Pregnancy loss is relatively common in horses. Only 80% of mares bred give birth to a live foal at term. The loss of the developing foal (fetus) during gestation can fall under one of two categories: early embryonic loss, in which the loss of a fetus occurs in the early stages of pregnancy (generally less than 40 days of gestation); and abortion--a loss later in gestation. Determining the cause of early fetal loss is very difficult, and in many instances the aborted fetus is reabsorbed or lost. The probability of determining the cause of an abortion later in gestation is greater, but many still go undiagnosed.

Lark’s tail


One research study revealed that 1,308 fetuses or fetuses and placentas were submitted to the University of Kentucky Veterinary Diagnostic Laboratory over a two-year period encompassing the 2008 and 2009 foaling seasons. Full-term fetuses that died from birth-related trauma, dystocia (difficult birth), or asphyxia (lack of oxygen) were excluded from the study, 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 (inflammation of the placenta), with 174 cases (19% of overall total), bacterial abortion/septicemia (48 cases, 5%), and viral abortion (25 cases, 3%). The most common noninfectious cause was torsion of the umbilical cord (126 cases, 14%). However, in 289 cases (31%) no cause for the abortion was found. The highest percentage of nondiagnosed abortions occurred from 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.

Abortion cases of undetermined cause can be frustrating to the owner/manager, veterinarian, and the pathologist. These cases can be referred to as idiopathic abortions, no diagnosis, or abortions of undetermined etiology. While these diagnoses are not a welcome sign on a report, all is not lost. It simply means 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 not found, many significant diseases and conditions can be ruled out. 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 might not be found by examination of the fetus and placental membranes.

For more information, contact Dr. Neil Williams,, Veterinary Diagnostic Laboratory, University of Kentucky, Lexington.

Reprinted from the Equine Disease Quarterly, University of Kentucky, Department of Veterinary Science.

Want more articles like this? Sign up for the Bluegrass Equine Digest e-Newsletter.

More information on Gluck Equine Research Center and UK's Equine Initiative.

Reprinted from the Equine Disease Quarterly, University of Kentucky, Department of Veterinary Science.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More