Mare Not Getting in Foal: What to Do, and in What Order

A mare that does not get in foal is not a rare case, but an expensive one: every cycle costs time, semen, vet visits and a slice of the season. Among problem mares, the foaling rate has historically been 15 to 50 %, against 80 to 85 % in the general population (Bosh et al., Equine Veterinary Journal 2009). The good news is that most causes can be worked out. One of the most common is overlooked only because the usual sample does not reach it.

This article goes through the causes in the order a veterinarian rules them out, and says at each step what you can do yourself.

Why is my mare not getting in foal?

In most cases for one of five reasons: the timing of covering and ovulation was off or the semen was weak, a twin pregnancy was lost early, the uterine lining has aged, the embryo was resorbed early, or there is an inflammation of the uterus (endometritis), which often runs without symptoms. In mares that repeatedly stay empty despite a clean swab, hidden endometritis is the cause to check first.

Step 1: Was the timing right?

The most banal cause is also the most common in young mares: covering or insemination was too far from ovulation. Fresh semen survives two to three days in the uterus, frozen semen only a few hours. Without follicle checks by ultrasound, hitting ovulation is luck.

What you can do: have the vet follow the cycle (follicle size, induced ovulation) and ask the stallion owner for the semen analysis of the dose your mare received. Two failed cycles with good timing and good semen are the point at which to look deeper.

Step 2: Was she in foal and lost it?

Not every empty mare was never pregnant. Some lose the pregnancy in the first weeks without anyone noticing: no bleeding, no discharge, the mare simply comes back into season. Two causes are common here.

Twins: two vesicles are the most common reason for early loss, and one that a timely scan on day 14 to 16 can prevent. The vet can then remove one of the two.

Resorption: if a single vesicle disappears between two scans, that is resorption. In a young mare with no history, one wait-and-see is reasonable. With repeated resorption she belongs to the problem mares for whom step 4 is mandatory. More on what the scan shows in Ultrasound of the empty mare: what the vet sees.

What you can do: insist on a scan on day 14 to 16, around day 25 to 30 (heartbeat) and around day 40 to 45.

Step 3: How old is the uterus?

With the years and with every pregnancy the uterine lining changes: more connective tissue, more cysts, poorer blood supply. A mare of 10 years or more, or with three or more foals, therefore carries a higher risk. That is not a verdict, it is a reason to look more closely.

How well the lining can still carry a pregnancy is shown by a biopsy. It is also the sample that reaches much further than the swab when looking for an infection, more on that next.

Step 4: Is there endometritis even though the swab was clean?

Here lies the cause that is overlooked most often, and for a simple reason: the standard swab samples only the surface of the lining. In a comparison study, swab culture found 34 % of uterine infections and biopsy culture 82 % (Nielsen, Theriogenology 2005). Two out of three infections stay undetected with the swab alone.

The reason is where the bacteria sit. In chronically infected mares, Streptococcus zooepidemicus sits in clusters 300 to 500 micrometers below the epithelium, not on the surface the swab reaches (Petersen, Nielsen, Lehn-Jensen, Bojesen, Clinical Theriogenology 2009). There the bacteria can switch into a dormant state in which they do not grow in culture (Petersen et al., Veterinary Microbiology 2015). The mare counts as clean, is covered, and the uterus is still no place for an embryo to stay.

How common that is, the largest German analysis shows: in 28,887 endometrial samples, beta-hemolytic streptococci accounted for 79.7 % of positive cultures, and 99.5 % of them were sensitive to penicillin (Köhne et al., Journal of Equine Veterinary Science 2024). The infection is usually very treatable. It just has to be found.

Signs that point to hidden endometritis:

• The mare repeatedly fails to conceive although swab and culture were clean
• Fluid in the uterus on the ultrasound
• Previous endometritis or abortion
• 10 years or older, three or more foals
• Antibiotics did not help, or only until the next season

What the swab shows and what it misses is covered in Why a clean swab does not rule out infection in a problem mare. On the organism itself: Streptococcus zooepidemicus in horses. The whole picture, from forms to treatment, is on Equine endometritis: causes, diagnosis and what the vet does next.

