The horse has an excellent immune system, on par with any other species. However, when nose-to-nose with a new pathogen (virus or bacteria), the horse's body has no defenses lined up. Without immunity, the horse will succumb to the full range of symptoms. Routine vaccination should be part of a comprehensive health maintenance program for all horses. The objectives of vaccination can be listed into three categories:
How Vaccines Work
Vaccines give your horse a specific immune response. A specific defense is one that recognizes and promptly destroys the invader. An initial vaccination alerts specific components of the horse's immune system, which leads to a much more rapid and effective response to the second exposure (booster) to the vaccine.
After vaccination, the concentration of antibodies (the "titer") eventually diminishes. Usually this takes 2 to 6 months or longer after several boosters. Boosters are generally given one time per year. However, horses that travel frequently should be boosted more often, for example every 3 months.
What Vaccines are Recommended for Adult Horses?
Most horses in the United States are regularly vaccinated against tetanus, influenza, eastern and western equine encephalomyelitis, rabies and rhinopneumonitis. Newer intranasal influenza vaccines are highly effective. Vaccines are also currently available to confer protection against strangles, botulism and Potomac horse fever. These vaccines are recommended in areas where these diseases are problems. A newer intranasal strangles vaccine appears to be effective assuming proper placement of the vaccine in the nasal cavity.
Despite the recent appearance of a vaccine for equine protozoal myelitis (EPM), we do not recommend it until there is further evidence that it is both safe and effective.
Vaccines should be administered by a veterinarian, as there can be serious mistakes in administration and occasionally allergic reactions. Your veterinarian is prepared to deal with these problems appropriately.
Special Consideration for Broodmares and Foals
Planning for the vaccination of foals should begin when the foal is developing inside the mare. Following birth, the newborn foal is essentially devoid of antibodies to fight infection. The foal derives all of his immunity by ingesting the mare's first milk (colostrum), which is a rich source of antibodies.
By vaccinating the brood mare on a regular basis, the colostrum will contain antibodies against the important diseases. It is recommended that the mare receive a booster vaccination about one month prior to her due date. In the case of rhinopneumonitis, vaccination every other month during pregnancy, starting at 3 or 5 months is recommended.
Foals should not be vaccinated until after these maternally derived antibodies have been depleted, since they block foal production of antibodies. It is generally recommended that, for most diseases, foal vaccination should not start any sooner than 8 to 10 weeks of age, and newer information suggests waiting until 6 months. Vaccinating young foals before 8 to 12 weeks fails to stimulate antibody production and may be stressful (painful) for the foal.
It is recommended that the horse owner should maintain good records of their horses' vaccinations. Before being allowed to compete at many open horse shows, the rider must often be able to demonstrate (using a vaccine certification passport that has been signed by a veterinarian) that the horse has been vaccinated on a regular basis. Needless to say, this is especially important with respect to the respiratory diseases. When purchasing or selling a horse, a well-maintained vaccination record will help to demonstrate to prospective purchasers that the horse's health has been carefully protected and there will be no need to start a new vaccination program all over again.
Why Not Just Measure Antibody Titers Before Deciding to Vaccinate?
Recently, there is much discussion on taking titers rather than just giving boosters. This is not well founded. Vaccines are generally short-lived, and not effective after a few months, so you shouldn't vaccinate less. A titer, which generally will be quite low after vaccination, will be hard to interpret. No one knows the titer that is "protective" against each disease – it might be small some times, and greater in the face of an outbreak. The treatment of the disease in an unvaccinated animal can be prolonged and more expensive. And last, in some cases the unvaccinated animal will die from the disease (e.g. tetanus, encephalitis), which was wholly preventable. It is important to stay with the program, even if the vaccines aren't perfect.