Stillbirth in Horses: Causes, Diagnosis, and Prevention

Stillbirth in horses is a heartbreaking event for breeders and owners alike. It refers to the birth of a foal that dies during, or just before, the foaling process, typically after 300 days of gestation. While some stillbirths occur suddenly and without warning, many are the result of underlying health problems, infections, or complications during pregnancy or delivery.

Understanding the potential causes and implementing preventive measures can help breeders reduce the risk and improve outcomes for both mares and foals in future pregnancies.

What Is Stillbirth in Horses

A stillbirth occurs when a foal is born dead at or near full term. In most cases, the mare has carried the pregnancy successfully for the majority of the gestation period, but something goes wrong in the final stages of pregnancy or during parturition. The foal may die in the uterus shortly before birth, during delivery, or in the first few minutes of life if it fails to establish breathing.

Stillbirth is different from abortion, which refers to pregnancy loss before 300 days of gestation. However, both can share similar causes, particularly infectious ones.

Common Causes of Stillbirth in Horses

There are many possible causes of stillbirth, including infectious diseases, placental abnormalities, umbilical problems, and foaling complications. In many cases, more than one factor contributes to the loss.

1. Infectious Causes

Equine Herpesvirus-1 (EHV-1)

EHV-1 is a major cause of late-term abortion and stillbirth in mares. It can cause the foal to die in the uterus just days before foaling. Mares infected with EHV-1 may show no clinical signs, or they may have mild respiratory symptoms earlier in pregnancy. The foal is typically fresh, with no signs of decomposition, and the placenta often appears normal. The virus spreads through respiratory secretions or contact with infected material.

Equine Viral Arteritis (EVA)

EVA can also lead to late-term foetal death or stillbirth. The infection can be transmitted through respiratory droplets or infected semen. Mares may show fever, limb swelling, or eye inflammation before losing the foal. Stallions can become long-term carriers and transmit the virus during breeding.

Leptospirosis

Leptospira bacteria can infect the placenta, reducing blood flow and oxygen supply to the foal. Infected mares may abort or give birth to stillborn or weak foals. This infection is often associated with exposure to water or feed contaminated with urine from infected animals such as rodents, cattle, or wildlife.

Placentitis

Infection and inflammation of the placenta (placentitis) are major causes of late-term foetal death and stillbirth. Bacteria or fungi ascend through the cervix and infect the placental tissues, disrupting nutrient and oxygen exchange between mare and foal. Common bacteria include Streptococcus zooepidemicus and Escherichia coli. Mares with placentitis may show premature udder development, vaginal discharge, or restlessness before foaling.

2. Non-Infectious Causes

Umbilical Cord Abnormalities

An excessively long or twisted umbilical cord can restrict blood flow to the foal, depriving it of oxygen and nutrients. This is one of the most frequent non-infectious causes of stillbirth. The problem is more common in certain breeds, such as Thoroughbreds, and may occur without warning signs.

Dystocia (Difficult Birth)

Complications during labour can result in stillbirth if the foal becomes stuck in the birth canal or if delivery is prolonged. Malpresentation (incorrect foal position), a large foal, or uterine inertia can all contribute. If the foal’s chest or umbilical cord is compressed for too long, it may die before or during birth.

Placental Separation

Premature separation of the placenta from the uterine wall, often referred to as “red bag delivery”, cuts off oxygen to the foal before birth. The condition is an emergency; without immediate intervention, the foal will suffocate. Prompt recognition and rapid assistance during foaling are critical for survival.

Twin Pregnancies

Mares carrying twins rarely deliver two healthy foals. One or both foetuses may die in utero due to insufficient uterine space or nutrients, sometimes resulting in a stillbirth at term. Early detection and reduction of twins by ultrasound are essential for preventing this complication.

Premature Placental Aging or Insufficiency

In some mares, especially older ones or those with previous uterine infections, the placenta may not function properly in late pregnancy. Poor blood flow and nutrient exchange can lead to foetal growth retardation, weakness, or stillbirth.

