Understanding Foal Immunity <em>In Utero</em> and Beyond
Foals have a functional immune system at birth. Actually, they have a functioning immune system in utero—but it’s one appropriate to an unborn foal in a sterile and protected environment. Once that baby hits the real world, he needs real-world immunity.

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.

When bacteria, viruses, fungi, and parasites bombard the foal, his health is challenged. But that challenge also stimulates the immune system, helping it mature.

Over time, the from-the-mare antibodies decay, so as critical as they were for the newborn, the 4-month-old foal no longer benefits from them. By then, he’s building immunity on his own.

The caveat comes when foals fail to get adequate colostrum or the colostrum doesn’t contain those critical antibodies. In this scenario, neonatal foals are highly susceptible to bacterial infections. Veterinarians might recommend vaccinating those young foals to protect against disease. While the young foal’s immune system won’t make antibodies as effectively as an older foal’s would, no adverse long-term effects of early vaccination have been noted, and it will help the foal mount a defense.

Bottom line: Vaccinating the mare is the No. 1 thing you can do to ensure a foal has the best chance at fighting off most of the bacteria and viruses that he’ll encounter in his early days.