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Also Known As

Miscarriage, Still birth, Misbirth


One of the more frustrating aspects of horse breeding is the fact that so many properly fertilized eggs fail to result in a live foal 11 months later. All abortions have one element in common: a dead foal, either during or at the end of the pregnancy.

It is not uncommon for horses to experience spontaneous abortions. A variety of medical reasons can cause the loss of the fetus, many of which depend on the gestational stage of the horse. In mares, abortion is defined as the failure of the fetus before it reaches the 300-day gestation period. Any loss of a fetus after that is considered to be an early delivery of the foal or early parturition.

Most abortions fall into the following categories:

  • Bacterial
  • Fungal
  • Viral
  • Hormonal
  • Twinning
  • Stress-Induced


  • Coughing
  • Fever
  • Depression
  • Swelling in legs
  • Formation of milk in the mammary gland
  • Swelling of the mammary gland
  • Vaginal discharge and bleeding
  • Asymptomatic

In many cases, the abortion will appear to be spontaneous, with symptoms of the contributing cause not having time to develop fully. In each case, whether the abortion is bacterial, fungal, viral, hormonal, stressed-induced, or the result of twinning, the mare may have the common symptoms associated with the disease or condition, or abortion may be the only sign. Premature labor, discomfort, or unusual activity require immediate attention, and steps should be taken to safeguard the health of the mare and, if possible, the foal.


Causes of spontaneous abortion in horses include viral diseases, ingestion of poisonous plants, mycotoxins produced by fungi, bacterial infections, stress of either the mare or the fetus, gene mutations, Mare Reproductive Loss Syndrome (MRLS), and  lack of sufficient nutrients to support the fetus, especially in the case of twins.

Umbilical cord abnormalities also cause abortion. Those abnormal conditions of the cord which can produce potentially lethal problems in the fetus are usually associated with excessive length and include:

  • Strangulation of the amniotic portion of the cord around parts of the fetus which causes deep grooves and local edema in the fetus. i.e. head, neck, back, thorax or legs.
  • Excessive torsion of the amniotic part of the cord with urachal obstruction and vascular obstruction.

Bacterial Abortions

In the case of bacterial abortions, several species of bacteria have been recognized as agents of equine abortion. The most common cause of bacterial abortion includes organisms of the Streptococci group.

Other bacteria frequently identified include Leptospir, Nocardia, Staphylococcal, and Klebsiella species. These agents gain entrance to the reproductive tract and move into the uterus, causing infection of the membranes and leading to abortion of the fetus. Often, retention of the placenta is an indication of a bacterial abortion. The mare may also have an infection of the uterus known as endometritis and/or metritis.

Leptospira infection is caused by motile bacteria called "spirochetes" which are very common in domestic and wild animals and can also infect humans. They invade the mucous membranes or damaged skin and migrate to various body organs. The diagnosis of leptospirosis will often be one of exclusion.

Fungal Abortions

Fungal abortions are caused by fungi that produce mycotoxins in the class of chemicals called "ergot" or "ergopeptine alkaloids." These alkaloids have a toxic effect on the reproductive tract and mammary gland of the mare and result in a depression of serum prolactin and progestagens, a prolonged gestation, and a thickened edematous placenta. The normal rise of progestagens during the last 40 days of gestation is inhibited, and foals born without these normal increases in maternal progestagens are small, weak, or stillborn.

The endophytic fungus Neotyphodium coenophialum lives inside the fescue plant and produces alkaloids which, when ingested, can result in the disease condition "fescue toxicity." These chemicals may cause an extended gestation length from 11 to 12 months. In addition, the mare may experience dystocia, with the mare sometimes trying to foal for many hours.

The genera Claviceps is a saprophytic fungus that lives on a variety of hays and pasture grasses. The Claviceps sclerotia, also known as "fruiting bodies," contain a large array of ergopeptine alkaloids similar to those involved in fescue toxicity. Thickened edematous placentas and agalacta are signs of ergotism.

Viral Abortions

Both equine herpesvirus and herpesvirus EHV-4 have the potential to cause abortions. The herpes family of viruses has the capacity to persist in the body of its host in a dormant state after a primary infection has been cleared up. The latent herpesvirus may cause new outbreaks of disease in its host, especially in stress-induced circumstances, as well as in stablemates. A fetus may become infected during the early gestation period. Most abortions due to this virus will occur between five and 11 months of gestation. The infected foal may be born alive at term, but will die shortly after birth due to the infection by the virus.

