Abortion is the term used to describe the loss of a foetus before term, in the horse this means that the mare loses the foal before 300 days. After 300 days the loss of the foal tends to be termed still-birth, prematurity or dysmaturity. If the abortion occurs very early in the pregnancy (less than 100 days) there may be no signs that the mare has aborted, she may just come back into season.

There are many of causes of abortion, and they can be divided into infectious and non-infectious causes.
Most abortions occur sporadically, but occasionally they occur in clusters if they are caused by an infectious agent.

Fetal factors

  • Twin conception can lead to abortion as the mare is unable to carry two foals to term.
  • Genetic defects in the foal can result in a foal that is not viable and dies in the uterus.
  • If the placenta is not big or healthy enough to support the foetus it will die.
  • Sometimes the umbilical cord twists cutting off the blood supply to the foetus causing it to die.

Maternal factors

  • The likelihood of abortion increases as a mare gets older.
  • If the mare is sick for another reason such as an infection she may abort.
  • If the mare is malnourished she is less likely to carry the foal to term.
  • If the mare becomes very stressed during pregnancy she may abort and should therefore be kept quiet and calm and not travelled if possible.

The most important infectious cause of abortion in the mare is Equine herpesvirus (EHV). This virus often causes several mares on the same premises to abort (an abortion storm). Mares can be vaccinated against this virus in late pregnancy.
Bacterial infections of the uterus or placenta also cause abortion.

If the mare aborts early in pregnancy she may not show any clinical signs. She may return to season, however sometimes the mare is only found not to be pregnant when she is actually expected to foal.
If the mare produces milk earlier than expected abortion should be suspected as this is often the only time.
Occasionally the mare may show signs of being in labour or have a vaginal discharge.

Contact your vet immediately.
It is prudent to separate the mare from other pregnant mares but you should not stress the mare unduly. Keep the mare quiet whilst waiting for your vet. If you have found the foal or placenta place it in a clean bag – wear gloves to do this.

Your vet will carry out a full clinical examination on your mare most probably including a rectal examination and examination of the reproductive tract. Your vet will also want to know details regarding your mares reproductive and medical history.
Your vet will also examine the aborted foetus and placenta so it is important that you keep this is a plastic bag. It may be necessary to send the foetus and placenta to a laboratory for further tests to determine what caused the abortion. If your vet suspects an infectious cause they may wish to take a blood sample from the mare and test for Equine herpesvirus (EHV).