Your Mare Is Activation-Positive: What Happens Next

A positive post-activation culture means that dormant bacteria, almost always Streptococcus zooepidemicus, were sitting in the uterus and have now been brought back into a growing state that a normal culture can find. For a mare that has been empty for seasons with clean swabs, this is the first time the cause has a name. This post explains what the result means, what the veterinarian does next, and when the mare is bred again.

What does a positive activation result mean?

The activation protocol has two cultures: one before bActivate and one 48 hours after. A mare is activation-positive when the second culture grows what the first did not. In the placebo-controlled study behind the method, that was the case in 64% of mares given bActivate against 8% given saline (Petersen et al., Veterinary Microbiology 2015). In the field series, 47% of the Hagyard problem mares (30 of 64) and 84% of the Kildangan mares (16 of 19) were activation-positive. The infection was there all along; the swab could not see it because the bacteria were dormant and sat 300 to 500 micrometers deep in the tissue (Petersen et al., Clinical Theriogenology 2009).

What does the veterinarian do with the result?

1. Reads the antibiogram. The laboratory tests the cultured organism against antibiotics. For beta-hemolytic streptococci, penicillin was effective in 99.5% of 28,887 German samples (Köhne et al., 2024), so the drug is usually a standard one.
2. Treats in the same estrus. Because the bacteria are now active, they are also reachable by the antibiotic. Treatment typically combines intrauterine and systemic antibiotics, often with a lavage first; the exact regime is the veterinarian's decision.
3. Does not breed in that cycle. The mare is bred in the following cycle, after treatment is complete.

When is the mare bred again?

In the next cycle. At Kildangan Stud, 19 problem mares with an average of 6.3 barren cycles were activated, treated and bred; 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). At Hagyard, 83% of 64 problem mares became pregnant (53 of 64) and 70% of the mares with a known outcome foaled (32 of 46); no placebo group was included (Petersen and Bojesen, Clinical Theriogenology 2014). For problem mares in general, published per-season pregnancy rates are 15 to 50%, against 80 to 85% in the general broodmare population (Bosh et al., 2009).

What about a negative activation result?

A negative second culture is also an answer. A dormant streptococcal infection is then unlikely, and the search continues elsewhere: timing, semen, anatomy, hormones. The step has ruled out the most common hidden cause without losing a cycle to guesswork.

Will she stay clear?

Treated after a positive culture, most mares in the field series went on to conceive in the following cycle. Whether a mare stays clear in later seasons depends on the same things as in any mare: cervical function, uterine clearance after covering, age. The when to use page lists the signs that justify checking again, and the how to use page has the full protocol for the veterinarian.

Frequently asked questions

Is a positive activation culture bad news?
It is an explanation. A mare that has been empty for seasons now has a named cause, an antibiogram, and a plan for the next cycle.

Can she be bred in the same cycle?
No. Activation, culture and treatment take one cycle; breeding is planned for the following one. At Kildangan the bred mares needed 1.1 covers on average.

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

Does bActivate treat the infection?
No. It makes the dormant bacteria grow so a normal culture can find them. The treatment is the veterinarian's, chosen from the antibiogram.

Sources

• Petersen MR, Skive B, Christoffersen M, Lu K, Nielsen JM, Troedsson MHT, Bojesen AM. Activation of persistent Streptococcus equi subspecies zooepidemicus in mares with subclinical endometritis. Veterinary Microbiology 2015;179:119-125. DOI 10.1016/j.vetmic.2015.06.006
• Petersen MR, Nielsen JM, Lehn-Jensen H, Bojesen AM. Streptococcus equi subspecies zooepidemicus resides deep in the chronically infected endometrium of mares. Clinical Theriogenology 2009;1:393-409
• Köhne M, Hegger A, Tönissen A, Heusinger A, Hader C, Görgens A, Sieme H. Frequency of potentially pathogenic bacterial and fungal isolates among 28,887 endometrial samples from mares. 2024.
• Petersen MR, Rosenbrock A, Osborne M, Bojesen AM. High prevalence of subclinical endometritis in problem mares: effect of activation and treatment on fertility. J Equine Vet Sci 2018;66:117
• Petersen MR, Bojesen AM. Equine subclinical endometritis caused by dormant beta-hemolytic streptococci. Clinical Theriogenology 2014;6(3):313-314 (congress abstract, 64 problem mares at Hagyard Equine Medical Institute, no placebo group).
• Bosh KA, Powell D, Shelton B, Zent WW. Reproductive performance measures among Thoroughbred mares in central Kentucky, during the 2004 mating season. Equine Veterinary Journal 2009;41:883-888. PMID 20383985

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.