Taming the Troublemaker: A Guide to Handling Endometritis in Mares
In short. Endometritis in mares is most often caused by Streptococcus zooepidemicus, and in problem mares the bacteria are frequently dormant, so a standard swab reads negative: swab culture finds about 34% of infected mares, biopsy culture about 82% (Nielsen 2005). Signs to act on are repeated failure to conceive, uterine fluid, early embryo loss and antibiotics that help for one cycle only. Activation before culture reveals the dormant infection so the veterinarian can treat what the antibiogram shows.
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 samples showed growth of potentially pathogenic bacteria, with beta-haemolytic Streptococci (primarily Streptococcus equi subsp. zooepidemicus) accounting for 79.7% of positive cultures. That is a sample prevalence: roughly one positive sample in four, before any dormant bacteria are counted.
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 estrus 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 estrus
- 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 estrus, 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 the bred mares (16 of 18) at Kildangan Stud conceived after a single activation cycle and targeted antibiotic treatment.
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 estrus, and the activation plus veterinary 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).
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