Essential Oils Fight Microorganisms Responsible for Endometritis in Mares

Veterinarians frequently trace subfertility in mares to endometritis, an inflammation of the uterine lining often caused by infection. Conventional antibiotic therapy aimed at persistent post-breeding and chronic endometritis often fails. Response to traditional therapies can be further blunted by increased incidence of antimicrobial resistance. In a recent study, Brazilian researchers tested three essential oils to determine their effectiveness against common microorganisms that trigger endometritis.^*

Researchers identified 36 mares with endometritis as evidenced by clinical signs such as intrauterine fluid accumulation, abbreviated estrous cycle, and cloudy uterine lavages. Intrauterine swabs were taken from these mares, and pathogenic microorganisms isolated in a laboratory. Specific pathogens selected for this study included Streptococcus equi zooepidemicus, Staphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa, Escherichia coli, and Candida albicans. Researchers chose three common essential oils to test—basil, rosemary, and lemongrass—and performed serial studies with increasing concentrations of each, noting antimicrobial activity.

The researchers found that lemongrass had the highest inhibition percentage (74%) followed by basil (67%) and rosemary (59%). Inhibition percentage is the rate of measured normal activity minus inhibited activity, divided by the rate of normal activity. A higher percentage indicates greater antimicrobial activity. The essential oils were effective against all pathogens except P. aeruginosa.

“According to the data of the present study, the essential oils of basil, rosemary, and lemongrass have presented in vitro antimicrobial activity, in different levels, against microorganisms causing endometritis in mares,” the researchers concluded.

Scientists know well the pressing need to look for alternatives to tried-and-true medicines for endometritis and other common infections. “Essential oils, with their well-known antimicrobial properties, are poised to be possible solutions to the ever-increasing problem of drug resistance among pathogens,” said Kathleen Crandell, Ph.D., a nutritionist at Kentucky Equine Research.

“The research reported here was performed in the laboratory, so the next steps involve looking closely at the compounds in the oils and their mechanisms of action, and eventually testing the efficacy of these essential oils in mares with endometritis,” she said. Additional research will, among other goals, determine the effect of essential oils when used in combination with other antimicrobial compounds.

As researchers pinpoint novel and efficient alternatives to traditional antimicrobial therapies, ensuring reproductive health through optimal year-round nutrition and appropriate supplementation is one way to stay on top of broodmare well-being. “While no dietary change can prevent endometritis, a robust immune system bolstered by sound nutrition and targeted supplementation will help keep reproductive tissues healthy and capable of nurturing an embryo,” Crandell opined. “For mares, I often recommend a marine-derived source of omega-3 fatty acids, such as EO-3.”

Benefits of marine-derived omega-3s in mares have been observed. In one study, for example, researchers recruited light-horse mares that were fed a diet supplemented with marine-derived omega-3 fatty acids starting 60-days before breeding.^** Researchers found that omega-3 supplementation resulted in better “endometrial scores,” which are indicators of uterine health, compared with unsupplemented mares, and the expression of certain genes important in the development of the embryo in pregnant mares was significantly different between supplemented and unsupplemented mares.

The findings suggested that the marine-derived omega-3 supplement used in the study fed near conception “may alter the post-ovulatory uterine environment and early embryonic development in the horse,” according to researchers.

^*Lisboa, F.P., W.P. Silvestre, J.O. Castro, G.V. Martins, L.G.T.M. Segabinazzi, G.F. Pauletti, and J.A. Dell’Aqua. 2022. In vitro antimicrobial activity of selected essential oils against endometritis-causing microorganisms in mares. Journal of Equine Veterinary Science 110:103840.

^**Jacobs, R.D., A.D. Ealy, P.M. Pennington, et al. 2015. Effect of dietary omega-3 supplementation on equine endometrial and embryonic gene expression. Journal of Equine Veterinary Science 35(5):438-439.