Management of Late Gestation Pregnancy Loss in the Mare

December 13, 2017

Breeding season in the northern hemisphere is over and hopefully all mares are in foal. Waiting for the arrival of the foals next year has begun. But what if something goes wrong? Abortion after day 45 of gestation occurs up to 3 to 15% in different horse populations. Infectious and non-infectious causes can be involved. It is important, that as long as the etiology of the pregnancy loss is undetermined, every abortion should be assumed as infectious until proven otherwise. How do you best handle the situation when a mare aborts and how do you prevent further damage to the mare or her fellow broodmares?


The first aim is to prevent further abortions. Therefore, the affected mare has to be isolated in a stall or a paddock from other horses, especially pregnant mares. Strict hygiene measures have to be followed to prevent spread of possible contagious materials to other horses. One should wear disposable gloves and protective clothing when handling the affected mare or the aborted fetus and disinfection of possible contaminated sites and equipment is mandatory. When the mare aborted in a paddock or on pasture, she should be taken out of that group of mares and be kept isolated until the reason for the abortion is established. The site of the abortion has to be separated, so that the other mares cannot get in touch with infectious materials such as fetal fluids, fetus and placenta. If there is enough space on the farm, the whole group (without the affected mare) can be moved to another paddock on the same premises. However, do NOT mix the exposed mares to a new group of mares.

If the mare aborted in a stall, there are different opinions how to handle the situation: some authors recommend to properly remove all bedding and to clean and disinfect her stall. However, this might provoke spreading of infectious materials between stalls and horses, so it might be better to delay this process until the cause of the pregnancy loss is determined (non-infectious abortion, placentitis) or until the quarantine period of three to four weeks is over (viral abortion). Direct contact with barn mates and spreading of infectious materials through living (humans, dogs, etc.) or dead (brooms, forks, etc.) vectors should be prevented. A footbath with disinfecting agents in front of the mare’s stall is recommended.


Late Term Pregnancy Loss_ ~11 months Gestation Once an abortion (photo right, 11 months gestation) is noticed, the affected mare should be presented to a veterinarian for general and reproductive health examination. The whole genital tract should be thoroughly evaluated for retained pieces of placenta or retention of a twin fetus and care should be taken to rule out severe trauma to the cervix or pelvic canal. Pathological findings, like retained placenta, should be treated immediately according to the severity. This situation is typically treated with oxytocin injections intravenously or intramuscularly, uterine lavage with saline or lactated ringer solution but may also require the use of broad spectrum antibiotics and anti-inflammatory drugs like flunixine meglumine to avoid the development of severe laminitis because of metritis and endotoxemia.

For diagnostic purposes, a serum blood sample should be obtained for detection of viral antibodies (Leptospirosis and EVA). A paired serum blood sample can then be taken 14 to 21 days later to demonstrate a rise in antibody titers if diagnosis could not be established until then. Additionally, two endometrial swabs (dry and with media) should be taken for microbiological (bacterial and fungal culture) and PCR (EHV 1 and 4, EVA, Leptospira spp. and Chlamydia spp.) testing if available.


The aborted fetus and attached placental membranes have to be handled very carefully. Shedding of infectious materials on the premises must be avoided. As previously mentioned, disposable gloves have to be worn at any time when handling the dead fetus and examination should take place in an area that can be disinfected. A gross evaluation includes

  • Inspection of the placental membranes (amnion and allantois) from both sides with notification of color
  • Exudates evaluated (culture/cytology)
  • Thickness or big avillous areas on the chorion can indicate a placentitis or a twin pregnancy.
  • Placenta should be weighed - Normal weight of a placenta near term is around 11% of the weight of the foal. Inflammation of the placental membranes and edema will increase the weight. 
  • Umbilical cord length should be measured - A very long cord (more than 81 cm) with excessive twisting might indicate umbilical cord torsion.
  • Crown-rump length of the fetus should be measured 
  • Fetus should be weighed
  • Gross inspection of the fetus for malformation of head, body wall and legs

Late Term Pregnancy Loss_ 7 months Gestation Whenever possible the whole abortus (photo left, 7 months gestation due to malformation of the head known as hydrocephalus) and placental membranes should be cooled and sent to a pathology laboratory immediately. The fetus should be packed in at least two strong plastic bags with appropriate sealing. If the shipment of the fetus is not possible, there are different possibilities to do a more or less intensive abortion diagnostics.

