Mare Absorbed at 30 Days: Signs, Causes, Next Steps

On day 15 after covering, the scan shows a vesicle. Two weeks later the vesicle is gone, the mare is back in season, and nobody noticed a thing. The mare has absorbed, and it is one of the most frustrating experiences in breeding because it happens in silence.

This post explains how you can tell that a mare has absorbed, which causes come into question, and why repeated losses should turn the search toward a cause the standard swab usually does not find.

Mare absorbed: what does it mean?

Breeders say the mare absorbed or reabsorbed; the veterinary term is early embryonic loss, or resorption. It means the pregnancy ends in the first weeks and the embryo is taken up by the mare's body instead of being expelled. There is no abortion, no bleeding, no visible tissue. The vesicle that was there on the scan is simply not there at the next appointment.

The line between resorption and abortion is a matter of timing. Up to about day 40 of pregnancy one speaks of an embryo and of resorption; after that, of a fetus and of abortion. From an abortion onwards there is usually something to see: fluid, fetal membranes, the fetus itself.

Signs: how do you know a mare has absorbed?

The honest picture is: usually by nothing at all. Resorption runs without fever, without discharge and without a change in behavior. It is almost always the veterinarian who finds it:

On ultrasound the vesicle seen earlier is no longer there, or it is smaller than it should be at that stage, irregular in shape, or without an embryo with a heartbeat.
The mare comes back into season although she was thought to be in foal. This is often the first sign breeders notice themselves.
Fluid in the uterus on the scan, before or after the loss. It is not proof of a resorption, but a sign that the uterus is not right.

Because resorption is so quiet, the check scans matter: the first around day 14 to 16 (also for twins), then a check around day 25 to 30 with a heartbeat, and another around day 40 to 45. If a mare drops out between two appointments, the vet at least knows in which window it happened.

Causes: why does a mare absorb?

A resorption rarely has a single cause, and not every one can be worked out afterwards. The usual suspects your vet goes through:

Age and the state of the uterus. With the years, the quality of the uterine lining declines, and fibrosis and cysts increase. A biopsy shows how well the lining can still carry a pregnancy; that is what the biopsy score, a IIA or a IIB, describes.
Twins. Two vesicles are the most common cause of an early loss that a timely scan can prevent.
Embryo quality. Part of the early losses come from the embryo itself, for instance in older mares or with older semen. That cannot be treated, only reduced by good timing of the cover.
Stress, transport, fever, medication in the sensitive window of the first weeks.
Inflammation or infection of the uterus (endometritis). This is the cause most likely to be missed, because the standard swab often does not find it. It is also the cause that can best be put right.

The overlooked cause: the infection the swab does not find

Many mares that absorb had a clean culture and cytology before covering. That is reassuring, but it says less than one thinks. A uterine swab only samples the surface of the lining. In a direct comparison, the standard swab found only 34 % of uterine infections, the biopsy culture 82 % (Nielsen, Theriogenology 2005).

The reason is where the bacteria sit. In chronically infected mares, Streptococcus zooepidemicus was found in foci 300 to 500 micrometers below the epithelium, not on the surface the swab reaches (Petersen et al., Clinical Theriogenology 2009). There the bacteria can go dormant, a state in which they do not grow in culture (Petersen et al., Veterinary Microbiology 2015). The mare is called clean, is covered, may even conceive, and the uterus is still not a place where an embryo can stay.

So the rule is: a mare that absorbs repeatedly although the swab was clean belongs to the problem mares where the search should go deeper. More on this in Why a clean swab does not rule out infection in a problem mare.

What to do when the mare has absorbed

After a first loss in a young mare with no history, waiting is reasonable; many mares conceive on the next cycle and carry to term. It is different for a mare that meets any of these points:

10 years or older
3 or more foals
Previous endometritis or abortion
Has absorbed before, or has failed to conceive more than once although the swab was clean
Antibiotics did not help
Fluid in the uterus on ultrasound

Then your vet, or a repro vet, should find out what is going on in the uterus before the next cover, with methods that reach deeper than the swab:

Low-volume lavage or biopsy instead of a swab. Both also pick up what sits below the surface. The biopsy additionally shows the state of fibrosis and inflammation.
Activation of dormant bacteria before the culture. The vet instills a diagnostic culture medium (bActivate) into the uterus, which wakes dormant streptococci. 48 hours later a second culture is taken. Now the bacteria grow, and the result is clear. bActivate is not a drug: it makes the infection visible, and treatment follows, targeted by sensitivity testing.
Treat, then cover. If the culture is positive, the mare is treated on the basis of the sensitivity test and covered on the following cycle. If it is negative, a dormant streptococcal infection is ruled out, and your vet can concentrate on the other causes. That too is a result.

How this can pay off is shown by a field study at the Hagyard Equine Medical Institute in Kentucky: 64 problem mares that had failed to conceive for at least three cycles were examined, treated and covered this way. 83 % became pregnant. The study had no placebo group, so the figure describes the whole sequence of diagnosis and veterinary treatment (Clinical Theriogenology 2014). For comparison: for problem mares of this kind, the historically expected foaling rate was 15 to 50 %, against 80 to 85 % in the general population (Bosh et al., Equine Vet J 2009).

Frequently asked questions

Can a mare get in foal again in the same year after absorbing?
Yes, often. After an early loss the mare usually comes back into season within a few weeks and can be covered again. After repeated losses the cause should be worked out first, otherwise the pattern repeats.

Does the mare notice anything herself?
As a rule, no. There is no pain and no visible sign. The first thing breeders notice is usually that the mare is back in season.

Is a clean swab enough before the next cover?
In a mare that has absorbed before, not necessarily. The standard swab finds only about one in three uterine infections (Nielsen 2005). If a dormant infection is suspected, a low-volume lavage, a biopsy or an activation before the culture are the more reliable routes.

Can I do the activation myself?
No. Activation, sampling and treatment belong in the hands of your vet. Our page How to use has the protocol, the evidence and the ordering details you can pass on.

Sources

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
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 the Hagyard Equine Medical Institute, 64 problem mares. Clinical Theriogenology 2014;6(3):313-314 (conference abstract, no placebo group). therio.org (PDF)
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

More on the bacterium: Streptococcus zooepidemicus in horses. All studies by level of evidence: Studies and effect. The condition itself: Equine endometritis.

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.