endometritis in mares

A healthy endometrium—the lining of the mare’s uterus—is important for achieving and maintaining a pregnancy. However, it’s a delicate structure, vulnerable to inflammation and capable of harboring bacteria deep in its layers. To boot, this inflammation, known as endometritis, can be challenging to treat effectively, leaving researchers on the hunt for new therapeutic options.

But there’s some good news: Veterinarians have tested a new antibiotic option for horses with endometritis, and the initial results are promising.

“Endometritis continues to plague the equine industry and is currently touted as the third-most-important medical problem diagnosed in adult horses,” said researcher Dale Paccamonti, DVM, Dipl. ACT, theriogenology professor and head of Veterinary Clinical Sciences in Louisiana State University’s (LSU) School of Veterinary.

“Improving our understanding of the endometrium and causes of endometritis will help devise targeted, effective therapies to minimize losses due conception and foaling failures,” he added.

Veterinarians frequently treat bacterial endometritis by infusing antibiotics directly into the uterus. Their goal is to apply the antibiotic directly to the endometrium to fight infection.

However, “there are a few drawbacks of uterine infusions that make using systemic antibiotics more appealing,” said lead author Gabriel Davolli, DVM, Dipl. ACT, who conducted the study while he was a theriogenology resident at LSU.

For example, studies show that veterinarians often choose their antibiotics based on results from uterine swabs, which capture bacteria residing on or near the endometrial surface. While those cultured bacteria can, indeed, be the culprits, other bacteria residing out of the swabs’ reach can also contribute to endometritis. One such bacterium is Streptococcus equi subsp zooepidemicus.

“This bacterium has the capability to chronically reside in the deep endometrium rather than close to the surface,” said Davolli. “This potentially makes these bacteria invulnerable to an antibiotic infused into the uterine lumen that would not penetrate deep enough into the endometrium.”

So, in looking at different treatment options for such bacteria, Paccamonti, Davolli, and colleagues tested a newer equine antibiotic specifically designed for oral administration. The researchers said this sulfadiazine and trimethoprim product comes as a suspension, is economical, and has “good and moderate susceptibility index against S. equi subsp zooepidemicus and Escherichia coli, respectively. These bacteria are considered the first- and second-most-commonly isolated organisms from cases of infectious endometritis.”

They administered the medication twice daily at a dose of 24 mg/kg of body weight to 41 nonpregnant mares in estrus and collected blood samples from 0 to 60 hours after starting treatment. They also obtained endometrial biopsies 60 hours after starting treatment. The blood and endometrial samples allowed the team to see if the medication was reaching the necessary tissue concentrations to effectively treat bacteria.

As hoped, the concentrations of both sulfadiazine and trimethoprim were present in blood and endometrial tissues at sufficient concentrations to fight endometritis-causing bacteria such as Streptococcus equi subsp zooepidemicus and E. coli.

“These findings indicate that the tested sulfadiazine-trimethoprim product should be an efficacious and viable treatment for bacterial endometritis,” said Davolli.

The study authors also noted that the sulfadiazine-trimethoprim product used in the current study is approved for use in horses.

The study, “Concentrations of sulfadiazine and trimethoprim in blood and endometrium of mares after administration of an oral suspension,” was published in the Journal of Equine Veterinary Science.