In all cases, the diagnostic laboratory should be contacted prior to submission of any materials, as different laboratories might have different diagnostic methods available and need special handling of the samples. Some laboratories offer special abortion sample kits, which can be ordered to have them ready on site, when a pregnancy loss occurs. The easiest way is to obtain an aseptic fetal lung aspirate for microbiological (bacterial and fungal culture) and PCR (EHV 1 and 4, EVA, Leptospira spp. and Chlamydia spp.) testing. A more complex method is to do a field necropsy and to submit the different tissue samples in adequate transport media for histological, microbiological, PCR and immunohistochemical testing. This will optimize the chance of obtaining a correct diagnosis for the abortion cause, but will also require a more experienced veterinarian to perform the necropsy.

In every situation involving an abortion, complete history should be submitted to the laboratory (stage of gestation, general health of the mare, vaccination program on the farm, contact to other horses and pregnant mares, previous abortions, etc.). It should be specifically noted to the laboratory if the affected mare had been in contact with other pregnant mares This may encourage the lab to obtain the results sooner in case further diagnostics are needed in the affected mare’s herd.

As in cases of diagnosed equine herpesvirus (EHV 1 or seldom 4) or equine arteritis virus (EVA) abortions, it is recommended to take deep nasal swabs in the very early stage of infection of all contact mares for quantitative PCR testing. This procedure allows to separate the contact mares in two groups: probably infected and likely to abort and probably not infected and unlikely to abort. With this method further abortions and spreading of the virus can be reduced to a minimum. If Leptospirosis has been diagnosed, serum titers or paired titers of mares exposed to the affected mare should be obtained to identify any additional exposed or previously infected mares that necessitate treatment.


There are many individual causes of abortion (photo below, 3 months gestation due to malformation of the body) in the mare. And to discuss them all would overstress this article. Just a short overview about infectious and non-infectious causes of late gestational pregnancy loss will be given. In addition, it should be noted that some causes of abortion in mares show special regional appearance. Late Term Pregnancy Loss_ 3 months Gestation Such as the mare reproductive loss syndrome (MRLS) in Kentucky / USA or amnionitis and funisitis (EAFL) in Australia which are caused by spiny caterpillars. Also, placentitis caused by Leptospira spp. or Nocardioform actinomycetes are rare in Europe but common in the certain regions of the USA.

Infectious causes

EHV 1 is one of the most frequently identified causes of infectious/contagious abortions. However, viral abortions, due to EHV 1 and equine arteritis virus, and abortion storms are not as frequently observed since vaccination protocols have been implemented. EHV 4 and the equine infectious anemia virus (EIA) may be isolated, but have lesser abortogenic potential.

Another common cause of abortion is bacterial placentitis. Most bacterial infections ascend from the vulva and vagina into the uterus. Pathogens that are isolated most frequently are Streptococcus zooepidemicus, Escherichia coli and Pseudomonas aeruginosa. An inflammatory response to the infection as well as infection of the fetus causes abortion.

Non-infectious causes:
  • Twins
  • Umbilical cord problems like excessive twisting of a long cord
  • Strangulation of the cord because of wrapping around the fetus’ legs or compromise through a large yolk sac remnant, etc.
  • Placental problems such as body pregnancy or ischemic necrosis
  • Hydrops of the amnion or allantois
  • Poor placental support of the fetus because of degenerative ageing chances, etc.
  • Severe congenital fetal malformations of the head, spine, body wall or legs 
  • Severe maternal illness like colic or pyrexia which can cause endotoxemia

For additional information about the causes of abortion in the mare you can read these articles on our website written by Dr. Karen Wolfsdorf, What Can Cause a Mare to Lose Her Pregnancy (April 2016) and Placentitis in the Mare – Causes, Diagnosis and Treatment (April 2014).


