It’s World Antimicrobial Resistance Awareness Week. Did you know that? Probably not. Antimicrobial resistance (AMR) is a huge global health threat with an unfortunately crappy marketing plan.

I’m on my way back from the BC One Health and Zoonoses Symposium where I was talking about AMR. It’s a complex issue and there are a lot of things we need to do to address the problem. One is development of clinical guidelines for antimicrobial use (including when not to use them), but a major challenge with guideline development in veterinary medicine is a profound lack of high-quality evidence to guide us.  We have few clinical trials, and even fewer that are properly designed and executed. That doesn’t mean we can’t create guidelines, after all we have to start somewhere, but it limits the certainty we have in the evidence that underpins our recommendations.

Fortunately, we are making progress in this field. Slowly. Trials are difficult to do and can be expensive, and it’s often very hard to find funding for this type of work, but progress is being made.

Peri-operative antibiotics are commonly used (and overused) in veterinary medicine to prevent surgical site infections. Sometimes they are needed, but often they aren’t. Continuing antibiotics post-operatively is still common too for some procedures, but there’s likely little to no reason to do so. We want antibiotics on board during the “period of risk” when contamination of tissues related to surgery is most likely: that starts with the surgical incision and ends soon after the surgical wound is closed. So we want antibiotics in the tissues at good levels during surgery, and potentially for up to 24 hours after, but that’s it. Once the period of risk is over, the benefit of antibiotics plummets and the costs (e.g. adverse events, selection for resistance) outweigh the risks.

Unfortunately, it can be difficult to convince people to stop giving antibiotics post-operatively, because it’s what many have gotten used to doing, and people are resistant to change. Post-op antibiotics is more psychotherapy for the veterinarian or owner…. it make us feel better if the animal gets an antibiotic, but it doesn’t make the animal feel any better (and may in fact do more harm than good).

A new study published just a couple of months ago (Vlhäinen et al. 2025) provides some good new evidence regarding the use of antimicrobials to prevent infectious complications following pyometra surgery in dogs. Good data like these can help convince people to change their habits and helps support our guidelines. Pyometra is an infection of the uterus that is not uncommon in unspayed dogs. In these cases, the uterus basically becomes a big pus-filled bag, and the most effective treatment is removing it (hysterectomy).

In this well-designed randomized controlled trial, 152 dogs undergoing pyometra surgery were enrolled and received an intravenous antibiotic (trimethoprim-sulfadoxine) pre-operatively. Then, they were randomized to either receive 5 days of oral trimethoprim-sulfadiazine (TMS) or a placebo. It was a non-inferiority study, so all they were aiming to do was to determine if no treatment was no worse than antibiotic treatment.

• Surgical site infections developed in 7.8% of dogs that got post-operative antibiotics and 2.7% of those that didn’t, so not giving post-op antibiotics was definitely no worse than giving them.
• Post-operative urinary tract infections developed in 2.7% of the treated dogs and none of the untreated dogs, so antibiotics didn’t help prevent these infections either.

In another interesting component, they looked at bacteriuria (bacteria in the urine without infection) of a subset of 43 dogs that were known to have been bacteriuric pre-operatively. They found 14/20 (70%) in the placebo group and 2/23 (8.7%) in the treatment group were still bacteriuric post-operatively.

• That’s not too surprising, since TMS is a great urinary drug, and 5 days of post-op treatment would be more effective for clearing bacteria from the lower urinary tract than a single dose (single dose treatment for urinary infections works for some drugs but not something I’d have a lot of confidence in for TMS). But remember that only 2 of those dogs had signs of cystitis, and both of them were dogs in the treatment group.  None of the dogs that had pre-operative and then post-operative bacteriuria had any clinical signs of infection. This supports the notion (and our guideline recommendations) that bacteriuria in the absence of disease (subclinical bacteriuria) doesn’t need to be treated.

This study provides some nice evidence supporting the lack of need for post-operative antibiotics in pyometra surgery, which is something we can also likely extrapolate to a range of other “clean-contaminated” procedures.