EQUINE | Equine Disease Update
Foal Health
Article reprinted from The Horse, March 2017, page 37. The
Horse is published by The Horse Media Group LLC.
Maureen Gallatin ;
Stacey Oke, DVM, MSC
Understanding Foal Immunity
F
oals have a functional 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.
David W. Horohov, PhD, of the University of Kentucky
Maxwell H. Gluck Research Center, in Lexington, de-
scribed aspects of mare and foal immunity that are use-
ful in everyday management.
With six layers of placenta separating the mare’s cir-
culation from the fetus, only small molecules can get
through to the foal. Large proteins, such as antibody
molecules, cannot. A lack of antibodies leaves the new-
born foal unprotected against infections. So it 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 de-
pends on the mare’s antibody levels, Horohov said. So,
boosting her antibodies by vaccinating according to
AAEP guidelines helps assure they’re sufficient. Mares
also develop antibodies as they encounter pathogens in
the environment, which ultimately helps foal immunity.
When bacteria, viruses, fungi, and parasites bombard
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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 once were, the 4-month-old foal no lon-
ger benefits. By then, he’s building his own immunity.
The caveat comes when foals don’t get adequate colos-
trum or the colostrum doesn’t contain those critical anti-
bodies. In this scenario, neonates are highly susceptible
to infections. Veterinarians might recommend vaccinat-
ing 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 him 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 he’ll encounter in
his early days.
Evaluating Neonatal Foals and Postpartum
Mares at Foal Heat
Newborn foals must transition from dependence on the
dam’s body to doing everything on their own. They have
to make huge physiologic changes to simply survive—
by beginning to breathe, digest milk, fight disease (as
• Equine Health Update •