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Vaccination of Horses

By: Dr. Philip Johnson

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Vaccination programs should be carefully planned by the veterinarian to suit the needs of individual horses based on local knowledge of the infections that are occurring out there. Certain diseases are more important in some geographical locations than others.

The need to vaccinate young horses is greater, since they are often being mixed with other young horses (of unknown vaccination background), for example at sales, auctions, breeding farms, and training centers. Certainly, over time, most healthy horses develop substantial immunity against common pathogens in the absence of vaccination. This immunity develops as a result of natural exposure. Unfortunately, natural exposure can result in a severe illness in some cases, most obvious in tetanus, encephalitis and rabies.

If you are thinking of vaccinating your horse, the prior history of vaccination is invaluable. If that prior vaccination history is unknown, you should start with the assumption that the horse has never been vaccinated before.

For most vaccines, the program has to entail the administration of at least two doses separated by several weeks. The first vaccination is a primary dose and the subsequent treatments (within the space of a few weeks) are secondary doses or boosters. The first dose acts to prime the immune system in the short term, but does not generate much of a response. It's like the first time you hear something in school – the concept is planted in your brain, but you need to go back and study it again or memory fades. Boosters come out of the same bottle and are identical doses of vaccines as the primary dose.

However, boosters massively stimulate the immune system, since the system is primed and it "remembers" the antigen. After the booster, the body produces billions of copies of the right information that it needs to make the right antibodies. Lymphocytes, the white blood cells that carry this information and make antibodies, multiply ("clonal expansion") and lodge in tissues, on body surfaces, and in the blood, just waiting for the foreign antigen to attack. Unless it's a Trojan horse, it is met by the antibodies. Because of the greater response from the booster, the antibodies last for several months. Without boosters, a horse would have little, or short-lived protection.

The concentration of antibodies (the "titer") in the blood gradually diminishes over time. Following the initial stimulus of initiation of immunity using a primary and booster vaccination program, the horse's immunity must be maintained on a regular basis using boosters. Boosters are generally given one time per year. However, horses that travel frequently should be given boosters more often, for example every three months. The frequency with which horses should have their immunity boostered varies between vaccines, local geographical need (relative risk of exposure), and the use and age of the horse. Your veterinarian is skilled at devising a vaccination program that is tailored to your horse's needs.

Antibody Titers

Recently, there is much discussion on taking titers rather than just giving boosters, but this is not well founded. Titers are the quantity of a substance needed to react with or to correspond to a given amount of another substance. Vaccines are generally short-lived, and inefficacious after a few months, so you shouldn't vaccinate less. A titer, which is generally quite low after vaccination, is hard to interpret.

  • Antibodies are not the only defense, so titers do not estimate the immune response from the arm of the immune system that is independent of antibodies.

  • No one knows the titer that is protective for each commercial vaccine. In a slight infection, the horse might need a few antibodies. In a serious exposure to a nasty strain of the pathogen, a greater number of antibodies might be needed. We also do not know what titer is protective. This is because it is not a requirement to show the protective level, for the approval of a vaccine. Indeed, one does not even need to show that vaccines are efficacious, so interpreting titers is a shot in the dark.

  • You will spend a lot of money testing the titers of all the potential pathogens, and this will cost much more than the vaccines themselves.

  • If you delay vaccination based on titers, your horse will probably not be adequately protected and you will be in part responsible for the spread of the disease.

  • The treatment of the disease in an unvaccinated animal can be more expensive.

  • 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.

    Special Consideration for Broodmares and Foals

    Planning for the vaccination of foals should begin when the foal is developing inside the mare. An excellent health maintenance program for the brood mare should include the regular use of appropriate vaccines. Following birth, the newborn foal is essentially devoid of antibodies to fight infection. The foal derives all of its immunity by ingesting the mare's first milk (colostrum) which is a rich source of antibodies. These colostral antibodies operate in the foal's blood to defeat pathogens during the first several weeks of life.

    By vaccinating the brood mare on a regular basis, the colostrum will contain antibodies against the important diseases. It is recommended that the mare receives 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 and replaced with the foal's own antibodies. It is generally recommended that, for most diseases, foal vaccination should not start any sooner than 8 to 10 weeks of age. Vaccinating young foals before this time fails to stimulate antibody production (the maternally derived antibodies neutralize the vaccine) and may be stressful (pain) for the foal.

    New information suggests that, for protection against the respiratory pathogens, vaccination should not be started until after 6 months of age. Ideally, vaccine should not be given to youngsters until after the maternally derived antibodies have disappeared. The time at which these antibodies disappear has not been known until recently and is currently a focus of much investigation.

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