The University of Kentucky Gluck Equine newsletter indicates that because of better veterinary care and better nutrition, horses are living longer than ever. Almost 30% of the U.S. horse population is over 15 years old. Many of these older horses are still actively involved in equine athletic events and because of this, it is important to study the health aspects of these older horses. There are many age-related diseases of these older horses and a decline of immunity is one of them. A decline in immune function decreases the horse ability to resist infection and also it decreases the ability of horses to respond to vaccines.
Research is being performed to determine which formulations of vaccines may stimulate the best immunity in older horses because if the horse does not respond to the vaccine, the vaccine will not prevent disease. And different kinds of vaccines stimulate the immune system in different ways as killed vaccines work differently than live vaccines. Killed vaccines are processed and recognized by the immune system as foreign proteins and the body produces antibodies against the foreign protein but the body produces poor cell mediated immunity, which is much more effective at preventing certain diseases. On the other hand, modified live vaccines are processed as antigens that are already in the horse's body and mimic a viral infection. Modified live vaccines not only produce an antibody response but also produce strong cell mediated immunity. Because of this, it is possible older horses may respond better to vaccination with modified live vaccines instead of killed vaccines. If you have an older horse, talk with your veterinarian about the types of vaccines recommended for your horse.