Abortion in Horses: Causes of Pregnancy Loss in Mares

Equine abortion and stillbirth refer to delivery of a non-viable fetus or foal. Most causes of abortion and stillbirth do not affect the mare’s future fertility; however, the loss of the foal can cause significant economic losses for the breeder.

Often, the only sign of pregnancy loss or miscarriage is the presence of a fetus and placenta in the mare’s environment. Before aborting, some mares may develop vaginal discharge or have udders filled with milk well before their expected due date.

There are many causes of equine abortion and stillbirth. In North America, issues in the development of the fetus or its placenta and bacterial infections are the most common causes. ^[1][2][3]

Other causes include nutritional deficiencies, traumatic injuries during birth, and viral infections. In 30 – 50% of cases, the cause of abortion is not identified, even with a thorough diagnostic work-up. ^[1]

Preventing abortion is a key component of managing a pregnant broodmare. Prevention strategies include providing an appropriate diet, vaccinating for infectious diseases, adopting a robust biosecurity strategy, and thorough examination of mares prior to and after breeding.

Equine Abortion

Abortion and stillbirth in mares are common causes of economic and time loss for horse breeders around the world. Many of the causes of abortion and stillbirth can be prevented through good husbandry, appropriate biosecurity measures, and working closely with an experienced veterinarian.

Abortion specifically refers to loss of the fetus after 40 days of gestation, up to approximately 320 days gestation. Stillbirth refers to fetal death after 320 days gestation, the gestational age where a foal is developed enough to survive on its own. ^[15]

The causes, symptoms, diagnosis, treatment, and prevention of abortion and stillbirth are similar.

Clinical Signs

The most common sign of abortion or stillbirth is the presence of fetal remains or placental tissues in the mare’s environment, such as the pasture or stall. Additional signs and symptoms may include: ^[4]

• Vaginal discharge
• Udder development with production of milk or colostrum
• Presence of fetal or placental tissues hanging from the mare’s vulva

In some cases, mares abort without showing symptoms, and the pregnancy loss may not be noticed until the mare either does not have a foal around her expected due date, or does not show normal pregnancy development such as a large abdomen.

Secondary Complications

Mares that experience a miscarriage have an increased risk of dystocia (difficulty foaling) and retained fetal membranes. ^[4] Mares must be monitored carefully before, during, and after foaling or delivering a fetus.

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Causes of Abortion

Broadly, causes of equine abortion can be divided into infectious and non-infectious causes. Infectious causes of abortion in horses are typically due to bacterial or viral infections. ^[1] Most of the time, the health of the mare is not directly impacted by these infections, and abortion may be the only symptom noticed by mare owners.

Non-infectious abortions most commonly arise from issues with fetal or placental development, traumatic injuries and nutritional disorders.

Fetal Abnormalities

Fetal abnormalities refer to structural or genetic defects in the developing fetus that can interfere with its normal growth and viability. Abnormalities may include genetic mutations and chromosomal abnormalities, congenital malformations, and developmental disorders.

Malformations in the developing fetus may be incompatible with life, resulting in pregnancy loss. ^[6] Common malformations include limb deformities, heart defects, and incomplete development of the intestinal tract. ^[6]

Some malformations may be due to known genetic diseases or toxin exposure, however the majority have no identifiable cause. ^[6]

Twin Pregnancies

Production of multiple eggs is estimated to occur in 8 – 15% of equine ovulations, leading to a high risk of twin pregnancy. ^[5] The equine placenta and uterus are not designed to sustain two fetuses, resulting in inadequate nutrient and oxygen supply for fetal growth.

This often results in death of the smaller twin or both twins. Rarely, mares can carry both twins to term successfully. The increased use of ultrasound to identify and remove twins has minimized this condition as a cause of abortion in horses.

Placental Abnormalities

Placental abnormalities refer to irregularities in the function or structure of the placenta, which can lead to complications and potential pregnancy loss in horses.

The placenta is a crucial organ that forms during pregnancy in mammals, acting as a bridge between the developing fetus and the mother. It enables the exchange of nutrients, oxygen, waste elimination, and hormone signalling to support fetal development.

Umbilical Cord Torsions

Umbilical cord torsions refer to the twisting or rotation of the umbilical cord during pregnancy, and are a common cause of equine miscarriage in North America. ^[1][2][3]

As the umbilical cord twists, it cuts off blood supply to the fetus, preventing adequate delivery of nutrients and oxygen. Ultimately, this results in fetal death and abortion. ^[7]

Torsions are thought to develop from fetal movement in utero; however, the reasons for increased fetal movement are currently unknown. ^[8] Excessively long umbilical cords may be predisposed to twisting by allowing the foal more movement within the uterus. ^[7]

Placental Insufficiency

Placental insufficiency refers to improper development of the placenta, which ultimately results in poor delivery of oxygen and nutrients to the growing foal. ^[6]

Most cases of placental insufficiency involve villous atrophy, where the placenta does not form enough villi during development. ^[6] The villi are the main connection between the maternal blood supply and the growing fetus.

