Addressing the Challenges of Problem Mares
Problem mares — those that cycle normally, accept covering, but consistently fail to conceive or maintain pregnancy — represent one of the most frustrating challenges in equine breeding. For breeders and veterinarians managing these cases, the combination of high costs, diagnostic uncertainty and repeated failure makes the problem feel intractable. Understanding the most likely underlying cause is the first step toward a practical solution.
How Common Is the Problem?
The scale of subclinical uterine infection in the mare population is larger than most people realise. A comprehensive German study published in the Journal of Equine Veterinary Science in February 2024 analysed 28,887 endometrial swab samples. Key findings:
- 25.9% of all samples showed growth of potentially pathogenic bacteria
- Of positive cases, 79.8% were Streptococcus infections — affecting 6,658 mares in the dataset
- This implies that approximately 1 in 5 broodmares carries a uterine streptococcal infection at any point during the breeding season
These figures represent detectable infections. The true prevalence is likely higher. Research by Prof. Anders Miki Bojesen and Dr. Morten Rønn Petersen indicates that a significant proportion of streptococcal infections in mares exist in a dormant persister state — one that evades detection by standard swab culture entirely. Their studies suggest that 50–75% of mares that remain open at end of season carry dormant infections.
Why Standard Diagnostics Miss the Problem
The standard diagnostic pathway — uterine swab culture — is reliable for acute, active infections. It is unreliable for chronic subclinical infections because of the biology of Streptococcus equi subsp. zooepidemicus.
When conditions are unfavourable for bacterial survival (including, potentially, the presence of antibiotics or a strong immune response), Strep zoo can enter a dormant persister state. Persister cells are not multiplying. They are metabolically inactive. A culture swab will not detect them. This is why many problem mares return culture-negative results yet fail to conceive — the swab is genuinely negative for active bacteria, but dormant cells remain embedded in the endometrium, ready to reactivate during the next oestrus and re-establish an inflammatory environment hostile to embryo survival.
Recognising a Problem Mare with Dormant Infection
No single clinical sign definitively confirms dormant infection, but a pattern of the following indicators should prompt investigation:
- Failure to conceive over two or more consecutive breeding seasons with a proven fertile stallion
- Culture-negative swabs in a mare with persistent infertility or recurrent uterine fluid on ultrasound
- Previous antibiotic treatment that resolved fluid temporarily but fertility did not improve
- Early pregnancy loss before day 15 in multiple seasons
- Older broodmare (10+ years) with declining reproductive performance
- History of repeated bacterial isolations (Strep zoo, E. coli) that recur despite treatment
Mares fitting two or more of these criteria are strong candidates for evaluation with bActivate.
The Activate-First Solution
The challenge of dormant infection requires a diagnostic step that does not yet exist in the standard protocol: forced reactivation. bActivate is a bacterial growth medium that, when instilled into the uterus during early oestrus, stimulates dormant bacteria to become metabolically active within 48 hours. A culture swab taken at that point detects the true infection. Targeted antibiotic therapy applied to now-active bacteria is effective in a way that empirical treatment of dormant cells never can be.
The protocol takes place one oestrus cycle before the intended breeding cycle. Results from field studies confirm its effectiveness in problem mare populations:
- 83% pregnancy rate in 64 problem mares — Hagyard Equine Medical Institute, Lexington, Kentucky (Dr. Kristina Lu)
- 89% pregnancy rate in 19 barren thoroughbred problem mares — Kildangan Stud / Godolphin, Ireland
Watch: Identifying and Treating Dormant Streptococci
Dr. Morten R. Petersen covers the clinical aspects of dormant streptococcal infection in mares, including best practice for diagnosis, treatment efficiency and the role of bActivate in the protocol.
Clinical aspects — Dr. Morten R. Petersen (DVM, PhD, Dipl. ACT): identifying dormant streptococci, culture-negative swabs, and treatment efficiency.
For further information on when and how to use bActivate, visit When to Use bActivate or How to Use bActivate. To review the full clinical study data, see Clinical Evidence.
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.
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.”