There are numerous different factors that can lead mares to abort fetuses at any time during the pregnancy. In most cases fetuses are aborted in the first part of the pregnancy due to a variety genetic and health factors that would prevent the foal from developing normally. Some abortions are also caused by injury to the mare and these can occur regardless of the health of either the mare or the breeding stallion.
Equine herpes virus or EHV-1 is the viral infection most often associated with abortion in horses. This condition is particularly problematic in herds and stables as the mares and stallions can live as carriers showing no outwards signs of the disease for months and even years before the symptoms develop. During this time they may be exposing other horses to the virus, resulting in abortion problems in herds of mares or even in stallions that are pastured or have access to mares that are carrying the virus. Generally abortions caused by viral infections of equine herpes virus will occur late term, usually between 8 to 11 months. Foals may also be live born but die shortly after birth due to respiratory complications caused by the virus.
Infections of Staphylococci and Streptococci groups of bacteria can also be trouble by infecting the fetal membranes causing an abortion. These bacteria may be present in the mare prior to conception or may gain access to the reproductive tract during or after conception. Often mares that abort due to a bacterial infection will retain the placenta, resulting in a much greater risk of serious infections including endometriosis as a further complication. Completely treating the bacterial infection is a must before breeding the mare again. Testing all mares for bacterial infections is a simple procedure that many breeders routinely perform before planning to use the mare in a breeding program. Leptospirosis, the same type of infection that causes moon blindness, can also be a cause of abortion in horses. Since this condition can be transferred between species through indirect contact in the environment this condition can be hard to manage if it is problematic in your location.
Twinning may result in abortion in mares as the reproductive system simply cannot support the development of two fetuses in the same uterus. Other causes of abortion that are not related to infections or viruses include a progesterone deficiency, which can be managed by progesterone therapy as well as torsion or twisting of the umbilical cord which cuts off blood flow to the fetus. This is most commonly noted in late term abortions and is relatively easy for a vet to diagnose. This is not a preventable situation but some mares seem more prone to the condition than others.
There are two different types of ergots or fungal components to feed that can cause abortion in mares. These ergots cause alkaloids to build up in the mare's body, resulting in toxicity. Often foals are stillborn or aborted very late in the pregnancy. Those that survive may have poor development, be more susceptible to respiratory infections that cause death and be generally weak and fail to thrive.
Any abortion, even those caused by a known injury to the mare, should be thoroughly checked by a vet within 24 hours. This can limit any possible problems with retained placentas, infections or other complications that may possibly pose a life threatening condition in the mare if left untreated.