Do Mares Really Need Antibiotics for Bacterial Endometritis?

One veterinarian calls for cultures, cytology, and antibiotics when indicated.
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Not all mares need antibiotic infusions to treat bacterial endometritis. | iStock
If you’re going to treat a mare for bacterial endometritis, be sure she has bacterial endometritis. It’s the first step in responsible antibiotic stewardship, said Tamara Dobbie, DVM, Dipl. ACT, associate professor of clinical large animal reproduction at the University of Pennsylvania, School of Veterinary Medicine, in Kennett Square. Dobbie spoke at the 2022 American Association of Equine Practitioners Convention, held Nov. 18-22 in San Antonio, Texas, about the responsible use of antibiotics when treating mares with bacterial endometritis.

Dobbie called for using an evidence-based approach to diagnosis; basing antibiotic therapy on current knowledge; reconsidering the use of post-breeding antibiotic infusions; and considering oocyte aspiration and intracytoplasmic sperm injection (ICSI, a form of in vitro fertilization) for chronically infected mares.

For diagnosis, Dobbie recommended sampling mares in estrus because that’s when they “have better uterine defense mechanisms.” She suggested obtaining samples through a vaginal speculum to minimize swab contamination.

“Alternatively, you can use a sterile sleeve or chemise to protect the swab from contamination,” she said. “You can reduce vestibular contamination by inserting a rolled feminine pad into the vestibule prior to rectal palpation and ultrasonography.”

To further avoid swab contamination, be cautious when transferring samples into the transport media. Dobbie said she prefers doing the transfer in a tack room or feed room to a barn aisle because they’re cleaner and not as drafty.

Most of the time, a routine culture is fine, but if the culture comes back with no growth and Dobbie suspects the mare has “something going on” based on clinical signs and history, she said she’ll obtain another sample and submit it for DNA analysis (Next Generation Sequencing).

Identification of bacteria alone, however, is not evidence of bacterial endometritis.

“That’s why we always recommend that we do cytology (examination of cells under a microscope) together with our culture,” said Dobbie. “Because if we have a positive cytology, and we isolate a pathogenic (disease-causing) bacterium, that is really good evidence that we have bacterial endometritis.”

While biopsy samples can be incredibly helpful, they’re not always practical, she added.

Once you have your culture results, look at the whole picture. Are there clinical signs of endometritis? These might include:

  • Anechoic (black on ultrasound) fluid in diestrus (when the mare’s not in heat).
  • Echogenic (gray on ultrasound) fluid anytime in the estrus cycle.
  • Fluid in the vagina.
  • Vulvar discharge.
  • Edema (fluid swelling) in the uterus of a diestrus mare or mares that come into estrus unexpectedly.

The two main culprits of bacterial endometritis are Escherichia coli and Streptococcus equi zooepidemicus. However, these pathogenic organisms are also normally found in the clitoral fossa and the vestibule.

“We want to make really sure that when we isolate these organisms, they are real, and they’re not contaminants,” Dobbie noted.

Veterinarians should also base antibiotic therapy on culture and sensitivity, Dobbie said. Decisions include whether to use intrauterine or systemic therapy and the appropriate dose and duration of therapy.

Benefits of intrauterine antibiotic therapy include higher drug concentrations in the uterine lumen and endometrium, where the bacteria tend to be. Intrauterine antibiotic therapy can, however, incite uterine inflammation. Moreover, treatment during diestrus can predispose the mare to fungal or secondary bacterial endometritis.

“When you’re doing intrauterine therapy, you need to really limit your treatment to estrus only,” Dobbie said. “And that’s really important.”

Intrauterine antibiotic therapy must be combined with uterine lavage, which removes exudate (pus) and inflammatory debris. The exudate and debris can dilute the antibiotics or make them inactive; lavage also helps ensure good contact with the endometrium and uterine lumen.

Little scientific evidence regarding intrauterine antibiotic dose and duration exists, Dobbie said.

While the cons of systemic antibiotic therapy include using higher quantities of the drug and potential widespread disturbances in the microbiome, the advantages include the ability to treat at any point in the estrous cycle and reduced endometrial irritation or iatrogenic (inadvertently caused by treatment) uterine contamination. You’d likely still perform a uterine lavage but would otherwise stay out of the uterus.

If a biofilm (a community of bacteria that’s difficult to treat) exists, then you’ll also need to decide on an approach that will disrupt it (potential producers include E. coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, and, rarely, Streptococcus equi zooepidemicus) in addition to antibiotic therapy. “Otherwise, your treatment is unlikely to be successful,” she added.

Dobbie was struck, she said, by “how few studies were actually out there that looked at antibiotics and how they concentrate in the endometrium appropriately, given how frequently we use them” and the lack of studies done in mares “in the disease state,” because infection and inflammation can alter antibiotics.

“Nowhere has it been shown that there’s a real significant difference in pregnancy rates between mares that receive post-breeding antibiotic infusions and those that received other therapies,” she said. “So, I really think that our job as veterinarians is to be able to identify the susceptible mare and use antibiotics as needed in addition to other therapies (uterine lavage, oxytocin, etc.).”

Still, not every mare should receive antibiotic infusions. For those mares that are chronically infected, a much more economical option, in the long run, is oocyte aspiration and ICSI, Dobbie said.

“I think for most of these mares, we have a really successful outcome,” she said. “And I think from an antibiotic stewardship point, it really makes sense that we should be pursuing other treatments because they’re out there and they’re available.”


Written by:

Karen Hopper Usher has a Master’s degree in journalism from Michigan State University, where she reported for Great Lakes Echo. She previously worked in local news and is a lifelong equestrian.

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