AAEP Reproduction Coverage: How Antimicrobial Stewards Approach Bacterial Endometritis in Mares

pregnant mare in field with mare and foal
mares with bacterial endometritis can still conceive foals
Studies show no significant difference in pregnancy rates between mares that received post-breeding antibiotics and those that did not. iStock

The days of dolling out antibiotics with wanton abandon for medical conditions—including post-mating-induced endometritis (PMIE)—have ended. Today’s practitioners must embrace ways of improving antibiotic stewardship using an evidence-based approach when treating bacterial endometritis in mares.

“First, we need to make sure the mare has a uterine infection, and we need to reconsider the routine use of antibiotics post-breeding,” said Tamara Dobbie DVM, DACT, from the University of Pennsylvania’s New Bolton at the 2022 AAEP Convention. 

Uterine Sampling

“Uterine sampling is incredibly important,” said Dobbie. “The samples need to be accurate and representative of the uterine environment, making sure they aren’t contaminants. In the normal mare, we can find Streptococcus equi zooepidemicus and Escherichia coli in the vestibule, so we need to make sure we are not picking up these organisms when we perform our uterine culture.”

Sampling during estrus is ideal because the mare is better able to resist contamination (i.e., veterinarians don’t want to contaminate the uterus when collecting their sample).

When collecting uterine samples, Dobbie recommended obtaining samples through a vaginal speculum or using a sterile chemise over the uterine swab. Placing a “plug” within the vestibule will help prevent vestibular—and even caudal vaginal—contamination during rectal palpation and ultrasonography.

“Admittedly, the plug is cumbersome and time-consuming, but the effort is well worth it,” advised Dobbie. 

To perform the plug technique, Dobbie scrubs the mare and dries her. She then “pops” a rolled feminine hygiene pad into the vestibule. Practitioners can then perform rectal palpation and ultrasonography with the vestibular plug in place without fear of vestibular contamination. The plug can be removed immediately prior to vaginal sampling and vulvar cleaning. 

Dobbie also urged attendees to consider swab contamination when transferring the sample into the transport media.

“You are in the barn! It is dirty. I never do this transfer in the barn aisle. Find somewhere cleaner, like the tack room,” Dobbie advised. 

Interpreting Culture Results

Even without fear of contamination, interpreting the culture results is difficult.

“Identification of bacteria alone is not evidence of bacterial endometritis. And even if there are no bacteria on culture, this doesn’t mean there is no uterine infection,” warned Dobbie. “You need to look at whole clinical picture, such as fluid in the uterus, vulvar discharge, uterine edema during diestrus, and premature luteolysis, for example.”

Once the presence of a bacterial infection has been confirmed, then antibiotics must be selected based on sensitivity results.

Intrauterine Therapy for Treating Bacterial Endometritis in Mares

“Intrauterine therapy can achieve high drug concentrations with a small amount of antibiotics compared with when antibiotics are given systemically,” stated Dobbie. “The intrauterine route results in minimal systemic absorption and little impact on other body systems, such as the intestinal microbiome.” 

Intrauterine infusions, however, must only be performed during estrus (this is in comparison to systemic antibiotics, which can be administered at any stage of the estrus cycle).

Focusing on PMIE, Dobbie said, “We know it is a normal physiologic response to breeding, but the fluid should be cleared within 24-48 hours. Up to 15% of mares have delayed uterine clearance and prolonged inflammatory responses. These mares will need post-breeding lavage and maybe antibiotics, but this is a small group of mares!”  

She then relayed that studies show no significant difference in pregnancy rates between mares that received post-breeding antibiotics and those that did not. 

“Only treat infected mares. We really shouldn’t be using antibiotics in every mare we breed,” Dobbie emphasized. “Using these principles, we can really improve our antibiotic stewardship.” 

Alternate Options for Infected Mares

Instead of relying on antibiotics, Dobbie recommended considering oocyte aspiration and ICSI (intracytoplasmic sperm injection) for chronically infected mares.

“Not only is this approach more economical in the long run, it’s associated with very successful outcomes and is a much better approach from an antibiotic stewardship point of view,” shared Dobbie.  

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