Diagnostic Approaches to Equine Abortion

The abortion of a foal can be a very costly occurrence. A potentially valuable individual is lost; a year’s production by the mare is lost, even though the fixed costs of maintaining the mare are unchanged; and psychological effects occur from the loss of an anticipated birth after considerable work and planning. Finding the cause of abortion is important. Some diseases causing abortion may require treatment of the mare or can pose a risk to other pregnant mares. Other conditions causing abortion may necessitate changes in management practices, and clusters or outbreaks of abortions can indicate unusual or new diseases or conditions. The University of Kentucky (UK) Livestock Disease Diagnostic Center is uniquely positioned to address this situation.

Each foaling season the UK Livestock Disease Diagnostic Center receives over 600 fetuses, placentas, and term foals. These cases represent approximately one-third of all equine submissions. Most involve fetuses from five months of gestation to term. The UK Livestock Disease Diagnostic Center performs a complete work-up using a standardized approach to the examination of these fetuses and placentas.

Following an abortion, the fetus and fetal membranes should be collected in an undisturbed state, kept cool, and submitted to the laboratory as soon as possible. A detailed history of the mare’s current and past reproductive performance is important in the evaluation of an abortion, and a completely filled-out accession sheet is a necessity.

As a part of the case work-up, the fetus is measured and weighed, and the weight of the membranes and length of the umbilical cord are obtained. Examination includes a complete necropsy of the fetus and membranes with fetal tissues and fluids (stomach content, heart blood, and pericardial fluid) obtained. Also, a representative set of tissue is placed in fixative to be processed for microscopic examination. The alantochorion is spread with the chorionic surface outward, and changes in texture, color, and thickness are noted. The amnion and umbilical cord are also examined for lesions. The laboratory does testing on every submitted fetus for bacterial infection by culturing lung, liver, stomach content, and placental membranes and by dark-field examination of stomach fluid. The fluorescent antibody (FA) test is performed on kidney, liver, and placenta for the presence of leptospires. Tissues are tested for equine herpesvirus by the FA test, and virus isolation is performed to test for the presence of arteritis virus and other equine viruses. In addition, body fluids are tested for the presence of antibodies to leptospira species.

A comprehensive study by pathologists at the UK Livestock Disease Diagnostic Center (Giles, RC, et. al., JAVMA, 203: 1170-1175) of causes of abortion, stillbirth, and perinatal death over a six-year period revealed that infection of the fetus and/or placenta caused by bacteria, equine herpesvirus, fungi, or placentitis of unknown cause was the most common cause of abortion and accounted for 34% of the diagnoses. The second most common cause involved birth complications, including asphyxia, dystocia, and trauma (19% of cases). Other common diagnoses were placental edema or premature separation of the placenta (7%), twins (6%), contracted foal syndrome (5%), other congenital abnormalities (5%), and umbilical cord abnormalities including torsion of the cord (3%). Less common conditions included placental villous atrophy, body pregnancy, fetal diarrhea, and other miscellaneous conditions. A diagnosis was not made in 16% of the cases.

Such a large caseload of aborted fetuses and placentas provides a rich resource of materials by which to study and monitor equine reproductive conditions. Testing of fetuses at the UK Livestock Disease Diagnostic Center has led to the recognition of leptospirosis as an important cause of abortion in horses and emergence of nocardioform placentitis as a cause of chronic placentitis in mares. It has also allowed for the rapid recognition and characterization of changes in fetuses associated with the Mare Reproductive Loss Syndrome (MRLS) outbreak in 2001.

Dr. Neil M. Williams, (859) 253-0571, nmwillia@uky.edu
University of Kentucky, Livestock Disease Diagnostic Center, Lexington, Kentucky