Abortion is the premature termination of pregnancy. There can be many causes of abortion, from infection to noninfectious causes such as exposure to toxins in the environment or genetics. It may be difficult to pinpoint the cause, as the abortion often occurs weeks or months after an infection or exposure to toxin, so the cause is no longer apparent. In order to diagnose the cause of an abortion, it may be necessary for your veterinarian to collect samples from the placenta or fetus for testing in a diagnostic laboratory.

Noninfectious Causes

The most common noninfectious cause of abortion in horses is a twin pregnancy. Most abortions related to twinning occur at 8 to 9 months of pregnancy (normal pregnancy length is about 11 months in horses) and may be preceded by premature lactation. Abnormalities of the umbilical cord, which connects the developing fetus to the placenta, (such as twisting) are another cause of abortion in horses.

Mare reproductive loss syndrome was first identified in 2001. There was a large increase in abortions affecting all breeds of horses in central Kentucky. Foals aborted in late gestation were dead or, if alive, were weak and dehydrated. Foals in the first trimester of gestation usually died in the uterus before being aborted. An infectious cause (such as a bacteria or virus) is not thought to be responsible. The outbreak occurred after a colder than normal March, followed by above-normal temperatures in April. During the third week of April, there was a frost followed by warm weather. Abortions increased after a similar weather pattern in 1980 and 1981, but not to the extent seen in 2001. The warm weather in early April resulted in rapid plant growth and unusually high numbers of eastern tent caterpillars. An absence of eastern tent caterpillars and feeding hay to the mares when they were on the pasture were associated with few or no abortions. In experiments to test whether the caterpillars were the cause, pregnant mares fed crushed caterpillars mixed in water aborted. Current recommendations for control of mare reproductive loss syndrome consist of controlling tent caterpillars, removing wild cherry trees (the principle food source for the caterpillars), frequently mowing pastures used by pregnant mares, and feeding hay to the mares when on pasture. Secondary measures include increasing the ratio of grass to clover in pastures and reducing the time mares spend on pasture when a hard freeze following a warm spell is expected.

Fescue grass poisoning occurs when horses graze on fescue grass that is infected with a fungus called Acremonium. The fungus lives in plant cells and produces a toxic chemical that causes prolonged pregnancy in mares and stillborn or weak foals. The placenta is thickened and does not separate normally during birth. The foal becomes trapped in fetal membranes during birth and dies because it cannot breathe. The source of infected fescue can be pasture, hay, or bedding. The best practice to prevent abortion of this type is the removal of mares from the fescue pasture or removal of infected fescue and replanting another grass.

Infectious Causes

Infectious causes of abortion include viral, bacterial, and fungal infections.

Equine rhinopneumonitis is caused by equine herpesvirus 1 and is the most important viral cause of abortion in horses. Abortion usually occurs after 7 months of pregnancy and there is no corresponding maternal illness. The disease is diagnosed by a blood test or isolation of the virus from fetal tissues. Prevention is based on vaccinating at 5, 7, and 9 months of pregnancy as well as preventing exposure of pregnant mares to horses that attend shows or other equine events where they may have been exposed to the disease.

Equine viral arteritis (see Disorders Affecting Multiple Body Systems of Horses: Equine Viral Arteritis) may also cause abortion in horses 6 to 29 days after signs of infection appear. Signs of the disease include fever, swelling of the limbs, poor appetite, nasal discharge, and swelling of mammary glands. Stallions can be infected with the virus and carry the disease. Equine viral arteritis can be spread sexually or through the air. Infected horses usually recover without treatment. Prevention is based on vaccinating both the mare and the stallion.

Potomac horse fever, caused by Ehrlichia risticii bacteria, can be followed by abortion in mid to late pregnancy. Inflammation of the placenta and retained placentas can occur. Aborted fetuses have been found to have the bacteria in them. It is not known how effective the vaccine for Potomac horse fever is in preventing abortions (see Digestive Disorders of Horses: Potomac Horse Fever).

Leptospirosis has recently been identified as an occasional cause of abortion in horses in Kentucky, England, and Northern Ireland. Most fetuses were aborted at 6 months of pregnancy. The mares were otherwise healthy. Usually only a single horse on a farm will abort in this manner—it does not seem to spread from horse to horse.

Abortion can also be caused by other bacteria, such as Streptococcus zooepidemicus, other Streptococcus species, Salmonella, Escherichia coli, Pseudomonas, and Klebsiella. These abortions are usually caused by the bacteria entering through the cervix and causing inflammation of the placenta (placentitis).

Infection by several types of fungi can also lead to abortion in horses. These include Aspergillus, Mucor, and Candida species. Fetuses aborted late in pregnancy may show signs of delayed growth. Fungi may be found in the placenta, or the lungs, liver, or stomach of the fetus.

Last full review/revision July 2011 by Robert O. Gilbert, BVSc, MMedVet, DACT, MRCVS; Fabio Del Piero, DVM, DACVP, PhD; R. J. Erskine, DVM, PhD; Paul Nicoletti, DVM, MS; Jerome C. Nietfield, DVM, PhD, DACVP; Donald Peter, DVM, MS, DACT; Patricia L. Sertich, MS, VMD, DACT; Katrin Hinrichs, DVM, PhD, DACT; Brad E. Seguin, DVM, MS, PhD DACT