Foals have a functional immune system at birth. | Photo: Thinkstock

The science behind immunity is complex, but the lessons it teaches us can be very practical. At the 2016 American Association of Equine Practitioners (AAEP) Convention, held Dec. 3-7 in Orlando, Florida, David W. Horohov, PhD, of the University of Kentucky Maxwell H. Gluck Research Center, in Lexington, described aspects of immunity that are useful in everyday mare and foal management.

With six layers of placenta separating the mare’s circulation from the fetus, only small molecules can get through to the foal. Large proteins, such as antibody molecules, cannot. And while a newborn foal’s immune system is competent in that all the players are present, aspects of it are immature. The lack of antibodies leaves the foal unprotected against bacterial and viral infections. As a result, the foal depends on the mare’s colostrum (her antibody-rich “first milk”) to provide the necessary antibodies through a process called passive transfer.

Of course, the amount of antibodies the foal gets depends on the mare’s antibody levels, Horohov said. So, boosting her antibodies by vaccinating her according to AAEP guidelines helps assure sufficient antibody levels. Mares also develop antibodies as they are naturally exposed to pathogens in the environment, which ultimately helps foal immunity