Taming the Troublemaker: A Guide to Handling Endometritis in Mares
Endometritis — inflammation of the uterine lining — is one of the leading causes of reproductive failure in broodmares worldwide. According to a 2024 German study of 28,887 endometrial swab samples, 25.9% of mares showed growth of potentially pathogenic bacteria, with beta-haemolytic Streptococci (primarily Streptococcus equi subsp. zooepidemicus) accounting for 79.7% of positive cases. That means roughly 1 in 5 broodmares carries a uterine infection at any given time during the breeding season.
The challenge is that many of these infections are subclinical — no visible discharge, no fever, no obvious signs of illness. The mare cycles normally, accepts the stallion, and produces embryos that fail to implant. For breeders and veterinarians, this is one of the most frustrating problems in equine reproduction.
What Is Subclinical Endometritis?
Subclinical endometritis is uterine inflammation without obvious clinical signs. The uterus harbours bacteria at levels too low to trigger a systemic immune response, but high enough to prevent embryo survival. Standard uterine swabs frequently return “clean” results even in genuinely infected mares, because the bacteria responsible — particularly Strep zoo — can enter a dormant persister state deep within the endometrial tissue.
In this state the bacteria:
- Do not multiply actively, so they are not detected by standard culture
- Are metabolically inactive, so they do not respond to antibiotics
- Can persist for years between breeding seasons
- Reactivate when conditions change — such as during oestrus or after foaling
Research by Prof. Anders Miki Bojesen and Dr. Morten Rønn Petersen, published in Veterinary Microbiology (2015), demonstrated that dormant Strep zoo organisms reside deep in the chronically infected endometrium and can be reactivated with a specific bacterial growth medium.
Signs That Your Mare May Have Hidden Endometritis
Because subclinical endometritis produces no dramatic symptoms, breeders and veterinarians must rely on a pattern of clinical signs over time:
- Repeated failure to conceive despite breeding to a proven fertile stallion
- Culture-negative swabs combined with unexplained infertility
- Recurrent uterine fluid detected on ultrasound after breeding or during oestrus
- Early embryo loss before day 15
- A history of antibiotic treatment that temporarily resolved fluid but did not restore fertility
- Older broodmares (10+ years) with a history of repeated barren seasons
If your mare fits two or more of these criteria, dormant uterine infection is a likely cause of her infertility.
Why Standard Antibiotics Often Fail
The instinct when a uterine infection is suspected is to treat with antibiotics. This is often ineffective for dormant infections for a fundamental biological reason: most antibiotics work by disrupting the cellular processes of actively dividing bacteria. Dormant bacteria are not dividing. They are, in effect, invisible to the drugs.
Even when bacteria are detected by culture, targeted antibiotic therapy can fail if a subpopulation of the same species remains in a dormant persister state. The bacteria that are detectable are eliminated; the dormant ones survive, repopulate at the next breeding, and the problem recurs.
This cycle of apparent treatment success followed by renewed infertility is a classic pattern in mares with chronic subclinical endometritis.
The Activate-First Protocol: A More Effective Approach
The scientific breakthrough behind bActivate is the concept of forced bacterial reactivation before diagnostic sampling and antibiotic treatment. By instilling a bacterial growth medium into the uterus during early oestrus, dormant bacteria are stimulated into an active, multiplying state. Within 48 hours:
- The bacteria are now detectable by standard culture
- They are metabolically active and susceptible to antibiotics
- Targeted treatment can be applied based on the culture result
In a field study at Hagyard Equine Medical Institute (Lexington, Kentucky) led by Dr. Kristina Lu, 83% of 64 problem mares treated with this protocol became pregnant. At Kildangan Stud (Godolphin, Ireland), 89% of 19 problem thoroughbred mares conceived after a single treatment cycle.
When to Discuss bActivate with Your Veterinarian
bActivate is a prescription veterinary product indicated for mares with reduced uterine defence mechanisms and suspected chronic subclinical infection. It is administered by a veterinarian during early oestrus, and the treatment protocol spans a single cycle prior to the intended breeding cycle.
If your mare has been open for more than one season despite normal cycles and clean swabs, or if she has a history of early pregnancy loss, ask your veterinarian whether dormant uterine infection could be the cause. See the clinical evidence or learn about the full veterinary protocol.
Written by the Bojesen & Petersen Biotech ApS team. Medical oversight: Prof. Anders Miki Bojesen DVM PhD (University of Copenhagen) and Dr. Morten Rønn Petersen DVM PhD Dipl. ACT (Rigshospitalet).
What our clients say
Real results from veterinarians and breeders who have made bActivate part of their reproductive protocol.
“We incorporated bActivate into our standard reproductive work-up for problem mares at Hagyard. Out of 64 mares that had failed to conceive for at least 3 cycles, 83% became pregnant following bActivate activation and targeted antibiotic treatment. Nearly half had a dormant Streptococcus infection that standard culture had completely missed. It changed the way we approach the problem mare.”
“We used bActivate on 19 of our most persistent problem mares — horses that had been barren for over a year despite every conventional treatment we tried. 89% of them got in foal. What really opened our eyes was how many had a hidden infection that standard swabs had never detected. It is now a routine part of our protocol at Kildangan.”
“We have been using bActivate on several mares — all got pregnant and most of them in first try with frozen semen!”
“bActivate is an excellent tool that allows us as reproductive vets to do our job as effectively as possible. When you compare the cost to the expense of a mare that fails to conceive — or worse, never produces a foal — bActivate is both a smart and cost-effective solution in the long run.”
“I used bActivate and after just one covering got a colt foal — after 3 years of hardship where the mare went in foal but never managed to produce a live foal. I cannot recommend bActivate enough.”