Researchers debate whether physical changes in older horses are the source of declining immune function or the result of it, referred to as inflamm-aging or immunosenescence. | Photo: iStock

Keep your horse’s protective immune system functioning at every stage of his life

Do you ever wonder how it is that you travel to clinics and events away from home and, for the most part, your horse returns in good shape, with not even a sniffle? It’s largely due to a well-functioning immune system defending him from a world teeming with microorganisms. Many factors affect your horse’s ability to mount an effective immune response, one of which is his age. Basic immune mechanisms are similar in foals and adults, but cell response, regulation, and response to pathogens and vaccines do differ in these populations. In this article we’ll examine immunity across all ages.

Priming Immune Systems for Life

A newborn foal acquires protective immunity from antibodies (proteins that target and eliminate foreign bodies called antigens) he obtains from his dam’s colostrum (first milk), through a process called passive transfer. The antibodies are ones the mare has produced in response to her environment and to immunizations received in the last month of pregnancy. The protein molecules in colostrum can pass easily through the intestinal tract lining immediately after birth. By 12 to 24 hours, however, specialized cells in the foal’s intestinal lining no longer pass these large molecules through into the bloodstream. Breeders can have their veterinarians determine the extent of foals’ protection via a blood test that measures concentrations of immunoglobulin G (IgG) antibodies in serum.

“Foals are capable of producing antibodies to foreign antigens while in the womb, starting around nine months gestation,” says David Horohov, PhD, director of the University of Kentucky’s Gluck Equine Research Center, in Lexington. “Once born, they have the capability of responding to any environmental antigen they encounter, but with two limitations. First, the ability to make specific subclasses of IgG is impeded for the first couple of months by the low expression of certain cytokines (molecules that affect and modulate the behavior of other surrounding cells, such as those necessary to mount an immune response). Second, and more importantly, the presence of maternal antibodies can interfere with a foal’s own immune system’s ability to