Immune Response and Early Pregnancy Loss in Horses

Even though a mare may be successful in conceiving after being bred, she may not still be in foal when checked a month or two later. This loss of pregnancy can be due to various factors including infection, morphologic defects in the embryo, or slowed embryonic growth. If the loss happens early in a breeding season, it is sometimes possible to schedule a later mating. Often, however, this mare will simply lose a year of production.

One possible cause of early pregnancy loss in mares is thought to be an overactive immune response. This might apply particularly to mares that repeatedly have early losses of pregnancy. A study led by Christine Aurich, D.V.M., a professor at the Graf Lehndorff Institute in Neustadt, Germany, examined the role of a specific kind of immune system cell that could be a significant factor in equine miscarriages.

Because it carries genetic material from the sire, an embryo that implants in the uterus is seen as foreign tissue by the dam’s immune system. This phenomenon is seen in all mammals as white blood cells are alerted to begin a defense against the intruder. In most cases, the pregnant female’s body begins a series of immune-related events designed to prevent rejection of the embryo. Special lymphocytes known as T regulatory cells, or Treg cells, are important in this interference with the normal rejection behavior. Led by Aurich, the research team investigated the action of Treg cells in equine pregnancies.

Using a group of 108 broodmares, the researchers tested Treg cell levels before the mares were bred. On day 40 after breeding, the mares were checked for pregnancy by ultrasound. Results showed that 17 of the mares had lost their embryos before that point. Tests of Treg cell numbers showed that these mares had significantly lower levels than mares that maintained their pregnancies and delivered live foals. Checking the reproductive histories of the mares showed that 91% of those that experienced early pregnancy loss had experienced the same problem in previous years. This led the team to suspect that embryonic abnormalities were probably not to blame; instead, the common factor was some condition in the mares that caused pregnancy terminations.

Differences in Treg cell levels were not great enough to be useful in successfully predicting which mares were likely to experience early pregnancy loss, according to the researchers. Not all mares that lose pregnancies have low Treg cell levels, and some mares with low levels are able to carry their foals to term. However, a breeder who knew that a particular mare had low levels might plan to breed earlier in the season, allowing time to rebreed if the mare lost an embryo from a first mating.

No approved treatment exists for raising the Treg cell count in broodmares. However, problem mares supplemented with progestin altrenogest have shown enhanced endometrial receptivity, and it is possible that this treatment also alters maternal tolerance, according to Aurich. Two recent studies have shown improvement if this treatment begins as early as day five after ovulation. Breeders should check with a veterinarian to learn more about this therapy.