Abortion in Horses: Causes, Diagnosis, Prevention, and Management

Abortion in horses is a distressing and often complex event for breeders and owners. It refers to the loss of a foal between 40 days and 300 days of gestation, after the mare has successfully conceived. Early embryonic loss (before 40 days) and stillbirth (after 300 days) are classified separately but may share similar underlying causes.

Understanding why abortions occur, how to identify them, and how to prevent recurrence is essential for maintaining the health of the mare and the overall success of a breeding program.

What Is Abortion in Horses

An abortion occurs when a pregnancy that had been confirmed fails and the foetus and placenta are expelled prematurely. The event may occur suddenly, sometimes without any obvious warning signs. Some mares may show behavioural changes or mild illness beforehand, while others abort unexpectedly. Abortions can happen sporadically in an individual mare or as part of an outbreak if an infectious agent is involved.

Common Causes of Abortion in Horses

Causes of abortion are broadly divided into two main categories: infectious and non-infectious. Identifying which type is responsible is vital for effective management and prevention.

Infectious Causes

Equine Herpesvirus-1 (EHV-1)

EHV-1 is the most common infectious cause of abortion in mares worldwide. It spreads via respiratory secretions, aborted material, or contaminated equipment and clothing. Abortions usually occur in the final trimester (between 7 and 11 months of gestation). In many cases, the mare shows no signs of illness before aborting. The aborted foetus, placenta, and fluids are highly infectious and can transmit the virus to other horses. EHV-1 may also cause respiratory disease or neurological symptoms in affected herds.

Equine Viral Arteritis (EVA)

Equine Viral Arteritis is another contagious viral disease that can cause abortion at any stage of pregnancy. It spreads through respiratory droplets or infected semen from carrier stallions. Affected mares may display fever, nasal discharge, swelling of the limbs or genitals, and eye inflammation. Stallions can carry the virus for extended periods, making testing and vaccination essential in breeding programs.

Leptospirosis

Leptospirosis, caused by the bacterium Leptospira interrogans, is an emerging cause of abortion in horses. It is typically associated with wet, muddy environments contaminated with urine from infected animals such as rodents or wildlife. Abortions usually occur in mid to late pregnancy, and the mare may appear otherwise healthy. The infection can also cause equine recurrent uveitis (moon blindness) and is zoonotic, meaning it can spread to humans.

Placentitis

Placentitis is an infection and inflammation of the placenta, often caused by bacteria such as Streptococcus zooepidemicus, Escherichia coli, or fungi. The infection usually ascends through the cervix from the vagina but can also occur through the bloodstream. Signs may include premature udder development, vaginal discharge, restlessness, or carrying the foal beyond the expected term. Placentitis can result in abortion, premature birth, or weak foals that struggle to survive after delivery.

Non-Infectious Causes

Twinning

Twinning is a major non-infectious cause of abortion. Horses are not designed to carry twins successfully, and competition for nutrients and space often leads to abortion of one or both foetuses, typically between 7 and 9 months of gestation. Early detection by ultrasound and reduction of one embryo is essential for preventing loss.

Umbilical Cord Abnormalities

Excessively long umbilical cords or twisted cords can compromise blood flow to the foal, resulting in abortion or stillbirth. This cause is more common in certain breeds, including Thoroughbreds.

Toxins

Certain plants such as fescue infected with endophyte fungus, pine needles, or some mouldy feeds can cause abortion or stillbirth. Proper feed and pasture management are key to prevention.

Hormonal Imbalance or Poor Uterine Environment

Mares with low progesterone levels or uterine abnormalities such as fibrosis or cysts may fail to maintain pregnancy. Age and reproductive history also influence this risk.

Trauma or Stress

Severe physical trauma, high fever, or extreme stress can occasionally trigger abortion, though this is less common than infectious or physiological causes.

Recognising Signs of Impending Abortion

While many abortions occur without obvious warning, some mares show subtle signs before foetal loss:

• Vaginal discharge or premature lactation
• Enlarged or prematurely developed udder
• Restlessness or colic-like discomfort
• Drop in abdominal size (loss of foetal fluid)
• Aborted tissue or foetus found in the stable or field

Mares that abort should always be examined by a veterinarian, even if they appear healthy afterward.

Diagnosis

Identifying the cause of abortion is vital to prevent future losses and protect other horses on the premises. Diagnosis typically involves:

• Veterinary examination of the mare and aborted material
• Laboratory testing of foetal tissues, placenta, and uterine swabs
• Blood tests for infectious agents such as EHV-1, EVA, or leptospirosis
• Review of management practices, vaccination history, and feed sources

The placenta and foetus should be handled with care, using gloves and protective clothing, as some causes of abortion (like leptospirosis) can infect humans.

Treatment and Care of the Mare

After an abortion, the mare should be monitored for signs of infection, fever, or retained placenta. Veterinary treatment may include antibiotics, anti-inflammatory medication, and uterine lavage to clear debris or infection. Most mares recover fully and can become pregnant again in future breeding seasons once the underlying cause is addressed.

Prevention Strategies

Preventing abortion relies on good management, biosecurity, and reproductive care.

Vaccination

• Vaccinate mares against equine herpesvirus (EHV-1) at 5, 7, and 9 months of pregnancy.
• Vaccinate breeding stallions and mares against equine viral arteritis (EVA) as recommended.
• In high-risk areas, vaccination against leptospirosis may also be beneficial.

Biosecurity and Hygiene

• Isolate new horses for at least three weeks before mixing with pregnant mares.
• Disinfect stables, foaling areas, and equipment regularly.
• Remove aborted material and bedding immediately and dispose of them safely.
• Prevent nose-to-nose contact between pregnant mares and horses showing respiratory symptoms.

Breeding and Management Practices

• Perform early pregnancy scans to detect and manage twins.
• Maintain mares on consistent diets with high-quality forage and balanced nutrition.
• Minimise stress, especially during transport or environmental changes.
• Monitor mares closely during late gestation for premature udder development or discharge.

Prognosis and Future Fertility

The prognosis for future pregnancies depends on the cause of abortion. Mares that lose a foal due to infectious disease or placentitis can often carry a healthy pregnancy once the infection is treated and controlled. Non-infectious causes such as twinning or trauma usually do not affect future fertility if properly managed.

However, mares that have aborted due to viral or bacterial infections should be isolated and retested before rebreeding, to ensure they are no longer shedding pathogens.

Conclusion

Abortion in horses is a challenging and often emotional issue for breeders, but with careful management, investigation, and preventive measures, many cases can be avoided. The key to success lies in maintaining a clean, low-stress environment, following appropriate vaccination schedules, and working closely with your veterinarian to monitor pregnancies.

By understanding the possible causes, both infectious and non-infectious, and taking proactive steps to reduce risks, horse owners can greatly improve reproductive outcomes and safeguard the health of both mares and foals.