Not all abortions can be prevented, but with proper management the incidence of pregnancy loss can be decreased considerably. To decrease non-infectious reasons for abortion is almost impossible. Older mares that show early pregnancy loss or abortions without infectious causes identified in consecutive years and where abortion due to degenerative uterine changes is likely, may benefit in an embryo transfer program.

The incidence of twin abortions has severely decreased because of the routine use of ultrasound for pregnancy examination. Unfortunately, there are still a few identified because there was no confirmed pregnancy examination with ultrasound done at all. Or, the circumstances of examination were not ideal (too early, too late, light surroundings which can impede the ultrasound image on the screen, unexperienced examiner). Not identifying a twin pregnancy early in gestation can be avoided with good management:

  • Correct examination date (12-14 days from ovulation)
  • Appropriate examination conditions
  • Repeated examinations especially when in doubt or a double ovulation was noticed.

The number of viral abortions can only be decreased by limiting possible exposure of the pregnant mare to the virus. This includes:

  • Keeping the number of new pregnant mares in the herd to a minimum. All horses, that are new on the farm should be quarantined for at least three weeks. Also, pregnant mares should be separated from young stock (virus shedders) and horses that go to competitions (introduction of virus).
  • Pregnant mares should be separated in smaller groups without physical contact between the groups. Stress should be avoided wherever possible. That includes enough feeding places for all horses in the group and a stable ranking order in the group. 
  • Appropriate deworming and EHV vaccination programs are mandatory and differ between countries and even farms. 
  • EHV1 vaccination will not prevent herpes abortions, but WILL reduce the incidence of abortions caused by EHV1 and vaccinated mares shed less amounts of virus with the abortion so that virus dissemination is limited. 
  • EVA vaccination is not recommended for all broodmares. Mares can get infected through respiratory secretions or infected semen. Some stallions can become chronic virus shedders through their semen after infection. Stallion owners should test their stallions for EVA and if negative, should vaccinate them. 
  • If a mare is going to be bred by an EVA shedding stallion, she should be tested for EVA. If negative, she can be vaccinated at least three weeks before the planned breeding date (vaccinated animals should be isolated from non-vaccinated horses) or, if not vaccinated, has to be quarantined at least three weeks after the breeding until she has seroconverted and does not shed the virus anymore. 
  • Pregnant mares should not be in contact with EVA shedding horses. In Germany, for instance, every stallion, that is breeding in an artificial semen program on an European Union certified stallion premises has to be tested negative for EVA in serum every 30 days or has to be vaccinated after a special scheme. Shedders are not allowed to breed.

Ascending bacterial or fungal infections that cause placentitis may be more common in older mares with deficient perineal conformation. If the vulvar seal is incompetent, an episioplasty should be performed to prevent air and feces from being aspirated into the vagina. Excellent breeding hygiene is mandatory. Care should also be taken that these mares are kept in a good body condition, so that the perineal conformation does not get worse during advanced pregnancy. Mares at risk for ascending placentitis should be checked daily for vulvar discharge or premature mammary gland development and can be monitored by trans-rectal ultrasonography screening on a monthly basis.


The take home message is to treat every late pregnancy loss as an infectious abortion until proven otherwise. Strict hygiene should be used to avoid spreading potentially contagious materials between horses or premises. Every abortus should be examined to identify the etiological cause to control and prevent epidemic pregnancy failures and to identify a treatment for the affected mare, so that the prognosis for future breeding performance is optimized. Not all abortions can be prevented, but good management can help.