Does Your Vaccine Program Need an Overhaul?

How would you like to spend less money, yet do a better job of protecting your horse against disease? Ongoing research suggests that traditional vaccination schedules might not be protecting our foals adequately. There is even controversy over the possibility that the current protocol of vaccinating foals early and too often could keep them from reaching their full potential to resist disease later as adults. Why can't experts just determine the best time to begin a foal's initial immunization series and write easy-to-follow guidelines for everyone?

First of all, laboratory methods in immunology can only measure antibody levels, not how protective those levels are. Second, it takes a great deal of time to test vaccine efficacy on real animals. Third, certain diseases are more common in certain geographic areas, making vaccination recommendations somewhat different for horses in various parts of the country.

What research at several universities is telling us now is that maternal antibodies in the foal (those obtained from the mare via the colostrum) might last longer than previously thought. Veterinarians have known for years that foals will not produce their own active antibodies following a vaccination if there are still antibodies from the mare present in their systems. Current studies have demonstrated the presence of maternal antibodies in foals as old as nine months, whereas before they were thought to diminish at three or four months of age. Therefore, vaccines administered before nine months might not stimulate an immune response in the foal.

A review of basic immunology is helpful for understanding this dilemma. Many farm animals, including the horse, receive no antibodies from the mother while in the uterus. Youngsters must acquire antibodies from the mare's first milk (colostrum). The quality and quantity of these maternal antibodies depend on the mare's immune status and the ability of the foal to absorb the antibodies. A mare which has been vaccinated for the appropriate diseases 30 days before foaling will provide more protection to her foal than a mare which was not properly boostered. Likewise, healthy foals which nurse within the first 24 hours of life will be better protected from disease than foals which fail to nurse or have poor antibody absorption because of illness.

Testing the one- to two-day-old foal for antibody absorption and supporting those whose antibody levels are deficient with either colostrum or plasma (depending on the foal's age) is prudent.

While these maternal antibodies are important in protecting foals from disease in the short term, they interfere with or delay our ability to protect foals from disease in the long term. Vaccines (as well as actual diseases) possess antigens, the substances that stimulate the body's immune system to produce its own antibodies. Maternal antibodies treat vaccine antigens like the real disease, "scooping them up" before they have a chance to stimulate the foal's immune system. The problem is knowing when the level of maternal antibodies in the foal is low enough to allow vaccines to be effective without being so low that the foal is unprotected from disease. Unfortunately, even when no maternal antibodies can be detected in a foal, some mechanism or component of the maternal antibodies still remains, which interferes with the foal's response to vaccination.

Current research recommends delaying foal vaccinations until it is believed maternal antibodies are greatly diminished. Researchers suggest owners begin vaccinating foals against influenza no earlier than nine months of age if using the modified live product, and waiting until 10 to 12 months of age if using killed products. Although studies on encephalitis vaccinations are less conclusive, early reports recommend waiting until foals are seven to nine months of age before beginning the initial series.

Recent findings indicate that foals of properly immunized mares also fail to respond to tetanus, rabies, and botulism vaccines when given before nine months of age.

It is important to remember that vaccine research recently has become a top priority at several universities, and recommendations might change quickly. That is why your local veterinarian is an invaluable source of information and advice regarding your specific vaccination and other preventive health care programs. Take a few minutes at the beginning of each year to sit down with your veterinarian and revise your vaccine schedule. The time you spend might save your wallet, and your horse's health.

About the Author

H. Steve Conboy, DVM

H. Steve Conboy, DVM, a 1970 graduate of the University of Georgia College of Veterinary Medicine, has been an active AAEP member since 1972. A private practitioner in Lexington, Ky., he has assisted in vaccine research at the University of Kentucky and has studied foals' response to influenza and strangles vaccines for many years.

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