The loss of a developing fetus during pregnancy can be a frustrating, emotional, and costly experience for horse owners, farm workers, veterinarians, and the public. A thorough evaluation of the aborted fetoplacental unit (the fetus and placenta) by a veterinary pathologist can help determine the cause of abortion; identify new, unusual, or foreign causes of fetal loss; rule out infectious agent involvement; and aid in the epidemiologic monitoring of abortifacients (factors that can result in abortion).
A two-year review of equine abortions, from the 2016 and 2017 breeding seasons, was conducted at the University of Kentucky (UK) Veterinary Diagnostic Laboratory to evaluate current abortion trends.
Dates listed below indicate data for the respective breeding season, not calendar year. A total of 898 cases of equine abortion, 570 from 2016 and 328 from 2017, were evaluated.
The majority of cases were considered sporadic and unrelated, except for one equine herpesvirus-1 abortion storm identified during the 2016 breeding season. Abortions during early gestation began in May of the 2016 breeding season and June of 2017 breeding season. The highest number of abortions occurred in March of both years, and the last abortions occurred in July of 2017 (2016 breeding season) and May of 2018 (2017 breeding season). Abortions were categorized into infectious (2016= 55% and 2017= 38%) and non-infectious causes (2016= 45% and 2017= 62%).
Infectious causes of fetal death were attributed to bacterial, viral, fungal, and unidentified (presumably bacterial) agents that resulted in placentitis (inflammation of the placenta) and/or systemic infections. Placentitis was the most common cause of infectious disease and was identified in 280 cases (24.6%) in 2016 and 102 cases (20.2%) in 2017. Approximately 5% of abortions each year were attributed to ascending placental infections through the mare’s cervix by bacteria such as Streptococcus zooepidemicus and Escherichia coli. Nocardioform/mucoid placentitis was diagnosed in 145 (12.7%) and 27 (5.3%) cases during 2016 and 2017, respectfully. Three cases of mycotic placentitis were diagnosed in 2016, and one case was diagnosed in 2017. Placentitis due to unidentified agents occurred in 79 (6.9%) cases in 2016 and 47 (9.3%) cases during 2017. Agents were not identified, presumably, due to the use of antimicrobial therapy, chronic resolved infections, or overgrowth by environmental organisms. Leptospiral abortion or perinatal death was identified in five cases (0.4%) during 2016 and 11 cases (2.2%) in 2017. Abortion due to fetal bacterial septicemia or pneumonia was diagnosed in 4.4% of cases in 2016 and 1.6% of cases in 2017. Equine herpesvirus-1 was the only viral agent identified in fetuses over the two-year period, and it was responsible for 16 (1.4%) abortions or perinatal deaths in the 2016 breeding season and 10 (2.0%) in the 2017 breeding season.
Noninfectious causes of abortion are considered sporadic events. They included abortion associated with umbilical cord torsion (2016= 3.9% and 2017= 7.9%), fetal stress (2016= 1.8% and 2017= 2.0%), placental “cervical pole” necrosis (2016= 0.5% and 2017= 0.4%), twin pregnancy (2016= 0% and 2017= 0.6%), miscellaneous causes (hydrops, tissue necrosis of unknown etiology, and maternal stress and disease; 2016= 1.2% and 2017= 2.0%), and abortion of undetermined cause (2016= 14.9% and 2017= 27.5%).
Abortion of undetermined cause occurs quite regularly and is frustrating to both clients and diagnosticians. Based on the human and veterinary literature, many of these occur due to physiologic abnormalities (e.g., fetal cardiovascular disease, hypoxia), stress and disease in the pregnant mare, autoimmune disorders, genetic irregularities, environmental exposures, and endocrine abnormalities; all of which cannot be easily assessed or tested for in the aborted fetoplacental units. A diagnosis of abortion of undetermined etiology isn’t completely without value, because infectious diseases and other possible causes of abortion storms can be readily ruled out.
In conclusion, equine abortion remains a common issue. Both infectious and noninfectious causes are frequently responsible. Evaluation of the aborted fetoplacental unit by your local veterinary diagnostic laboratory can aid in determining the cause of abortion, help to monitor and track known abortifacients, and identify new and possibly emerging causes of abortion.