, it wasn’t well established whether dogs with moderate systemic signs could safely be managed without postoperative antimicrobials, or whether only afebrile dogs with minimal clinical or laboratory abnormalities should be considered uncomplicated. For this study, we defined complicated pyometra narrowly—limited to dogs with sepsis, peritonitis, uterine rupture, or comorbidities or medications known to increase infection risk (e.g., diabetes mellitus, immunosuppressive corticosteroid use). Because including dogs with moderate systemic signs in the uncomplicated group didn’t result in higher infection rates, I’m now comfortable refraining from postoperative antimicrobials in these patients as well.
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Q: In this study, 34% of dogs showed signs of illness postoperatively; yet most were mild and resolved without treatment. Recognizing that a veterinarian must use sound clinical judgment to discern clinical signs and signs of infection, how would you advise veterinarians to tailor their clinical recommendations surrounding antimicrobial therapy?
What I’ve found helpful in practice is starting with evidence-based protocols, then adapting them based on individual patient risk factors. In our study population, postoperative antimicrobials were not necessary even when mild clinical signs occurred as these were often unrelated to infection. The most common postoperative finding we recorded was a reduced appetite lasting 1 to 3 days, which is a fairly typical response following surgery or anesthesia.
When deciding whether antimicrobials are warranted, I would encourage clinicians to prioritize objective indicators of infection over transient, nonspecific changes. Clear communication with clients about what is expected postoperatively—and what should prompt concern or re-evaluation—is also essential in supporting these decisions and ensuring patient safety.
Q: Were you surprised by any findings from this clinical trial?
I was somewhat surprised by how low the infection rates were overall, and particularly by the fact that we observed even fewer infections in the placebo group than in the antimicrobial-treated group despite low levels of antimicrobial resistance among the isolated bacteria. I think this challenges the assumption many of us have carried—that extending antimicrobial therapy necessarily adds an extra layer of protection. While this finding can only be applied to a specific patient population for now, it raises interesting questions about whether routine antimicrobial use always provides the benefit we expect; and it might even suggest that routine antimicrobial use could disrupt normal recovery.
Q: Is there anything you would like our readers to know that has not been mentioned?
I would emphasize that antimicrobial resistance is a global concern, and every prescribing decision contributes to it. Studies like this are not about telling clinicians what they should be doing but about providing better data to support the decisions we already make every day in practice. The results of our study suggest that we can maintain excellent patient outcomes while safely reducing unnecessary antimicrobial use, and continued research is vital in helping us protect the health of our patients, our profession, and the effectiveness of antimicrobial therapies moving forward.
Read the full study
Placebo is non-inferior to postoperative antimicrobial treatment in uncomplicated canine pyometra – A double-blinded randomized controlled trial.
Ylhäinen A, Rantala M, Thomson K, et al. Vet J | doi:10.1016/j.tvjl.2025.106449