Breeding Series: How to Deal with Abortion in Mares

Abortion in mares is defined as the expulsion of a foetus and its membranes at less than 300 days gestation. When it occurs it may be potentially life-threatening for the mare and emotionally distressing for the owner. The costs involved can be significant and can involve lost stud fees, loss of the foal and veterinary costs for treating the mare. In a 2008 survey, the percentage of UK and Thoroughbred mares that aborted was 2.1%.

Equine AbortionThis article will outline some of the more common causes of abortion in mares and what to do in the unfortunate event it occurs.


Historically, twinning used to be the most common cause of abortion.

The vast majority of mares are unable to sustain a twin pregnancy to term because the surface area of the uterus is not large enough to provide sufficient nutrients to 2 foeti.

More recently, the use of veterinary ultrasound allows diagnosis of twin pregnancies at 14-16 days post­-ovulation and early, appropriate management of twins. As a result, the incidence of abortions due to twinning has fallen over the past 20 years and in a 2001 survey of Thoroughbreds was less than 3%.

Currently, the most common causes of equine abortion are:

  • Umbilical cord abnormalities (eg: excessive twisting)
  • Placental Infections (placentitis)
  • Equine Herpes Virus infections

Umbilical Cord Abnormalities

Umbilical cord abnormalities are one of the most common causes of abortion and in one survey accounted for 46% of abortions. The umbilical cord is a relatively long structure and can vary in length from less than 60cm to over 85cm. During foetal development it is normal for foetal movements and rotations to cause a degree of cord twisting. Excessive twisting, however. can compromise the blood supply to the foetus and result in abortion. Unfortunately there is little an owner or vet can do to prevent this in practice, although research into the condition is ongoing.

Umbilical cords over 80cm in length predispose to excessive twisting and kinking. Factors that influence a longer cord-length include:

  • A genetic predisposition
    The stallion appears to exert a particular influence.
  • Older mares produce foals with longer cords
  • Mares that have had several foals produce longer cords
  • Male foals tend to have longer cords.

The diagnosis of abortion due to umbilical cord abnormalities may be made by your vet at the time of examining the mare or upon submitting the aborted foetus and placenta to a specialist laboratory for post-mortem.

Placental Infections

Placentitis is defined as infection of the placenta and accounts for approximately 10% of cases of abortion. The infection may be bacterial or fungal and is usually an ascending infection whereby micro-­organisms gain access to the uterus via the cervix. The main predisposing factor is poor vulval conformation which causes the mare to suck air and micro-organisms into the vagina. Abortion can occur as early as 3 months of gestation.

Placentitis is an important condition for the veterinary surgeon because some cases are treatable if the infection is noticed early enough.

Clinical Signs of Placentitis 

Visible clinical signs may (but not always) include:

  • Vaginal discharge
  • Premature udder development i. e. bagging up, waxing up or running milk

The infection often results in thickening of the placenta. Ultrasound examination by a veterinary surgeon can be performed to measure the combined thickness of the uterus and placenta to aid diagnosis. Trans-abdominal ultrasound may be used to determine if the foal is alive by measuring foetal heart rates and monitoring foetal movements.

Once placentitis has been diagnosed, treatments your vet may prescribe include antibiotics to combat the infection, anti­-inflammatories to reduce the inflammation and drugs to relax the uterus and reduce contractions. If abortion does occur. the placenta may be noticeably thickened and be covered with a brown-coloured exudate.

Equine Herpes Virus Abortion

Equine Herpes Virus (EHV) is the most important cause of infectious abortion in the UK. EHV can cause three different disease presentations;

  • abortion
  • respiratory disease
  • neurological disease

Abortion may be caused by two sub-types of the virus; EHV-1 and EHV-4.

The majority are caused by EHV-1 which can cause devastating ‘abortion storms’ in a stud farm situation, whereby most in­-contact mares will abort unless early action is taken.

The majority of horses in the UK carry both EHV-1 and EHV-4 as latent infections. They are usually contracted through respiratory infections as young horses. The virus often lies ‘dormant’ for many years but may be reactivated by stress such as transport, mixing with new horses and concurrent disease, and shed in respiratory secretions. If the horse that is shedding the virus is in-contact with pregnant mares, abortion may result.

