A review of two breeding seasons in Kentucky has cast a spotlight on the causes of abortion in mares, with about half the losses assessed as being from non-infectious causes.
Equine abortion remains unfortunately common, according to Associate Professor Alan Loynachan, a veterinary pathologist with the University of Kentucky’s Veterinary Diagnostic Laboratory in Lexington.
Loynachan says the loss of a developing fetus during pregnancy can be a frustrating, emotional, and costly experience for those involved.
He says a thorough evaluation of the aborted fetus and placenta by a veterinary pathologist can help determine the cause, identify new, unusual, or foreign causes of fetal loss, rule out involvement by infectious agents, and aid in monitoring factors that can trigger an abortion.
Loynachan reports in the latest issue of Equine Disease Quarterly on a two-year review of equine abortions conducted at the laboratory from the 2016 and 2017 breeding seasons in the state, which boasts a strong thoroughbred breeding industry.
A total of 898 cases of abortion, 570 from 2016 and 328 from 2017, were evaluated.
Most were considered sporadic and unrelated, except for one Equine Herpesvirus 1 abortion storm identified during the 2016 breeding season.
Abortions were categorized into infectious (55% of cases in 2016 and 38% of them in 2017) and non-infectious causes (45% in 2016 and 62% in 2017).
“Infectious causes of fetal death were attributed to bacterial, viral, fungal, and unidentified (presumably bacterial) agents that resulted in placentitis and/or systemic infections.”
Placentitis was the most common cause of infectious disease, being identified in 280 cases (24.6%) in 2016 and 102 cases (20.2%) in 2017.
Around 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, respectively.
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 deaths around the time of birth in the 2016 breeding season and 10 (2.0%) in the 2017 breeding season.
Non-infectious causes of abortion are considered sporadic events. They included abortion associated with umbilical cord torsion (3.9% in 2016 and 7.9% in 2017), fetal stress (1.8% in 2016 and 2.0% in 2017), placental “cervical pole” necrosis (0.5% in 2016 and 0.4% in 2017), twin pregnancy (none in 2016 and 0.6% in 2017), miscellaneous causes (hydrops, tissue necrosis of unknown cause, and maternal stress and disease; (1.2% in 2016 and 2.0% in 2017), and abortion of undetermined cause (14.9% in 2016 and 27.5% in 2017).
“Abortion of undetermined cause occurs quite regularly and is frustrating to both clients and diagnosticians,” he writes.
“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 a commentary in the same issue, Shavahn Loux, a postdoctoral scholar with the University of Kentucky’s Maxwell H. Gluck Equine Research Center, describes late-term abortion as one of the most devastating issues horse breeders face.
“Every pregnancy represents a labor of love, with a substantial amount of time, energy and money put into achieving the perfect foal. Every pregnancy loss raises questions concerning our ability to have prevented that loss and whether we could have done more.”
Loux says Loynachan’s review is important in helping to identify areas where management techniques can be improved, and indicating where future research efforts should be focused.
She urged breeding farms to send their aborted foals to a veterinary diagnostic laboratory, even if the cause of abortion appears obvious.
“Approximately 50% of the abortions evaluated were deemed non-infectious. Of these, most are not likely to be management related with the exception of twin pregnancies; however, twins did not comprise a large percentage of the abortions submitted.
“This is likely due to better management techniques, namely identification and reduction of twin pregnancies early in gestation, but also likely reflects the failure of owners to submit abortions with an obvious cause to the diagnostic laboratory.
“While this is understandable, it also makes it difficult to accurately measure the frequency of these losses. Infectious abortions comprised the other 50% of submitted abortions, with placentitis representing the majority of these cases.
“Unfortunately, we know little about what predisposes a mare to develop placentitis, and still have trouble with early, accurate and specific diagnosis.
“Even so, there are steps that owners can take to aid in early diagnosis, including endocrine monitoring, regular ultrasound evaluation of the placenta as well as daily checks for premature mammary gland development, premature lactation, and purulent vulvar discharge.
“If anything out of the ordinary is noted, a veterinarian should be called to examine the mare and start treatment if indicated.”
Loux says the laboratory of Dr Barry Ball at the Gluck Center has been focusing on better understanding the causes and progression of placentitis.
“By utilizing state-of-the-art techniques to look at changing gene expression, we have identified several potential targets which we believe will function as diagnostic aids and/or treatment options.
“Although more work is still needed to confirm our findings, we are optimistic that better options for dealing with placentitis will be available soon.
“Again, I cannot stress enough the importance of sending all aborted foals, including fetal membranes and maternal serum, to a veterinary diagnostic laboratory.
“When breeders fail to submit abortions, it becomes more difficult to spot trends and, in turn, becomes more difficult to identify and respond to emerging threats.
“It’s easy to justify only submitting abortions without an obvious cause; however, the overall health of the equine breeding industry relies on the submission of every abortion, every time.”
Equine Disease Quarterly is funded by underwriters at Lloyd’s, London.