Pregnancy Loss in Mares

Most losses occur early in pregnancy and 15-20% of mares that conceive will lose the embryo before day 50. If the embryonic loss is detected, the mare can be served again later in the breeding season and this is an important reason for repeating pregnancy tests. An ultrasound examination can detect mares that are in danger of early embryonic loss, enabling treatment or further monitoring. Some mares, particularly older mares and mares with uterine inflammation, have an increased risk of early embryonic loss. Stress and disease also may increase the rate of early embryonic loss.

Abortion is the abnormal expulsion of a fetus any time from the first month to full term. Up to 30% of mares that conceive will lose the fetus before foaling. Most of these losses occur in the first 35 days of pregnancy and the embryo is resorbed, after which the mare may come back into heat at a longer interval after the last estrus. There are procedures that should be followed when a mare aborts. All other mares in the paddock or yard where the abortion occurred should be moved to an isolated, empty paddock. The fetus and the membranes should be collected in a strong plastic bag and taken to a veterinary laboratory for examination. If that is not possible, your vet should be called to do a postmortem of the fetus and take tissue samples as well as a cervical swab from the mare for laboratory examination. To minimize the spread of contagious abortion, the affected mare should be kept isolated until the results of the laboratory test are known, generally in two weeks. The area should be disinfected. Any people who handled the mare or the aborted fetus should thoroughly wash their hands, disinfect their boots, and change their clothes before handling other horses, especially pregnant mares.

Some mares will show signs of impending abortion, such as bagging up or having a vulval discharge. Other mares will abort without warning. It may be possible to delay an imminent abortion, so consult your veterinarian.

Equine herpesvirus-1 (EHV-1) infection may cause abortion, stillbirths, or death of a foal within three days of birth. Viral abortion occurs in late pregnancy. The mare will show either mild symptoms or none at all. All horses are susceptible to EHV-1 and the disease is spread by coughing, sneezing, or coming in contact with nasal discharge in the environment. Mares that recently aborted are highly contagious.

Weanlings and yearlings are very susceptible to EHV-1 and could become future carriers, so they should be kept away from pregnant mares. Some mares may also become carriers. To prevent outbreaks of viral abortion, pregnant mares should be run in small groups to minimize cross-infection. Reduce the movement of mares between properties. Isolate new mares for 7-14 days after arrival at a new property. Mares on properties known to be infected should be left there or if moved, they should be isolated on arrival at the next property until they have foaled.

In horse breeding, twins are not good news; 2% of abortions in Australian Thoroughbreds are caused by twinning and up to 10% of conceptions may be twins. Usually, one fetus develops more rapidly, progressively assuming the major portion of the maternal blood supply and causing the other fetus to die from lack of blood. The death of one fetus usually results in the abortion of both. Abortion due to twinning is most common between the fifth and ninth months. Live twins are born occasionally, but most are premature and are not likely to survive. Mares with a history of producing twins should have an ultrasound pregnancy test between 12 and 25 days after the last service, preferably between 14 and 16 days, and then one embryo can be destroyed so that the other can continue to develop normally. Sometimes one twin will fail to develop and be resorbed without any veterinary intervention.

Mares that have a history of aborting unrelated to infection or other known causes are thought to have a hormonal imbalance. Progesterone, oral or injected, may help the mare maintain the pregnancy to term. Orally administered progesterone is more effective than injections, but must be given daily to at-risk mares.

The stress of long-distance transport, feed shortages, or illness in late pregnancy may predispose the mare to abortion.