Multivariable analysis to determine risk factors associated with abortion in mares

Risk factors associated with equine reproductive efficiency have been identified along with those associated specifically with early pregnancy loss (EPL). In contrast, no studies have reported risk factors associated with abortion (loss between days 70 and 300 post-cover). Given the causes of abortion differ from those of EPL, likely too will the risk factors. A retrospective cohort study was carried out to identify risk factors associated with abortion in UK- and Irish-based Thoroughbreds, collecting data on 20 exposure variables over a 5-year period. A generalized linear mixed model was utilized to evaluate the associations between exposure variables and abortion, with clustering of observations accounted for at the mare and farm level. Variables with a likelihood ratio test (LRT) P value < 0.2 were entered into the model in a forward stepwise approach. Pregnancy outcome was available on 4439 pregnancies from 2510 mares. Having had two or more prior abortions (odds ratio (OR): 7.91, 95% CI: 2.86, 21.88), conceiving on the second or subsequent covered estrous cycle (OR: 1.84, 95% CI: 1.22, 2.78) and conceiving multiple conceptuses (OR: 1.68, 95% CI: 1.02, 2.76) were associated with an increased risk of abortion compared to null parous, first estrous cycle covers and singleton conceptions, respectively. Increasing paternal age (OR: 0.95, 95% CI: 0.90, 0.99) was associated with a decreasing risk of abortion. Mare and farm variance were not significant in the final model, LRT P = 0.43. These findings provide evidence-based data to inform Thoroughbred breeding management practices to help mitigate abortion risk. LAY SUMMARY: This is the first study to identify the risk factors (characteristics which change the chance of an event) for abortion (miscarriage between days 70 and 300 of pregnancy) in the horse. Statistical models were used to account for the interactions between 20 different factors. The factor which increased the mare’s risk of having an abortion the most was when she had had two or more abortions prior to the pregnancy. Additionally, when the mare was initially pregnant with twins but one of those pregnancies was reduced, the remaining pregnancy was at an increased risk of aborting. Older mares were not at an increased risk of abortion like in humans; however, pregnancies fathered by older stallions were less likely to abort than those from younger stallions. The findings of this study can inform horse breeding practices to help reduce the chance of an abortion.