Placental insufficiency can also result from implantation of the embryo in an unusual location within the uterus. Normal implantation occurs in the uterine horns. Implantation in the uterine body can result in abortion because the developing placenta does not grow large enough to support the growth of the fetus, resulting in inadequate nutrient exchange. ^[9]

Premature Separation

Premature separation of the placenta, also known as placental abruption, is a condition where the placenta detaches from the uterine wall before the birth of the fetus.

Premature separation from the mare immediately cuts off blood supply to the fetus, rapidly causing fetal death. In most cases, this separation occurs around the time of foaling, resulting in a “red bag delivery.” Prompt intervention is required to deliver the foal as it no longer has access to oxygen. ^[10]

The cause of premature placental separation is usually unknown; however, some cases have been linked with ingestion of fescue grasses during pregnancy. ^[2]

Nutritional Causes

The mare’s nutrition directly impacts the health of the growing fetus. In particular, vitamin E, selenium, and iodine levels have been correlated with abortions.

White Muscle Disease

White muscle disease is a disease of the heart and skeletal muscles caused by low vitamin E and selenium levels in the mare. ^[11] Of the two nutrients, the trace mineral selenium appears to be the most critical, as most cases of white muscle disease occur in mares grazing in areas with selenium-deficient soil. ^[11]

Both nutrients play a role as antioxidants by removing harmful substances produced during normal muscle activity. Without adequate removal, muscle degeneration and dysfunction occurs, including within the heart. ^[11] This damage can result in fetal death due to heart failure.

Iodine

The link between the trace mineral iodine and equine abortions is poorly understood at this time. Iodine deficiency is thought to be the major cause of congenital hypothyroidism and dysmaturity syndrome, which has been described in Western Canada and Europe. ^[12][13]

Although this condition primarily affects full-term foals, it has also been identified in stillborn foals, likely due to complications of fetal development. ^[6]

Traumatic injuries

Traumatic injuries to the fetus or foal are most likely to occur during dystocia, which refers to difficult or abnormal labor and delivery in mammals. Dystocia occurs in 4 – 10% of foalings and primarily arises when the foal’s long legs or neck interfere with the delivery process. ^[14]

During dystocia, the foal can lose its connection to the maternal blood supply through separation of the placenta from the uterine wall. Without available blood supply, the foal is deprived of oxygen, resulting in rapid fetal death and stillbirth. ^[15]

Infectious Causes of Abortion

Infectious causes of abortion in horses refer to various pathogenic microorganisms, such as bacteria, viruses, and parasites, that can invade the mare’s reproductive system and lead to pregnancy loss.

Bacterial Infections

Bacterial infections usually cause abortion by inducing placentitis, or inflammation of the placenta.

In most cases, the infection results from contamination of the vulva and vagina by bacteria, which then enters the cervix and contaminates the placenta. Once the placenta is contaminated, the bacterial infection spreads to the fetus, causing fetal death. ^[16]

The most common bacteria associated with placentitis include: ^[16]

• Streptococcus
• E. coli
• Pseudomonas

Viral Infections

Almost all cases of viral equine abortion are due to equine herpesvirus. The mare usually acquires the virus through inhaling viral particles or directly contacting infected horses. From there, the virus spreads to the reproductive tract and can infect the fetus, resulting in fetal death. ^[17]

Since equine herpesvirus is highly contagious, it is a common cause of “abortion storms” in equine breeding facilities. ^[17] Despite the development of effective vaccine protocols, equine herpesvirus remains a very common cause of equine abortion. ^[1]

Viral abortions can also occur due to equine arteritis virus, however extensive testing of breeding horses and increased vaccination has reduced the risk of this virus within North America. ^[18]

Diagnosis

Diagnosis of equine abortions can be challenging. Between 30 to 50% of miscarriages in horses receive no final diagnosis, despite thorough investigation into potential causes. ^[1]

Diagnostic tests may include:

• Physical examination of the mare
• Ultrasound of the reproductive tract
• Autopsy of the fetus and placenta
• Bloodwork
• Testing for specific infectious agents, such as equine herpesvirus
• Uterine biopsy or culture
• Hormone level testing in the mare
• Analysis of the mare’s diet

Cases where the full fetus and the placenta are submitted to a diagnostic laboratory for analysis are more likely to achieve a diagnosis. ^[1]

Treatment

After the abortion occurs, treatment is not typically necessary for the mare. ^[4] In some cases, mares with placentitis may require uterine lavage or antibiotic therapy to clear the infection.

Mares that experience secondary complications, such as retained fetal membranes often require intensive treatment. Prolonged retained fetal membranes can cause systemic inflammation and laminitis in the hooves.

Prior to the next breeding season, your veterinarian may recommend a breeding soundness exam, including a physical exam, uterine biopsy and uterine culture to rule out maternal factors causing placental insufficiency or placentitis. ^[4]

Additionally, a thorough review of the mare’s husbandry, including diet and vaccination schedule, may be recommended.

