Equine abortion is a devastating and very costly event in the horse industry with loses totaling in the millions annually. As an owner or manager of pregnant mares, it is important to be aware of the issues that can develop during the long gestation period as well as knowledgeable about what options are available for prevention and treatment. There are many causes of equine abortion which are split up into two categories; infectious and non-infectious causes.
The most common infectious causes of equine abortion are bacterial infections, Equine Herpes Virus-1, and Equine Viral Arthritis. It is important to know how to detect and prevent these infections before they cause any serious issues.
Bacterial causes of abortion are brought on by an infection. Broodmares should have uterine cultures performed prior to breeding to ensure there no infections in the uterus. Uterine infections can usually be treated with the appropriate antibiotics. Poor breeding conformation can cause infections, in this situation a surgical procedure called a caslicks that sutures part of the vulva together can be an option to prevent infection. Sterile breeding techniques and equipment are essential when breeding mares.
The leading cause of pre-term births as well as the cause of one-third of all late term abortions in mares is placentitis. Placentitis is an inflammation of the placenta, which is typically caused by an ascending bacterial infection or a residual uterine infection. The signs of placentitis are premature udder development, vaginal discharge, and thickening of the placenta. Udder development is considered within the normal range anywhere from 2-6 weeks prior to foaling, keep in mind each mare will be different. Tran-abdominal ultrasonography is used to measure the thickness of the placenta and with the other symptoms is used to diagnose placentitis. The main goal when treating mares with placentitis is to extend gestation length and prevent pre-term labor. The average gestation length of a mare is 340 days and pre-term labor is considered to be anything prior to 320 days. The treatment plan includes daily use of broad spectrum antibiotics to treat infection, anti-inflammatory drugs to reduce inflammation and prevent contractions, and progesterone supplementation to maintain the pregnancy.
Equine Herpes Virus (EHV)
EHV has two strains, EHV-1 and EHV-4. The most dangerous strain, EHV-1 can cause abortion, respiratory disease, and in some cases, neurological disease. EHV-4 is known as the non-abortive form, but can cause respiratory disease.
The EHV virus’ main mode of transport is through inhalation, but can also be transferred between infected horses through infected tissue, fetuses, placenta, and fluids. Mares infected with EHV-1 usually give no warning signs prior to abortion which usually takes place in the eighth or ninth month of their gestation. Diagnostic testing of the fetus is required to determine if EHV-1 was the cause of the abortion.
It is important to remember that any mare having a pre-term abortion should be quarantined and sanitation is necessary to prevent further spread of any possible disease. Vaccination serves as the best prevention method for the herpes virus. All pregnant mares should be vaccinated for Equine Herpes Virus at 5, 7, and 9 months of gestation. In high risk situations where either the mare or the farm has had cases of EHV in the past, the broodmares should receive an additional dose at 3 months.
Equine Viral Arthritis (EVA)
EVA is a viral disease found in horses that is known for causing abortions as well as respiratory issues. This disease is transferred sexually and through the respiratory tract. Once again there are usually no warning signs prior to abortion in infected mares. This disease can be managed using a vaccine and controlled breeding. EVA is unique in that both the mare and breeding stallions need to be vaccinated. This disease can be spread through semen so stallions must be tested for EVA prior to breeding. Diagnostic tests are the only way to determine if EVA was the cause of abortion.
Non-Infectious Causes of Abortion
Many of the non-infectious causes of abortion can be prevented with close management of the mare prior to and throughout her pregnancy. This includes twinning, abnormal umbilical cords, malnutrition, stress, hormonal imbalances, and toxins.
Twinning used to be the most common cause for equine abortion prior to the use of ultrasound technology which has become routine for most veterinarians and equine breeders. Twins are the product of double ovulation which can happen at rates as high as 20 percent in some breeds. If left alone, up to 90 percent of twins will result in abortion either from natural regression of one or both embryos or through stillbirths, premature foals, dystocia or death. Dystocia is simply a term defining a difficult and complicated birth; at this point both the offspring and the mare are at risk for serious injury or death. Only 1-2 percent of twin pregnancies result in two live foals. Most equine breeders have their initial pregnancy check performed at 14 days, pregnancy can be detected a few days earlier but two weeks is the norm. If twins are present at two weeks, there are a few options not including natural regression. The most commonly used options are pinching, aspiration, and chemical abortion.
Manual pinching of one embryo is very successful and will only leave one viable embryo. Aspiration of one or both embryos is also an option. Chemical abortion via prostaglandin injection is the cheapest and easiest method; however, both pregnancies will be terminated.
Ideally you would like to terminate one or both embryos prior to the heartbeat check which takes place around day 28 and at the very latest prior to the formation of the endometrial cups which happens around day 40. The reason being at this point you should still be able to cycle your mare back and get another chance at breeding her so you don’t miss a year. Pasture bred mares are at the highest risk for abortion due to twins and usually the only way to diagnose twins in this case is when it is too late.
Umbilical twisting happens at some point during gestation and this can cut off the blood flow to the fetus. The cause of these is mainly unknown and when blood flow is completely cut off abortion is the end result.
Poor nutrition in pregnant mares can cause abortion and usually happens early on in pregnancy. Studies have shown that mares in good condition prior to breeding have the best conception rates.
Severe stress can cause abortion as well. Strenuous training and riding in early pregnancy can result in pregnancy loss. Sickness, disease, environmental factors and surgical procedures can also be a cause for abortion.
Hormone imbalance is commonly overlooked as a cause for a failed pregnancy. Pregnant mares should have blood drawn and progesterone levels checked to determine if they are producing enough progesterone naturally to maintain pregnancy. Some mares will fail to reach these levels on their own and will need progesterone supplementation throughout pregnancy.
Fescue grass has toxins which, when ingested by a pregnant mare, can cause late term abortions. These ergot alkaloid toxins can also cause placental abnormalities, and retained placenta in a large number of mares. Also, a mare ingesting fescue late in gestation can suffer from an extended gestation length. Prevention for fescue toxicity is simply not allowing your mare access to fescue grass or hay, or dosing them with product designed to prevent the toxins affects.
In summary, when managing or owning pregnant mares it’s important to be aware of the norms during pregnancy as well as what is not normal. In some cases prevention begins prior to breeding, in others it’s dependent on how quickly the issues can be addressed and treated. Being knowledgeable about problems during pregnancy and knowing how to diagnose and treat those issues is the best way to ensure that your mare has a healthy foal.