The Crucial Balance: Responsible Antibiotic Use in Equine Reproduction

In a compelling presentation at the 2024 American Association of Equine Practitioners Convention held in Orlando, Florida, Dr. Mats Troedsson, a prominent equine reproduction expert from the University of Kentucky's Maxwell H. Gluck Equine Research Center, highlighted the pressing issue of antimicrobial resistance (AMR) in the context of equine reproduction.

Veterinarians frequently use antibiotics not just to treat infections among mares, but also to promote reproductive efficiency, often without the presence of an active infection. These antibiotics are typically administered during procedures such as post-breeding intrauterine infusions, embryo transfers, and within semen extenders for artificial insemination.

Interestingly, it has been established that breeding-induced or physiological endometritis, an inflammatory response occurring in the uterus post-mating, is primarily triggered by the sperm itself rather than by bacteria. This reaction is essential for clearing excess sperm and any potential contaminants to create an ideal environment for embryo implantation.

Recent studies reveal a remarkable fact: nearly 30% of mares are capable of resolving uterine inflammation within six hours after breeding or insemination, with roughly 85% of broodmares showing resistance to persistent endometritis. "This underscores the effectiveness of the uterus's natural defense mechanisms," stated Dr. Troedsson.

However, for those mares struggling to clear inflammation, further investigation is warranted. According to Dr. Troedsson, these mares often exhibit impaired myometrial contractility due to an accumulation of nitric oxide and an imbalance of pro- and anti-inflammatory cytokines linked to breeding. Factors contributing to these issues may also include anatomical conformations.

When bacteria persist in the uterus, indicative of bacterial endometritis, the use of antibiotics becomes necessary, guided by culture and sensitivity results. Treatment typically spans three to five days and may include uterine lavage if indicated. Importantly, mares diagnosed with this condition are not bred within the same cycle, allowing time for effective treatment based on lab results.

One innovative approach discussed by Dr. Troedsson involves the use of a sterile growth medium called "Activate," which is capable of reviving dormant bacteria like **Streptococcus zooepidemicus**. This medium allows for post-treatment cultures to confirm whether the uterus is clear or still infected.

Persistent cases of infectious endometritis that resist conventional treatments could be attributed to biofilms, which enable bacteria to evade both the immune system and antibiotic effects. "In these scenarios, bacteria may be subjected to adequate antibiotic concentrations on the biofilm surface, yet still evade treatment due to diminished penetration through the biofilm structure," explained Dr. Troedsson. This environment also facilitates the sharing of genetic material related to AMR among bacterial populations.

To combat biofilm-related infections, veterinarians must first identify the presence of a biofilm. Dr. Troedsson suggested developing in vivo assays to detect specific proteins or lipids unique to biofilms, offering pathways for improved diagnosis.

While antibiotic treatment is warranted for infectious endometritis, Dr. Troedsson emphasizes alternative strategies for breeding-induced endometritis, such as using ecbolics and immune modulators. "Antibiotics should not be the first response," he noted, asserting that inflammation is typically caused by sperm, not bacteria.

Dr. Troedsson concluded with a call for the equine community to shift towards more responsible practices concerning antimicrobial use. "We must seek alternatives to antibiotics and ground our decisions in scientific evidence, moving away from traditional practices that involve routine antibiotic use post-mating," he advised.

As the conversation around AMR intensifies, acknowledging and addressing these challenges in equine reproduction is essential for the health and well-being of our equine companions. The veterinary community must prioritize evidence-based practices to ensure a sustainable future for both equine reproduction and antimicrobial stewardship.