Abortions due to EHV-1 usually occur in the last third of pregnancy. The abortion tends to be rapid and unexpected, although there may be a history of respiratory disease in young stock on the premises in previous weeks. In this situation, a full post-mortem with lab tests is essential to confirm the diagnosis. The virus can survive outside the horse for several weeks although is susceptible to certain disinfectants (e.g. Vircon).
Transmission between horses is achieved in two ways:

  • via the respiratory route
  • through contact with the products of abortion (the foetus, the placenta and the foetal fluids)

However, transmission requires close contact; therefore separation of horses is effective at limiting the spread. The risk of EHV abortion may be reduced by avoiding mixing horses of different age groups because the older horses (particularly yearlings) encountering the virus for the first time will spread the virus to the pregnant mares and the foals.

One extremely important point is that the virus is shed in the products of abortion (the foetus, placenta and foetal fluids). It is often difficult to persuade an owner to isolate a mare which has recently aborted because “She’s been with the others for the past 8 months so it’s too late to stop them getting it now”. However. the mare is unlikely to be infectious until abortion occurs. Isolation of an aborting mare is absolutely essential to reduce the spread of the virus to other pregnant mares. In fact, the consequences of an EHV outbreak can be so severe that it is wise to assume any equine abortion is due to EHV until lab tests prove otherwise.

A vaccine is available to reduce the chances of abortion due to EHV. It is given at months 5, 7 and 9 of gestation and is an extremely worthwhile investment for the mare owner.


It is essential to contact your vet if the worst happens and your mare aborts.

There are several important considerations:

  • Prevention of further abortions
  • Assessing the mare’s health
  • Investigating the cause of the abortion

Prevention of Further Abortions

  • Isolate the mare in the paddock or stable in which she aborted.
  • Move other mares in the same paddock to a clean paddock and keep them in batch isolation until the cause of the abortion is established.
  • Horses should not move on or off the premises (especially if it is a stud farm) until the risk of infectious, contagious disease has been established.
  • Disinfect and destroy bedding. Clean and disinfect the premises, equipment and any vehicles used to transport horses
  • Have different personnel attend to the aborted mare, the pregnant mares and the other stock
  • Implement strict hygiene measures; e. g. disinfect clothes, use of foot-dips.

Remember, the mare is not likely to be infective to other horses until she aborts, so isolation is essential!

Assessing the Mare’s Health

Equine Abortion 2Your vet will perform a complete physical examination in order to detect any complications arising as a result of the abortion. These may include:

  • Fragments of retained placenta These can rapidly become ‘rotten’ inside the mare and cause a uterine infection (‘metritis’). This often results in toxins being absorbed into the mare’s bloodstream. In turn, this can make the mare extremely sick (‘endotoxaemia’) and cause severe, life-threatening laminitis.
  • Injuries to the cervix and vagina Often the cervix in not completely dilated as the mare aborts. Tears can occur which may need veterinary attention.
    These can severely impair the mare’s future fertility.
  • The presence of a twin, still inside the mare – Specific treatments depend on the findings and professional judgement of your vet, but may include
    antibiotics, anti-inflammatories, oxytocin (to aid uterine contractions) and uterine lavage to flush out fragments of retained placenta and debris from the uterus.

Investigating the Cause

It is important to investigate the cause of the abortion for several reasons. Firstly, it enables your vet to screen for infectious contagious disease such as Equine Herpes Virus which may affect other in-contact horses and the local equine industry.

Secondly, it enables a diagnosis of the cause of the abortion so specific treatment can be directed to the mare and to give the mare owner an idea of future breeding performance. Mares which abort are often more difficult to get in foal at subsequent breeding.

Ideally, the whole foetus and placenta should be submitted to a specialist laboratory for a full post-mortem. Visual examination alone is often insufficient to obtain a diagnosis so specialist techniques including histopathology (examining the organs under a microscope), virus isolation and bacterial culture are necessary. Where it is not possible to submit the whole foetus and placenta for logistical reasons, your vet can take samples on the premises and submit them for testing (although this is not as ideal). It is crucial to retain the placenta as well as the foetus because many of the diagnostic changes occur in the placenta and/or the umbilical cord.

In summary, if abortion occurs, contact your vet immediately for advice and isolate the mare. Swift action can reduce the likelihood of other horses aborting, prevent or reduce serious disease in the mare and improve her chances of future breeding. 

Monitoring Future Pregnancies

Active monitoring the health of pregnant mares is becoming increasingly common. Experienced stud vets are able to perform a variety of tests including:

  • ultrasound of the placenta, both by rectal and transabdominal ultrasound.
  • ultrasound of the foetal fluids.
  • assessment of foetal movements.
  • assessment of foetal heart rate.

These tests may be performed either electively in a mare that has previously aborted, or when a problem such as premature lactation is reported. Please contact your vet for further information.

Roger Dixon BVM&S CertAVP(ESM) MRCVS



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