A recent study published in the Canadian Veterinary Journal examined the causes of abortion in horses.
Led by Dr. Madison Ricard at the Western College of Veterinary Medicine at the University of Saskatchewan, researchers collected and analyzed 901 cases of equine abortions submitted to veterinary diagnostic laboratories across Canada between 2015 and 2020.
A key result is that no diagnosis was reached for 52 percent of all abortions in horses. This result is similar to other studies. A relatively low success rate is unfortunate, leaving horse owners without knowing the root cause of the pregnancy loss, while still on the hook to pay for the testing. Cases in which the placenta was submitted with the fetus were statistically more likely to achieve a diagnosis.
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In an ideal situation, when a mare aborts, the fetus and placenta are kept cool and immediately submitted together to the diagnostic laboratory and any supportive tests such as checking for infections is pursued. While this may not always be practical, a complete submission with the placenta improves the chances of obtaining a diagnosis.
Among the cases that had a diagnosis, most were from causes other than infections. The main non-infectious causes were abnormal twisting of the umbilical cord and an insufficient placenta.
About 10 percent of cases were caused by bacterial and viral infections, respectively. Streptococcus and E. coli were the most common bacterial infections, while equine herpesvirus-1 caused all the viral abortions.
Only rarely did fungal infections cause abortions in Canadian mares, which is similar to other studies.
Importantly, there were no abortion cases caused by equine viral arteritis virus.
As is the case with many scientific investigations of animal health, made-in-Canada studies are important to understanding our local experiences. It is not always accurate to extrapolate from American or European cases.
Indeed, the pattern of abortion causes in Canada appears to most closely resemble those in the United Kingdom, rather than the United States.
This study also represents an incredible effort of co-operation between the veterinary diagnostic laboratories across Canada. It would be great to see more of these types of cross-Canada studies in the future, in which the laboratories share data to come to a better understanding of specific animal health issues.
Retrospective studies such as this one are not without their limitations. The study could only include cases submitted to veterinary diagnostic laboratories. While these cases may be representative of the general horse breeding population, it is hard to discern if this is true. It is highly likely there is a population of aborted fetuses that are not sent to a laboratory.
Probably the data is biased toward more economically valuable horses and breeders, who have financial means and interest to pursue a diagnosis.
Another bias is that individual laboratories and pathologists may vary in the criteria they use to reach a diagnosis, which is nearly impossible to sort out with this type of study.
The time frame of the research is important. By choosing a recent 13-year period, the researchers have selected a time frame that includes modern diagnostic methods, including molecular diagnostic techniques that weren’t available decades ago.
Another related result of this study is the lower number of abortions attributed to twin pregnancies. With modern ultrasound techniques, twin pregnancies in mares are readily diagnosed and one of the twin fetuses is sacrificed for the safety of the other fetus and the mare. Older studies had much higher proportions of abortion associated with twins, likely because ultrasound was not yet in widespread use by veterinarians.
By better understanding the factors that cause pregnancy loss in mares, we can hopefully prevent more cases from happening in the future.
If you are unfortunate enough to experience an abortion in one of your mares, try to collect and submit the placenta along with the fetus to increase the chances of a reaching a diagnosis.
