Equine abortion is a common and frustrating condition with a variety of infectious and noninfectious causes. Evaluation of the fetoplacental unit can aid in understanding the cause of individual foal losses and identify emerging or changing trends between seasons.

Causes of abortion are categorized as infectious and contagious (equine herpesvirus, equine arteritis virus, leptospirosis); infectious (bacterial and mycotic infections, Potomac horse fever); and noninfectious (congenital malformations, twins, placental infarction, umbilical cord torsion, idiopathic). This is not an exhaustive list of causes of abortion but represent the most common diagnoses within the University of Kentucky Veterinary Diagnostic Laboratory, in Lexington.

In 2025-26, the laboratory received 374 equine abortion cases. Of these, there were 172 of undetermined cause, 27 umbilical cord torsions, 23 cases of equine herpesvirus-1, six cases of leptospiral abortion, 17 congenital anomalies, and five cases of cervical pole necrosis. Cases of placentitis are further categorized as bacterial ascending placentitis, mycotic placentitis, mucoid placentitis (most commonly associated with nocardioform bacteria), and placentitis not otherwise specified. In this breeding season, there were 41 cases of nocardioform placentitis identified, 47 cases of ascending bacterial placentitis, three cases of mycotic placentitis, and 17 cases of placentitis not otherwise specified. Nine cases of bacterial abortion were diagnosed. Rare cases of fetal neoplasia, in utero fracture, and hepatopathy were also observed.

In addition to abortions, placentas can be submitted for evaluation following foaling. These might be submitted due to observed abnormalities, a history of ultrasound abnormalities, high risk pregnancy, or to confirm the chorioallantois is complete and normal. Fifty-four cases of mucoid placentitis were diagnosed. Of these, there were 20 cases identified as Amycolatopsis sp, 24 cases as Crossiella equi, six cases as Streptomyces sp, and one case that was positive for Crossiella equi and Streptomyces. Two cases cultured a gram-positive branching bacillus but PCR was negative for Amycolatopsis sp., Crossiella equi, and Streptomyces sp. Of the 41 cases of abortion diagnosed with nocardioform placentitis, there were three cases where Amycolatopsis sp was identified, 28 cases with Crossiella equi, six cases with Streptomyces sp, two cases with both Crossiella equi and Amycolatopsis sp, and one with Streptomyces and Amycolatopsis sp.

Compared with other years of abortion, cases of mucoid placentitis continue to be variable between years, highlighting the need for continued investigation of the factors that make some seasons a higher incidence year than others.

This is an excerpt from Equine Disease Quarterly, Vol. 35, Issue 3, funded by Equus/Standardbred Station Inc. and M&J Insurance. It was written by Rebecca Ruby, Dipl. ACVP, ACVIM, of the University of Kentucky’s College of Agriculture, Food, and Environment, in Lexington.