Age-related changes in the equine immune system haven’t been extensively studied, but it is known that the function of T-cells---a type of white blood cell integral to fighting pathogens---is reduced in older horses. As a result, an elderly horse cannot respond to infections as quickly as he might have when he was younger, which makes him more susceptible to disease and slower to recover when he does become ill. Compromised immune function also means that an old horse may not get full protection from vaccination and may be at a higher risk for significant internal parasite infestations. What you can do: Keep your aging horse on a consistent and appropriate vaccination schedule. His specific immunization needs depend on his lifestyle and location, so work with your veterinarian to develop a customized plan. Parasite control is also important. Regular fecal egg count tests will be needed to determine how often your horse needs to be dewormed and with which anthelmintic agents.
Biosecurity measures will also help protect an older horse from disease. Quarantine new arrivals to your property until you are certain of their health status, and do not share buckets or other equipment among horses---especially when you travel with an older horse. When to worry: If an older horse develops a fever, runny nose or other signs of illness, call your veterinarian right away. Even minor ailments can hit an old-timer harder than his younger pasture-mates and you’ll want to intervene as soon as possible to ensure a quick recovery.
An older horse can’t respond to infections as quickly as he might have when he was young, which makes him more susceptible to disease and slower to recover when he does become ill.