Horses have a low birth rate, with many pregnancies failing within the first few weeks after conception. The reason is currently unknown but recent research by the team of Christine Aurich at the University of Veterinary Medicine, Vienna (Vetmeduni) suggests that a particular class of blood cells may be involved.
Finding the answers for low birth rate in horses
Horses have a low birth rate with many pregnancies failing within the first few weeks after conception and new research may have found the reason.
The fact that horses have a considerable number of pregnancies failing to progress beyond the first five weeks has not been well known. Now, although the explanation is still unclear researchers are suggesting it may relate to a problem with the mare's immune system.
The fertilized egg, or conceptus, contains contributions from the stallion as well as the mare, which means it somehow has to avoid being recognized and attacked by the mare's immune system.
Horse breeders talk about maternal tolerance of the conceptus and horses -- as other animals -- have a variety of mechanisms to enable fertilization and a successful pregnancy. Maternal tolerance probably depends on wide-ranging changes to the maternal immune response. But what makes some mares better than others at adapting their immune systems?
There are indications from work in humans and mice that a particular class of immune cell, the so-called regulatory T cells or Tregs (pronounced "tea regs"), might somehow be important in maternal tolerance.
Aurich and her colleagues have now developed a highly sensitive assay to measure the proportion of Tregs in the blood of horses. They used their new technique to investigate over 100 mares that were presented for artificial insemination, correlating the results with the outcome of the resulting pregnancies.
The scientists could find no association between the level of Tregs in the mares' blood with the success of the insemination procedure.
However, they did observe clearly lower numbers of Tregs in the blood of horses that suffered an early loss of pregnancy compared with those that either aborted much later (for whatever reason) or gave birth to live foals.
The results suggest that high numbers of Tregs might somehow be required to suppress the mare's natural immune reaction against the conceptus. In other words, low levels of Treg cells in mares might cause pregnancy loss in the five weeks after fertilization.
The differences in the levels of Tregs between horses in the "successful pregnancy" and the "unsuccessful" groups were too small to be useful to predict whether an individual mare is likely to suffer early pregnancy failure. Nevertheless, the results may help breeders solve the problem.
Evidence from human medicine shows that oestradiol may enhance the function of Tregs during pregnancy, and treatment with this hormone could possibly benefit mares prone to losing the conceptus.
Another possible course would be to expose mares to the stallion's semen before implantation, which might increase the number of Tregs in the blood and the animal's tolerance to the conceptus, thereby helping avoid early pregnancy losses.
Aurich is optimistic: "A number of reasons have been proposed to account for early pregnancy failure in the horse but our work suggests that a low level of Tregs may actually be among the most important factors. It can only be a matter of time before we find out how to solve the problem."