Prognosis

The long-term prognosis for the mare’s fertility depends on the cause of pregnancy loss. An abortion event typically has no impact on fertility; however, the cause of abortion may be indicative of other fertility issues. ^[4]

For example, placental insufficiency may indicate that there is previous damage or irritation to the uterine lining, preventing proper development of the placenta. These mares are predisposed to having future cases of placental insufficiency with subsequent pregnancies. ^[4]

Mares that develop secondary complications such as dystocia or uterine infections may suffer irreparable damage to the uterus, compromising future fertility. ^[4]

Prevention

Many of the causes of equine abortion are preventable with appropriate management. Strategies for prevention include vaccination, nutritional analysis, and reproductive examinations of mares prior to and after breeding.

Nutrition

A balanced feeding program that provides sufficient energy, protein, vitamins, and minerals is the foundation to maintaining a healthy broodmare and supporting the growth and development of the foal. Providing adequate nutrition is particularly important during mid to late gestation, when the fetus is growing rapidly. ^[19]

Work with a qualified equine nutritionist to formulate your broodmare’s diet based on the nutrition composition data provided by a hay analysis. A nutritionist will assess your horse’s individual needs by considering factors such as forage quality, nutrient deficiencies, and any special requirements to create an appropriate feeding regimen.

Ensuring adequate intake of iodine, vitamin E and selenium may help prevent nutrition-related causes of abortion. Fescue grasses should be avoided due to their link to red bag deliveries and poor milk production. ^[10]

Vaccination

Vaccination is an important part of broodmare management, particularly for prevention of equine herpesvirus infection. ^[9] The equine herpesvirus vaccine is given at five, seven and nine months of gestation, and helps prevent “abortion storms” in facilities with large populations of broodmares. ^[20]

Some mares may be vaccinated prior to foaling to increase the foal’s immunity against the virus. Furthermore, all horses coming into contact with pregnant broodmares should receive a herpesvirus vaccination every six months. ^[20]

Equine viral arteritis is another vaccinable disease that can cause abortion in mares. Mares are typically only vaccinated if they are being bred to a known carrier stallion. ^[21]

Vaccination takes place at least three weeks prior to breeding, and vaccinated mares must be kept separate from non-vaccinated mares for three weeks to prevent potential infection of other mares. ^[21] Mares bred to stallions that test negative for equine viral arteritis do not require the vaccine.

Biosecurity

Not all infectious diseases can be vaccinated for, and in some cases, vaccines are not completely effective. Therefore, biosecurity protocols should be implemented on breeding farms to ensure the health of broodmares and reduce the risk of pregnancy loss.

Infectious diseases can spread by direct contact between horses, or exposure to contaminated surfaces such as stalls, mats, water buckets, and feed tubs. ^[22] Keeping broodmares separate from other horses, especially new arrivals to the farm, is the best method for reducing the spread of infectious agents. ^[22]

Regular disinfection of shared areas and avoiding sharing tack and equipment can also reduce the risk of transmission. ^[22]

Pre-Breeding Examinations

Your veterinarian may recommend a breeding soundness examination prior to attempting to breed a mare. These examinations help identify maternal factors that can potentially result in infertility, embryonic loss, or abortion.

Typically, a pre-breeding examination includes a uterine biopsy (a small section of uterine tissue) or cytology (a sample of uterine cells), a uterine culture, and an evaluation of the mare’s vulvar conformation.

Chronic bacterial infections within the uterus is a common cause of infertility in mares. In mares that become pregnant, the infection may result in placentitis and subsequent abortion. A uterine culture is used to identify any bacterial populations within the uterus prior to breeding, so that they can be treated appropriately through uterine lavage containing antibiotics. ^[23]

Mares with poor vulvar conformation are predisposed to uterine infections, as they may contaminate the vulva with fecal material during defecation. These mares may benefit from a Caslick’s procedure, where the lips of the vulva are surgically closed to reduce the size of the opening as a preventative measure. ^[23]

Mares with infertility may have structural changes within their uterine lining that prevent embryos from implanting and may result in placental insufficiency if implantation is successful. These changes are diagnosed using uterine cytology or a uterine biopsy. ^[23]

These samples provide the veterinarian with information about any uterine inflammation or degeneration of the uterine lining that may be present. ^[23]

Many mares with chronic changes identified in the uterine lining cannot be treated, although some may successfully carry to term when given supplemental progesterone. ^[23]

Twin Reduction

Examining mares for twin pregnancy shortly after breeding is an important preventative measure for the health of the mare and fetuses. Typically, mares are examined between day 11 and 14 of gestation to identify multiple embryos. ^[5]

At around this time period, twin embryos can be “pinched” to remove one of the embryos, while leaving the other intact. In many cases, this procedure successfully allows the intact embryo to be carried to term, without the risk of twinning.

For twin pregnancies identified later in gestation, other techniques include euthanizing of one fetus, inducing abortion of both fetuses, or surgically removing a fetus. ^[5]