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 malf