How common is equine abortion?

According to Dr. Neil Williams of the University of Kentucky, loss of a pregnancy is a relatively common occurrence in horses. It is generally accepted that only 80% of mares bred will give birth to a live foal at term.

The loss of the developing foal (fetus) during gestation is somewhat arbitrarily divided into two groups. The loss of a fetus in the early stages of pregnancy is referred to as an early embryonic loss (generally less than 40 days of gestation); a loss later in gestation is referred to as an abortion.

Causes of equine abortions

Determining the cause of early fetal losses is very difficult, and in many instances, the aborted fetus is reabsorbed or lost. The success of determining the cause of an abortion later in gestation is greater, but unfortunately the actual cause in many of these cases goes undiagnosed as well.

in a significant number of cases, a precise cause of abortion may not be found

According to Dr. Williams, a review of equine fetuses submitted to the University of Kentucky Veterinary Diagnostic Laboratory over a two-year period encompassing the 2008 and 2009 foaling seasons revealed that 1,308 fetuses/fetuses and placentas were submitted. Full-term fetuses that died from birth-related trauma, dystocia, or asphyxia were excluded, leaving a total of 921 aborted fetuses.

The most common diagnosis category was abortion due to an infectious cause, with 301 cases, or 33%. Of these, the more common diagnoses were placentitis, with 174 cases (19% of overall total), bacterial abortion/septicemia (48 cases, 5%), and viral abortion (25 cases, 3%). The most common non-infectious cause by far was torsion of the umbilical cord (126 cases, 14%). However, there were 289 cases (31%) in which no cause for the abortion was found.

The highest percent of non-diagnosed abortions occurred in the months of July to October, when the fetuses were typically of younger gestational age (Figure 1). As the fetuses aged and approached their due dates, the likelihood of a diagnosis increased.

Cases of abortion in which the cause is not determined are frustrating to the owner/ manager, veterinarian, and the pathologist. These cases can be referred to as an idiopathic abortion, no diagnosis, or an abortion of undetermined etiology. While these diagnoses are not welcome on a report, all is not lost. Such a diagnosis simply means that there is no explanation for the abortion by examination and testing of the fetus.

A positive outcome is that an infectious disease was not found in the fetus or membranes. Diagnostic laboratories are very adept at diagnosing infections of the fetus/membranes, and if pathogens are not found, the likelihood of an infectious abortion is low. Since infectious agents are often those that can result in multiple abortions or “abortion storms” in a herd, this determination allows the farm owner and staff to rest easier.

Likewise, a number of other common causes of abortion can be excluded through routine testing and pathology. Therefore, even if the etiology of the abortion is still not found, many diseases and conditions can be documented as not existing.

Often the pathologist also will note additional information about the case that can be helpful. When no fetal reason for the abortion is found, other possible explanations are considered. In addition to undetected genetic components or as yet unknown fetal factors, maternal problems should be considered, including those of genetic, metabolic, anatomic, endocrinologic, immunologic, and microbiologic origin.

Horse breeders generally recognize the importance of diagnostic testing on all abortions but should also realize that in a significant number of cases, a precise cause of abortion may not be found by examination of the fetus and placental membranes.

How to reduce equine abortion

1. Only breed healthy, mature mares that exhibit exceptional conformation, good temperament, and proven record of healthy live births.
2. Work closely with a veterinarian to establish a breeding plan, including pre-breeding examination and selection of an appropriate stallion.
3. Follow the veterinarians recommendations for feeding and supplementation to account for essential nutrition during fetal development. This is very important if you live in an area known for lacking important nutrients such as selenium.
4. Follow the veterinarians recommendations for appropriate vaccinations and other medical treatments that prepare the mare for parturition.
5. Reduce the chances of twin fetuses by early ultrasound monitoring by your veterinarian.
6. While most equine deliveries proceed quickly and without problem, make sure your veterinarian is on-call just in case something goes wrong.