Toxins and Nutritional Deficiencies

Consumption of endophyte-infected fescue grass, mouldy feeds, or certain toxic plants can interfere with hormone regulation, placental function, or foetal development. Nutritional imbalances such as selenium or vitamin E deficiency may also increase the risk of stillbirth.

Recognising Signs and Risk Factors

While stillbirth can occur without warning, some mares show signs that something is wrong in late pregnancy, such as:

• Premature udder development or lactation
• Vaginal discharge or foul odour
• Restlessness or discomfort before the expected foaling date
• A prolonged gestation period or changes in foal movement
• History of previous abortion or stillbirth

Mares with placentitis or other risk factors should be monitored closely under veterinary supervision during the final weeks of pregnancy.

Diagnosis

Determining the cause of a stillbirth is important for preventing future losses. A thorough veterinary investigation typically includes:

• Examination of the stillborn foal and placenta for signs of infection, umbilical issues, or trauma
• Laboratory testing of foetal tissues, placenta, and mare’s blood for infectious agents such as EHV-1, EVA, or Leptospira
• Culture and histopathology of placental tissues
• Review of mare’s vaccination history, nutrition, and breeding management
• Assessment of foaling conditions and timing

Samples should be collected as soon as possible after the event, as tissues deteriorate quickly and may no longer provide reliable results if delayed.

Care of the Mare After Stillbirth

After delivering a stillborn foal, the mare should be examined by a veterinarian to ensure that no placental tissue remains in the uterus and that there is no internal trauma or infection. Treatment may include:

• Antibiotics to prevent uterine infection
• Anti-inflammatory medications to reduce discomfort
• Uterine lavage (flushing) to remove debris and bacteria
• Monitoring for fever, depression, or unusual discharge

Most mares recover physically within a few weeks and can often be rebred successfully in the next season, depending on the cause.

Prevention

While not all stillbirths are preventable, good management practices greatly reduce the risk. Prevention focuses on controlling infection, managing foaling conditions, and maintaining mare health.

Vaccination

• Vaccinate mares against equine herpesvirus (EHV-1) at 5, 7, and 9 months of gestation.
• Consider vaccination for equine viral arteritis (EVA) where it is known to occur.
• In regions where leptospirosis is common, discuss vaccination with your veterinarian.

Biosecurity

• Isolate pregnant mares from horses that travel or attend shows.
• Avoid introducing new horses late in pregnancy.
• Clean and disinfect foaling areas and equipment thoroughly.
• Dispose of placentas and foetal material safely to prevent disease spread.

Nutrition and Management

• Provide balanced, high-quality nutrition throughout pregnancy.
• Avoid endophyte-infected fescue pastures during the last trimester.
• Maintain a healthy body condition score, neither underweight nor obese.
• Provide clean water and regular exercise.

Monitoring Late Pregnancy

• Observe mares daily in the final month for changes in udder development, discharge, or behaviour.
• Use ultrasound to assess placental thickness and foetal heartbeat in mares with a history of complications.
• Have a clear foaling plan, with emergency veterinary contact information readily available.

Prognosis and Future Fertility

The outlook for a mare following a stillbirth depends on the underlying cause. If infection or placental disease was responsible, treatment and recovery are often successful, and many mares go on to have healthy pregnancies. However, if structural issues or chronic uterine scarring are involved, fertility may be reduced.

Mares that have had stillbirths due to infectious causes should be isolated until cleared of infection and re-examined before rebreeding.

Conclusion

Stillbirth in horses is a devastating event but, with proper investigation and management, future losses can often be prevented. Infectious agents such as equine herpesvirus, placentitis, and leptospirosis remain major concerns, but non-infectious factors like umbilical abnormalities and foaling complications also play significant roles.

Good breeding management, regular veterinary monitoring, appropriate vaccination, and clean foaling environments are the foundation of prevention. With attention to detail and timely intervention, most mares can recover and successfully produce healthy foals in the future.