Allantochorion showing indications of placentitis

Late Term and Habitual Abortion in the Mare

– Pregnancy loss in the mare is a constant source of concern among breeders. Here some common causes are discussed.

By Jos Mottershead

Late term and habitual abortion in the mare is a constant source of concern among breeders. It may be as a result of identifiable repetitious causes, or sometimes random. Any time that one experiences a mare abortion without obvious cause, it is worthwhile presenting the fetal membranes and the aborted fetus to a pathology lab for necropsy. Your attending veterinarian should be able to provide you with sources for such a service. Sometimes one receives a report indicating it was an “idiopathic abortion” – no identified cause – but if one receives a report indicating something which can be corrected in a future pregnancy – or even indicating recommending protecting the rest of the broodmare herd in the case of an infectious cause – it will have been money well spent! Below we look at some of the more common causes of abortion. This should not however be considered an all-inclusive list!

EHV-1, sub 1: “Rhinopneumonitis”

One of the most obvious causes to be considered is Rhinopneumonitis (Equine Herpes 1, sub-1 virus). This can be an alarming condition if identified as a cause of abortion, as it can affect many animal in a broodmare herd if they are not protected by vaccination or recent prior exposure (pre-pregnancy) which raised antibody levels. The timely use of “Pneumabort”, “Prodigy” or a similar EHV-1 (sub 1) protective vaccination has been shown to reduce the likelihood of an “abortion storm”, but improper storage of the vaccine may reduce its efficacy, particularly following exposure to excess heat or cold for example during shipping or storage. EHV-1 is considered to have a latent condition in the equine, being able to surface as an active case at a later date. Stress may trigger this reappearance. It has been estimated that around 60% of horses in North America carry the EHV as a latent condition. For this reason, breeders need to consider that a “closed herd” (which is often offered as a reason for not vaccinating) is unlikely to be extant, and vaccination should be considered essential at the appropriate timing and intervals. Mare which experience an abortion caused by EHV-1 (sub-1) are considered likely to have elevated antibody levels preventing repetition for at least the next and possibly some subsequent breeding seasons.


Late Term and Habitual Abortion - due to placentitis

Fetus aborted due to placentitis. Note discoloured amniotic and allantoic fluid and discolouration of allantochorion with visible pus

Placentitis – technically “inflammation of the placenta” – can be caused by infection by a bacteria, fungus or virus. The origins may be external or internal. The end result is often abortion with the death of the foal. Internal causes of placentitis may be blood borne pathogens or as a result of previous unresolved endometritis.

With an external source of infection causing placentitis, the pathogen gains access through the cervix into the uterus, and most commonly initially affects the “cervical star” area of the allantochorion (placenta). From there, the infection ascends up through the uterus (hence being referenced as “ascending placentitis”). This may be more commonly associated with mares that have poor reproductive conformation (“tipped” perineal region or uterus and/or sunken anus). Bacterial causes are most commonly similar to those isolated during pre-breeding uterine cultures – Streptococcus spp., E-coli, Klebsiella spp., Staphylococcus spp., Pseudomonas spp., and (less commonly in pre-breeding “work- ups”) Actinobacillus spp., and Salmonella spp. Post mortem cultures may indicate the presence of these organisms. In some areas, there is an increase being seen of abortions due to Nocardioform placentitis of which Crossiela equi and Amycolatopsis seem to be common causative agents. We discuss placentitis in greater detail in our article “Placentitis – Etiology, Diagnosis and Treatment

Cervical Incompetence

Cervical incompetence may be a consideration and is related to the “tipped” aspect of the reproductive tract. One then gets into the perpetual question of whether the cervix became “incompetent” because of the impending abortion (due to other causes), as a result of previous damage, or because of lowering of progestogen levels (which may also have been lowered as a result of the impending abortion due to the other causes). Supplementary progestogens may be beneficial in tightening an incompetent cervix (see below).

“Low Progesterone”

It is worth noting that the question of exogenous progesterone or progestogen supplementation is a difficult one to address. Progesterone (specifically) levels drop significantly around day 100-120 of pregnancy, but a complex system of PROGESTOGENS becomes active prior to that point. The progestogens are multiple and an analysis of blood samples for progesterone specifically will probably indicate low levels, whereas an assay for the progestogen complex (which is not commonly performed) would show “high”. To compound this, even progesterone levels have been shown to vary greatly during the course of any given day, and up to 4 samples over the course of a couple of days may be required to establish an accurate average level. There has been no published documentation to support the concept that Altrenogest (“Regumate”) or progesterone therapy has proved efficient in pregnancy maintenance in anything but ovariectomised mares. The main reason for this lack of research is the next-to-impossible chance of collecting enough suitable and similar candidates for a sample study (including “controls”). The value of widespread use of such supplementation in alleged “problem mares” therefore remains questionable. There is much anecdotal evidence presented that such supplementation may aid in these situations, but the true benefit – or lack of – in these reports is often lacking scientific clarity. We discuss this in greater detail in our article “Does My Mare Need Regumate?” In late pregnancy habitual aborters, some veterinarians have prescribed the use of “Regumate” until as late as day 300 of pregnancy or even through to term. One important note: If progesterone is given with the intention of preventing the abortion by keeping the cervix tight, it must be remembered that if there is another factor (such as an already dead fetus), that may be retained for a more prolonged period as a result of treatment. It should be noted that in most cases the mare will foal or abort “through” progestin supplementation, however there have been exceptions. Progesterone and progestogen therapy has been shown to reduce the uterus’ ability to resist infection and could even be a contributory factor to higher placentitis rates. Research is ongoing in this area, but it is becoming more apparent that progestin supplementation may not be the benign panacea it was once thought to be.

Some Other Common Causes

  • Late Term and Habitual Abortion - aborted twins

    Aborted late-term twins. Note significant size difference, which is common

    Twinning may be a cause of pregnancy loss at any stage of pregnancy as well as late term and habitual abortion. Early loss is more common, then occurrence increases later on, when the placenta becomes unable to provide sufficient nutrition. 60% of fetal growth occurs in the last trimester, so fetal nutritional draw increases accordingly in that time frame. A failure of adequate fetal nutrition levels results in one or both of the fetuses dying in-utero followed by abortion. With the advent of ultrasound as an early pregnancy diagnostic tool, a reduction in the numbers of twin foals causing abortions during late pregnancy has occurred, but still happens.
  • Umbilical torsion, whereby the fetus gets wrapped in its own umbilical cord – more commonly a hind leg – and cuts off or severely reduces its oxygen/blood supply causing its death. Repetition has been suggested owing to some mares producing a fetus with a longer umbilicus.
  • Equine Viral Arteritis may be a cause of abortions, but unlike the stallion, who can become a “shedder” of the virus in his semen, the mare when recovered from the clinical signs of the disease has resistance. She is hence not likely to loose another foal as a result of it for some subsequent years, until such time as her titer levels drop below the point of protection, so the virus cannot be considered a cause of repeated abortion in the same mare. We discuss EVA in greater detail in our article “Equine Viral Arteritis“.
  • Leptospirosis abortions – caused by a spirochete bacteria – are rare, but do occur. They may be regional and may also be more commonly seen following abnormally wet weather in the fall. Mares which abort due to leptospirosis rarely show any signs ahead of the abortion.
  • Genetic conditions. These can be breed-specific, so genetic testing and responsible selective breeding practices can eliminate such risks. Failure to heed these factors could result both late term and habitual abortion.
  • Fetal deformity is another possibility, but rarely seen in the equine and unlikely as a repeat offender.

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