The Problem Mare: Why She Stays Empty and What to Check First

A problem mare is a mare that keeps failing to conceive despite normal cycles, a fertile stallion and clean routine swabs. In the published activation series, 47 to 87% of these mares carried a dormant Streptococcus zooepidemicus infection that the standard swab had missed (Hagyard 2014, Kildangan 2018, Copenhagen 2025). This post explains why the swab misses it, which signs point to it, and what your veterinarian can do before another season is lost.

What makes a mare a problem mare?

The working definition is a mare that fails to conceive over three or more consecutive estrus cycles despite breeding to a stallion of proven fertility. She often looks healthy. She cycles, she accepts the stallion, the ultrasound may show a little fluid after breeding, and the swab comes back negative. Published per-season pregnancy rates for this group are 15 to 50%, against 80 to 85% in the general broodmare population (Bosh et al., Equine Vet J 2009).

Why does a clean swab not rule out infection?

A standard uterine swab culture finds only about 34% of infected mares, while a biopsy culture finds about 82% (Nielsen, Theriogenology 2005). The reason is physical. In chronically infected mares, Streptococcus zooepidemicus sits in distinct foci 300 to 500 micrometers beneath the surface of the endometrium, where a surface swab never reaches (Petersen et al., Clinical Theriogenology 2009). The bacteria are also metabolically inactive, so even material that reaches the plate does not grow. A negative culture in a problem mare therefore means "nothing grew", not "nothing is there".

The three sampling options side by side, with their detection rates: swab culture vs biopsy vs bActivate.

How common is uterine infection in mares?

The largest recent dataset is German: 28,887 endometrial swab samples, of which 25.9% grew potentially pathogenic bacteria, and beta-hemolytic streptococci made up 79.7% of the positive cultures (Köhne et al., 2024). These are sample counts, not individual mares, and they include only bacteria that grew on the plate. The dormant fraction is not in that number at all.

Which signs point to a dormant infection?

Ask your veterinarian about activation before the next culture if your mare shows two or more of these:

• failure to conceive over two or more consecutive cycles with a fertile stallion
• a negative routine swab that does not explain the fertility failure
• uterine fluid on ultrasound around estrus or after breeding
• early embryo loss, typically between day 14 and day 25
• a previous episode of endometritis, treated or untreated
• antibiotic courses that helped for one cycle and then did not
• age 10 years or older, or three or more previous foals

None of these proves an infection. Together they describe the profile in which the activation series found the dormant bacteria most often.

What does activation change?

bActivate is a diagnostic growth medium, not a treatment. The veterinarian takes a culture, instills 10 ml of bActivate into the uterus in early estrus, and cultures again after 48 hours. The medium brings the dormant streptococci back into an active, growing state, so the second culture can identify them and an antibiogram can guide the treatment. The treatment itself is the veterinarian's, chosen from the culture result. The mare is not bred in that cycle but in the following one. In the placebo-controlled study behind the method, activation revealed Streptococcus zooepidemicus in 64% of treated mares against 8% of saline controls (Petersen et al., Veterinary Microbiology 2015). The full protocol is on the how to use page.

What did the field studies find?

At Hagyard Equine Medical Institute in Lexington, Kentucky, Dr. Kristina Lu's team applied the protocol to 64 Thoroughbred mares that had been barren for three or more seasons. 47% were activation-positive (30 of 64), 83% became pregnant (53 of 64) and 70% of the mares with a known outcome foaled (32 of 46). No placebo group was included, so the authors do not claim that fertility was significantly increased; the historic foaling rate in this group of mares is 15 to 50% (Petersen and Bojesen, Clinical Theriogenology 2014).

At Kildangan Stud in Ireland, operated by Godolphin, 19 problem mares with an average age of 12.7 years and an average of 6.3 barren cycles were activated. 84% were activation-positive (16 of 19), and 89% of the bred mares became pregnant (16 of 18) at an average of 1.1 covers per mare (Petersen, Rosenbrock, Osborne and Bojesen, J Equine Vet Sci 2018;66:117).

The founders of bActivate co-authored both reports and disclose their commercial interest. All results are on the clinical evidence page.

When should I talk to my veterinarian?

After the second failed cycle in a mare with no other explanation, and before the season starts for a mare that was empty last year. The whole protocol fits inside one estrus cycle, so testing first replaces a repeat of last season with a diagnosis. The when to use page lists the indications in full.

Frequently asked questions

Is bActivate a treatment?
No. It reveals a dormant infection so it can be cultured, identified and treated by your veterinarian. bActivate contains no antibiotics, no hormones and no live bacteria.

Can a mare with a positive activation culture still be bred that season?
Yes. Activation and treatment take one cycle; breeding is planned for the following cycle. At Kildangan the bred mares needed 1.1 covers on average.

What if the post-activation culture is negative too?
Then a dormant streptococcal infection is unlikely to be the cause, and your veterinarian can look elsewhere with that possibility ruled out rather than assumed.

Sources

• Nielsen JM. Theriogenology 2005;64:510-518. PMID 15978661.
• Petersen MR et al. Clinical Theriogenology 2009;1:393-409.
• Petersen MR, Bojesen AM. Clinical Theriogenology 2014;6(3):313-314.
• Petersen MR et al. Veterinary Microbiology 2015;179:119-125. DOI 10.1016/j.vetmic.2015.06.006.
• Petersen MR, Rosenbrock A, Osborne M, Bojesen AM. J Equine Vet Sci 2018;66:117. DOI 10.1016/j.jevs.2018.05.162.
• Köhne M et al. 2024, 28,887 endometrial samples.
• Bosh KA et al. Equine Vet J 2009;41:883-888.

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.