Endometritis (inflammation of the uterine lining) remains one of the most common causes of subfertility in broodmares. While uterine cultures are still the gold standard for diagnosis, results can take several days, leaving veterinarians to decide whether to start antibiotics before confirmation or wait. Josefina Ghersa, DVM, a theriogenology resident at the University of Saskatchewan’s Western College of Veterinary Medicine, in Canada, presented her clinical findings on using Gram-stain-guided assessment of endometrial cytology when treating endometritis in broodmares during the 2025 American Association of Equine Practitioners Convention, held Dec. 6-10, in Denver, Colorado.

In her retrospective analysis Ghersa said she evaluated how combining a traditional modified Wright-Giemsa stain with a Gram stain could inform veterinarians’ treatment choices in mares suspected of having endometritis. Gram-stain-guided point-of-care testing allows clinicians to start antimicrobials promptly when warranted while avoiding unnecessary use.

Testing Stallside Gram Stain

The researchers prepared two slides from the same endometrial sample—one stained with a modified rapid Wright-Giemsa stain to assess inflammation and cell health, and the other Gram stained to identify and classify bacteria as Gram positive or Gram negative. “The entire Gram stain process takes about five minutes and can be done stallside,” said Ghersa.

Not every mare with bacteria present in the uterus needs antibiotics. In this study Gram stain revealed bacteria in nearly half of the noninflammatory cytology samples, yet only a small percentage yielded a known pathogen on culture. For these mares, uterine lavage, sometimes paired with a biofilm-disrupting agent, might be sufficient while waiting for culture results. “This helps avoid unnecessary antibiotic use, a key concern as antimicrobial resistance continues to rise,” said Ghersa.

Mares with inflammatory cytology were far more likely to have clinically significant infections. In those cases Gram-stain results helped veterinarians choose an appropriate antibiotic class while awaiting culture and sensitivity data, which is particularly important in postpartum (because foaling contaminates her uterus) mares or those showing systemic illness, said Ghersa.

Take-Home Message

When a veterinarian detects bacteria on a uterine test, clinical context helps determine whether the mare needs antibiotics. Ghersa’s findings reinforce the idea that pairing routine cytology with Gram staining provides a clearer, faster picture of what’s happening inside a mare’s uterus and supports more thoughtful, evidence-based treatment decisions. She stressed that Gram staining does not replace culture and sensitivity testing. Instead, it acts as a bridge to help clinicians make informed choices in the critical window before lab results arrive.