Sometimes, in spite of all the care given to a pregnant mare, an abortion occurs. Many abortions are naturally occurring, and some are caused by infectious agents. A few of these causes are discussed here.
The most common cause of naturally occurring abortions is twinning. These usually occur at 8-9 months of gestation and may be preceded by premature lactation. The abortion of twins is caused by placental insufficiency. Another natural cause can be an abnormality of the umbilical cord. For example unusual length can cause torsion on the umbilical cord and can cause the mare to abort.
There is a syndrome call the Mare Reproductive Loss Syndrome (MRLS), which has occurred in several states, including Kentucky, New York, Florida, and in Australia. This is usually seen as an explosion of early and late term abortions, stillbirths, and weak foals born that die in a few days. While this does not occur every year, and the exact causes are unknown, it is interesting that pasture exposure to the eastern tent caterpillar is considered an important risk factor. It is thought that there is an unknown toxin in the exoskeleton that may be a causal factor, as early and late term abortions have been caused by ingestion of whole caterpillars or the exoskeleton only. The horses in the Australian outbreak of MRLS were exposed to processionary caterpillars. To help prevent MRLS, pastures should be managed to prevent exposure of pregnant mares to eastern tent caterpillars.
Fescue toxicosis is known to cause reproductive problems in some mares, including prolonged gestation, agalactia, edema, premature separation of the placenta, and perinatal death. Abortion may happen in the last two months of pregnancy due to severe edema and premature placental separation.
The most common causes of abortion in mares are infectious. Of these, the most prevalent viral agent causing abortions is Equine Rhinopneumonitis, or Equine Herpesvisus 1 (EHV-1). This is spread mainly through direct contact with nasal secretions, reproductive tract discharge, or the aborted fetus. Mares do not exhibit any illness, but will abort after about seven months of gestation. Prevention measures include preventing exposure of pregnant mares to horses attending horse shows, trail rides or other equine events, and by vaccinating pregnant mares at 5, 7, and 9 months of gestation.
Another viral cause of equine abortion is Equine Viral Arteritis (EVA). Abortion may follow clinical cases by 6 – 29 days. Stallions can be persistently infected and can spread the disease venereally – including natural service or artificial insemination with semen shipped cool or frozen. Diagnosis is made by checking the history of EVA, by isolating the virus, and by seroconversion of the mare. Prevention can be achieved by managing the breeding population to minimize viral transmission, and preventing stallions from becoming carriers of the virus.
Bacterial placentitis is the most commonly occurring form of abortion. There are several bacteria species that can cause this, including Streptococcus equi, E. Coli, Pseudomonus aeruginosa, Enterobacter ssp, and Klebsiella pneumoniae. Other bacteria species that can cause placentitis are Staph aureus, Strep equisimilis, Enterobacter agglomerans, and Actinobacillus spp.
Leptospira placentitis can cause abortion at 6 to 9 month of gestation, and is thought to be the cause of up to 13% of bacterial abortion in endemic regions. Several species of wildlife are known to harbor some leptospira species, including skunks, raccoons, and red foxes. There is not a vaccine labeled for horses, though some veterinarians have used vaccines labeled for cattle on horse farms with severe problems with Lepto. Please not the key words “not labeled for horses” and “veterinarians”. Vaccines formulated for one species should not be used on another species without veterinary oversight.
A couple of other causes of bacterial placentitis include Nocardioform placentitis, thought to be caused by various groups of gram positive, filamentous, branching bacteria, and Potomac Horse Fever (PHF). Abortions from PHF occur mid- to late gestation, and often have retained placentas. While there is a vaccine for PHF, the effectiveness for preventing abortion is unknown.
In spite of all these potential causes of equine abortion, most foals are carried full term and have a normal birth. Using good management practices such as keeping stalls and paddocks clean, minimizing horse-to-horse contact during pregnancy, having a good client-veterinarian-patient relationship and a good health program, and a good nutrition program to keep horses healthy and well-nourished all combine to make sure that potential problems such as equine abortion are kept to a minimum.
For more information on equine abortion, the Merck Veterinary Manual is a great resource. It contains more detailed information for those wanting to research this topic further.