These information sheets are provided for your interest. They should not replace veterinary advice from your veterinary surgeon.

Whilst every effort is taken to ensure the accuracy and completeness of the information provided, your specific circumstances must be discussed before advice can be given.

Fri, 20/05/2022 - 11:40 -- Brendan John MRCVS

Abortion in horses is defined as the delivery of a fetus and its membranes before the stage at which it is capable of independent survival. This point is usually taken to be between the 300th and 310th day of gestation. A dead foal delivered after this point is described as still-born. Following a positive pregnancy diagnosis at 6 weeks after service, around 10% (1 in 10) of foals are aborted. There are many causes for this which can involve an infectious agent or not.

Non-infectious causes

Umbilical cord torsion

This is now the most common cause of abortion in Thoroughbred mares in the UK. The cord can become twisted on itself or around the foal, cutting off the blood supply to the growing fetus. These abortions usually occur in the final trimester, and seem to have an association with cords which are longer than normal.

Congenital and developmental abnormalities

These are rare and the cause is usually unknown. The fetus develops with an abnormality which prevents its survival beyond a certain stage of gestation.


This used to be an important non-infectious cause of abortion. The embryos are undernourished, having to share the uterine space and maternal resources. In most cases one fetus will die and both will be aborted soon after. Occasionally one foal reaches developmental maturity and is born alive, although it will never reach its maximum potential body size. It is very rare, and dangerous for the mare, for both foals to be born alive. Routine ultrasound scanning usually results in the early detection of twinning, which can be reduced by manual destruction of one of the embryos per rectum by a veterinarian. Although this is a common practice, it has uncertain genetic justification, since the disposition to twinning is hereditary and the phenotype is being continued through the generations by allowing foals to be born with the genotype.

Other non-infectious causes

of abortion include stress, toxification, malnutrition or trauma to the mare.

Infectious causes

Equine Herpesviruses (EHV) 1 and 4

These pathogens are known to cause respiratory disease, patricularly in young horses. EHV-1 can cause foaling difficulties ranging from abortion, through still-birth to the birth of a weak, non-viable foal. Where mares are not vaccinated EHV-1 infection can lead to a 'viral abortion' storm in in-contact pregnant mares. Rarely single abortions can occur as a result of EHV-4 infection. EHV abortions usually occur suddenly and without warning, any time from one week to several months after exposure to the virus. The third month after successful mating is considered to be a high risk time for EHV infection, which occurs by the respiratory route. Isolation, which may be with other mares at a similar stage of gestation, is a sensible precaution at this time. Vaccination is recommended from the age of 4-6 weeks in all horses, to cover against respiratory disease in horse populations. One vaccine is licensed for use in pregnant mares for the reduction of risk of abortion storms (given in 5th, 7th, 9th months of gestation). Vaccination in all horses also reduces the prevalence of the virus in the population which decreases the risk of EHV abortion. The aborted material from an EHV abortion is highly infectious. If EHV is suspected as a cause of abortion, the foal and membranes will be sent for post mortem examination. The mare should be isolated and horses should not leave the premises until the cause of the abortion is determined.

Equine Viral Arteritis (EVA)

This is rare in the UK, but we should all be aware of it, since the population is susceptible to infection from mainland Europe. The disease is closely monitored by DEFRA and any suspected cases must be reported immediately by the owner/veterinarian. EVA can cause systemic illness with clinical signs including fever, depression, nose and eye discharge, swelling of the eyelids, lower abdomen and scrotum. Conjunctivitis may be seen. EVA is found in the accessory sex glands of infected stallions, which may or may not show clinical signs, and is shed in semen at service. Mares may spread the disease via the respiratory route. Aborted material is highly infectious. There is no specific treatment for EVA infection, but steps can be taken to prevent spread of infection. There is a blood test for the virus, and this should be applied to all animals before and after import, and all breeding animals at the start of the breeding season. The test cannot distinguish between vaccinated and infected animals so a careful record should be made of a negative test result prior to vaccination. In the event of a positive blood test, mares should be retested in a further 2 weeks, to show a static or lower antibody level, indicative of recovery from infection; stallions or colts with positive results and no evidence of vaccination must be resported to DEFRA. The options, after confirmation by further tests, are export, castration or euthanasia. An inactivated vaccine is licensed for use in the UK. It is only indicated for use in breeding stallions, since EVA has a very low incidence in the UK and vaccination complicates the surveillance programme which relies upon negative blood test results to track incidences of the virus. EVA is a notifiable disease. DEFRA must be informed and the horse and all in-contacts must be isolated.

Bacterial and Fungal Infections

Entering the uterus via the vagina, infection of the placenta usually results in abortion. Examples of agents include:

  • Streptococcus spp.
  • Escherichia coli
  • Klebsiella pneumoniae
  • Pseudomonas aeruginosa
  • Taylorella equigenitalis
  • Fungi.

Sometimes mares will show early signs of bacterial placentitis. Premature mammary development or a purulent vaginal discharge may forewarn of problems soon to become all too evident. Although some systemic antibiotics may reach the uterus and tackle the infection, they may not be able to restrict the damage which is done. Stallions and mares should be swabbed for pathogens prior to mating. Mares should have a Caslick's stitching if the vulval conformation disposes to infection. Abortion will never be entirely eliminated, but hygiene and management can play a huge part in reducing its impact.