Loss of Pregnancy (Abortion) in Horses

Key Takeaways

Abortion is the loss of a fetus before the expected due date, resulting in delivery of a non-viable foal. 

• There are many causes of abortion in horses, including infectious causes such as equine herpesvirus, and non-infectious causes such as twinning

• Symptoms of abortion include premature udder development, labor before the expected due date, and observation of fetal or placental tissues in a mare’s environment

• In most cases, diagnosis requires submission of the fetal and placental tissues to a diagnostic laboratory for evaluation

• Treatment depends on the underlying cause of the abortion, but may involve uterine lavage, antibiotics, dietary changes, or supplemental progesterone for future pregnancies

• Most causes of abortion do not affect a mare’s long-term reproductive capacity

• Vaccination, thorough veterinary examination, and careful mare management are the best strategies for preventing equine abortions

A Closer Look: What is Abortion in Horses?

Abortion is common in horses, and can result from numerous potential causes. Most causes of abortion do not affect a mare’s long-term ability to breed, unless complications develop. Mares that have aborted require prompt veterinary examination to identify the cause of abortion and begin appropriate treatment. Submission of the fetal and placental tissues to a diagnostic laboratory is recommended to maximize the chances of diagnosis. 

Mares that have aborted may experience retained placenta, a serious condition that can lead to life-threatening toxic shock and laminitis. Mares suspected of having a retained placenta require emergency veterinary care.

Possible Causes

Causes of abortions in horses are either infectious or non-infectious. Infectious causes of abortion result from bacterial, viral, or fungal infections that are typically acquired from a mare’s environment. 

Examples of infectious causes include:

• Viral infections, such as equine herpesvirus or equine viral arteritis

• Bacterial or fungal placentitis

• Mare reproductive loss syndrome associated with Eastern tent caterpillar ingestion

• Maternal illness, such as from leptospirosis or Potomac horse fever

Non-infectious abortions primarily result from abnormalities in the fetus or placenta that prevent proper growth and development.

Common non infectious causes include:

• Twin pregnancy

• Umbilical cord torsions

• Excessively long umbilical cords

• Premature placental separation (red bag delivery)

• Improper placental development 

• Maternal starvation

• Maternal nutritional deficiencies or exposure to toxic plants

• Repeated corticosteroid use in mares

Risk Factors

Equine abortion is either related to infectious or non infectious causes. Horses in contact with other animals are more likely to be exposed to infectious agents leading to abortion. Non infectious causes may be related to nutrition of the mare, but may also be unpredictable.

Infectious agents may affect multiple mares on a breeding farm at one time, producing an abortion storm. These abortion storms are particularly common with equine herpesvirus and mare reproductive loss syndrome.

Abortions associated with non infectious causes typically occur sporadically, unless there is an underlying nutritional or environmental problem that affects multiple mares on the same farm.  Most cases of abortion do not affect a mare’s long-term reproductive capacity after treatment, unless serious complications develop. Some types of abortion, such as placental insufficiency, may indicate an underlying issue with the mare’s uterine lining. These mares require examination by a veterinarian and a uterine biopsy to determine their likelihood of carrying a foal to term.

Testing and Diagnosis

Diagnosis of the underlying cause of equine abortions is notoriously difficult, with only around 50% of cases achieving a diagnosis.

Diagnostic tests include:

• Submission of the fetus and placenta to a diagnostic laboratory for examination

• Physical examination

• Blood work

• Uterine culture, cytology, and/or biopsy

• Blood testing for specific infectious diseases

Treatment depends on the underlying cause of abortion, and the health status of affected mares. 

Treatments may involve:

• Intrauterine lavage to flush out the uterus

• Antibiotic therapy

• Treatment of underlying infectious conditions, which may include hospitalization or other intensive treatment

• Dietary changes to treat deficiencies or malnutrition

• Supplemental progesterone for future pregnancies

Similar symptoms

Equine abortion is self-evident and not likely to be confused with other symptoms.

There are many preventative measures available for equine abortions. Infectious abortions are easiest to prevent, through management strategies such as:

• Routine vaccination, particularly for equine herpesvirus

• Segregating pregnant mares from other horses

• Prompt isolation of mares that abort from other pregnant mares

• Management of factors that predispose to placentitis, such as performing a Caslick’s procedure

• Careful observation of pregnant mares to identify signs of placentitis early

Although most non-infectious abortions are sporadic conditions that cannot be prevented, some conditions are linked to specific risk factors that can be managed by breeding operations. Strategies include:

• Early diagnosis of twins and “twin pinching” via ultrasound

• Uterine biopsy of mares prior to the breeding season to evaluate uterine health

• Reducing fescue grass intake (associated with red bag deliveries)

• Eliminating toxic plants in pastures

• Developing an appropriate diet under guidance from an equine nutritionist

Associated Symptoms

Symptoms of abortion in horses include:

• Labor before the expected due date

• Swelling of the udder and/or dripping milk before the expected due date

• Mild colic signs

• Discharge from the vulva

• Observation of aborted fetal or placental tissues in the stall or pasture

Some causes of abortion result in maternal illness, particularly infectious causes of abortion. Mares may show additional symptoms around the time of abortion, such as:

• Fever

Nasal discharge

Reduced appetite

• Lethargy

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