Some veterinarians hypothesize that overactive immune responses in mares can cause early pregnancy loss, and new research results show that a reduction in a certain kind of immune system cell could be a red flag for diagnosing equine miscarriage.

“Irrespective of the age of the mare, the majority of conceptus losses occurs in early pregnancy (until Day 35 after ovulation),” said Christine Aurich DVM, PhD, professor at the Graf Lehndorff Institute in Neustadt, Germany, and at the University of Veterinary Sciences in Vienna, Austria. “Morphologic defects, inadequate embryonic growth, or an inappropriate embryonic signal are suggested to cause failure of maternal recognition of pregnancy in the horse. However, early pregnancy loss often occurs repeatedly in the same mare, and it seemed worthwhile to look into immunological causes.”

In horses, as in all mammals, an implanting embryo carrying paternal cells is a foreign intruder in the uterus. However, pregnancy usually sets of a series of immune-related events that block the immune system from rejecting the embryo. Scientists have found one particular kind of lymphocyte (a type of white blood cell that pays a central role in directing and coordinating immune response), called “T Regulatory” (or “Treg”) cells, to be particularly helpful in allowing mammal mothers to tolerate their embryos instead of destroying them. So Aurich and her research team set out to investigate how Treg cells relate to the success or failure of equine pregnancies.

In their study the researchers tested 108 broodmares’ Treg levels just before pregnancy. The team checked the mares’ pregnancy status via ultrasound on Day 40 to determine which animals experienced early pregnancy loss. Pregnancy loss after 40 days in horses is not considered "early" and is not assumed to be related to the mare’s intolerance of her embryo, Aurich said