Source: Equine Disease Quarterly
THE OLD SAYING “An ounce of prevention is worth a pound of cure” certainly applies to the current practice of vaccination in order to prevent infectious disease. Most horse owners recognize that it is much easier and less expensive to protect their animals through vaccination than it is to treat them once they become infected. For vaccination to be effective, however, it must be administered correctly and at the proper time. In terms of the adult horse, this means administering the proper dosage(s) well in advance of exposure to the infectious agent. This timing is particularly necessary for primary immunizations, since full protection may not be obtained until a month or more post-vaccination. More rapid protection will be seen following administration of a booster to a previously immunized horse, though even in that case complete protective immunity may not be in place until several days post-vaccination.
The need to vaccinate foals in order to protect them from infectious disease is also widely appreciated; however, the timing of the initial series of vaccinations is a contentious issue. Foal vaccination is complicated both by the immature status of the foal’s immune system and by the presence of maternal antibodies obtained through the ingestion of colostrum. The newborn foal, like its human counterpart, possesses a fully functional but naïve immune system. The neonate is thus capable of responding to a vaccine or infectious agent, but because this represents a primary immune response, it may take a month or more for protective immunity to develop. This delay of protective immunity post-vaccination is why colostral transfer of maternal antibodies is so important to the foal. The maternal antibodies provide the foal with protection for several months and allow time for the foal’s immune system to develop its own protective responses. Ironically, these antibodies can also inhibit the foal’s ability to respond to vaccination by eliminating the antigens in the vaccine before an immune response has the opportunity to develop.
This is particularly true in the case of modified live vaccines in which the infectious agents can be neutralized by maternal antibodies. This phenomenon of “maternal interference” is well recognized in puppies and kittens and is only now being recognized as a significant issue in foal vaccination. In light of these findings, it is now recommended that no vaccine be administered to foals before 4 months of age. The concentration of maternal antibodies in the foal’s blood diminishes by approximately 50% every month of the foal’s life, and by four months the levels should be low enough so that maternal interference is no longer a problem. It may also be necessary to administer at least three doses of vaccine in the primary series for foals in order to give the foal’s immune system adequate stimulation with the antigen. Vaccination prior to 4 months of age will likely be ineffective in inducing protective immunity in the foal and could potentially interfere with the immune system’s ability to respond later to the same antigens. A better strategy is to enhance the quality of colostral antibodies by vaccinating the mare four to six weeks prior to foaling.
Dr. David Horohov, (859) 257-4757, email@example.com
Department of Veterinary Science, University of Kentucky, Lexington, Kentucky.