Step 5: Wake the dormant bacteria, then culture

If dormant streptococci are suspected, there are three ways to look deeper: a low-volume lavage, a biopsy, or activation of the bacteria before culture. In the third, the vet instills a diagnostic culture medium (bActivate) into the uterus. It wakes the dormant bacteria, and 48 hours later a second culture is taken. Now the bacteria grow, and the result is clear. In the study behind the method, Strep zoo was detectable after activation in 64 % of mares and without activation in 8 % (Petersen et al. 2015).

bActivate is not a medicine and not a treatment. It makes the infection visible. Treatment follows, targeted, with antibiotics chosen from the sensitivity test, and the mare is covered in the next cycle.

What can come of it, a field study at Hagyard Equine Medical Institute in Kentucky shows: 64 problem mares that had stayed empty for at least three cycles were examined, treated and covered this way. 83 % got in foal, and 70 % of the mares with a known outcome produced a live foal (32 of 46). The study had no placebo group, so the figures describe the whole pathway of diagnosis and veterinary treatment (Clinical Theriogenology 2014). At Kildangan Stud in Ireland the pregnancy rate of bred mares was 89 % (Journal of Equine Veterinary Science 2018).

Protocol, evidence and where to order are on the page How to use bActivate. You can simply pass the link to your vet.

What you can do yourself

1. Write down the history: every cycle, every cover, every swab result, every treatment, every scan. Your vet can do more with that list in ten minutes than with three phone calls.
2. Insist on a scan on day 14 to 16 after every cover.
3. After two failed cycles, or at once with any of the signs above, ask whether lavage, biopsy or activation before culture makes sense.
4. Do not repeat the same cycle with the same plan. Every failed cycle that shows nothing new is a lost month.

Frequently asked questions

How many times may a mare stay empty before looking deeper?
After two failed cycles with good timing and good semen. With any sign of hidden endometritis (fluid on the scan, previous inflammation, repeated resorption, 10 years or older), at once.

Is a clean swab enough to rule out endometritis?
No. In the comparison study, swab culture found 34 % of infections and biopsy culture 82 % (Nielsen 2005). Dormant streptococci deep in the lining do not grow in the standard culture.

Can an older mare still get in foal?
Often yes. Age is a risk factor, not a verdict. What matters is the state of the uterine lining, which a biopsy shows, and whether a hidden infection is found and treated.

Is bActivate a treatment?
No. bActivate is a diagnostic culture medium. It wakes dormant bacteria so they grow in a normal culture. Treatment follows with antibiotics chosen from the sensitivity test.

What does the work-up cost?
That depends on the vet and the route. For an activation, count on two visits 48 hours apart plus laboratory costs for two cultures. The price of bActivate itself is on the shop page; your vet orders it through the veterinary wholesaler.

Sources

• 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(9):883-888. doi.org/10.2746/042516409X456068
• Nielsen JM. Endometritis in the mare: A diagnostic study comparing cultures from swab and biopsy. Theriogenology 2005;64(3):510-518. doi.org/10.1016/j.theriogenology.2005.05.034
• 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(1):393-409. clinicaltheriogenology.net/index.php/CT/article/view/12588
• 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(1-2):119-125. doi.org/10.1016/j.vetmic.2015.06.006, full text on ResearchGate
• Petersen MR, Bojesen AM. Field study at Hagyard Equine Medical Institute, Kentucky: 64 problem mares. Clinical Theriogenology 2014;6(3):313-314 (congress abstract, no placebo group).
• Petersen MR, Rosenbrock A, Osborne M, Bojesen AM. High Prevalence of Subclinical Endometritis in Problem Mares, Effect of Activation and Treatment on Fertility. Journal of Equine Veterinary Science 2018;66:117 (congress abstract). doi.org/10.1016/j.jevs.2018.05.162
• Köhne M et al. Frequency of potentially pathogenic bacterial and fungal isolates among 28,887 endometrial samples from mares. Journal of Equine Veterinary Science 2024;133:105008. doi.org/10.1016/j.jevs.2024.105008

All studies by level of evidence: Studies and effect.

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.