Hormonal Abortions

Progesterone deficiency is the most common form of hormonal abortion. Abortions at the fourth or fifth month of pregnancy are often attributed to a lack of sufficient hormones to maintain pregnancy, although this conclusion is open to debate. Some veterinarians recommend treating mares with synthetic progesterone to help maintain pregnancy.

Stress-Induced Abortions

Stress-induced abortions fall into two categories: 1] conditions where the fetus is stressed because of direct contact with infectious agents or conditions that either affect development or have deadly effects on the fetus itself; and 2] situations or conditions where the mare is physically stressed to the point of aborting the fetus.

A mare may be stressed by severe illness or infection, internal parasites, vaccination, change of diet, long or uncomfortable transport, ingestion of Eastern tent caterpillars, and other conditions that result in a shock of some kind to the mare's system.

Twinning Abortions

With the advancement of ultrasound technology, it is common knowledge that many equine pregnancies start out with two ova. Often, one will disappear or be absorbed during the first 30 days of pregnancy. In other cases, one ovum will be pinched off in hopes that the other will remain viable. The mortality rate for twin pregnancies is very high, and the statistics on reaching a full-term delivery are noted to be one in 200,000 to 500,000, depending on the study.

Usually one of the twins will develop more slowly and be smaller. The smaller twin will eventually be crushed by the larger twin and die. The toxins that form in the dead fetus may kill the larger, healthy foal, and the mare will abort at about nine months.

If both fetuses survive, the birth process is often complicated, and one or both twins will involve malpresentation with resulting problems. If they survive, they often have the same issues as premature foals in that they have small frames, poor muscle development, and low birth weight, as well as other problems. These fragile foals will require extra medical and physical care and may never develop the stamina and healthy condition of single-birth foals.


Before considering breeding your mare, a veterinarian should evaluate your mare and provide information about her reproductive potential. During this breeding soundness examination the veterinarian will do a visual inspection of the vulva and vagina of the mare and a palpation of the cervix, uterus and ovaries.

In some cases a uterine biopsy may be called for and your veterinarian may suggest ultrasonography, cytogenetic evaluation, measurement of reproductive hormones or possibly an evaluation of the reproductive tract using a fiberoptic endoscope.

The evaluation process may determine that your mare is ready for breeding or it may detect problems that will limit fertility and, on occasion, the examination may reveal chronic or serious problems that would indicate it would not be wise to attempt to breed the mare both to prevent abortion and the health of the mare.

Before breeding, it is important to know everything possible about the stallion. Contagious Equine Metritis (CEM) also known as Taylorella equigenitalis and outbreaks of Equine Viral Arteritis (EVA) underscore the importance of doing diligent homework when choosing a stallion to prevent abortion in the mare.

If any questions exist as to the health and safety of the stallion, tests should be done to determine the state of the stallion's health to make sure that the health of the mare is not compromised as a result of breeding.

Prevention of situations, conditions, and illnesses that can result in abortion is paramount in producing a healthy foal. A healthy mare receiving ongoing health care, living under satisfactory conditions, and being carefully observed for any signs of distress has a very good chance of carrying a healthy foal to term.

A proper schedule of vaccinations to prevent illness, plenty of fresh air, good nutrition, and a clean, comfortable environment will contribute to healthy mares and foals.

Although only 40% of the equine abortion cases submitted to veterinary diagnostic laboratories are diagnosed as to the exact cause of abortion, to prevent reoccurrence of abortion in a mare, the cause should be determined in consultation with a veterinarian, if possible.


When an infectious agent is suspected, the help of the veterinarian to diagnose and promptly treat the condition is of paramount importance in preventing an abortion.

Once a mare has aborted the fetus, treatment is often needed to assure the health of the horse before she can be rebred successfully. It is a good practice to swab mares to determine if harmful bacteria are present in the uterus. A veterinarian should be called on to do any necessary tests, and his advice should be followed to make sure the mare does not have an on-going condition that might render her sterile or have any negative effects on her health and her ability to carry a foal to term in the future.

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