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Vaccination of Horses

By: Dr. Philip Johnson

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Most vaccines contain two essential components: an antigen (or antigens) and an adjuvant. The antigen is that part of the vaccine that specifically mimics a "piece" of the pathogen. Antigens can be protein or other chemical fragments of the pathogen of interest, or they can be whole dead pathogens. Some viral vaccines contain live active viral particles that have altered enough to render them harmless, but they carry enough fragments of the original pathogen that they stimulate the immune system ("MLV", modified live vaccine). There are three kinds of vaccines:

  • Component vaccines, usually coupled with an "adjuvant" to enhance their effect
  • Killed vaccines, which contain killed but whole pathogens, usually mixed with an adjuvant
  • Modified live vaccines (MLV)

    Generally, MLV vaccines provoke the best immune response of the three.

    The immune system recognizes and reacts to the antigen contained in the vaccine if it's been exposed at least once before. This is referred to as an anamnestic response. The immune system has almost a perfect memory, except that it fades, sometimes within weeks to months for many antigens.

    Once your horse is vaccinated, if he is faced with actual disease or the pathogen, the immune system is already prepared to mount a rapid and sufficient response to impede the infectious process. Adjuvant is mixed with antigen to enhance the immune response to the vaccine. An enormous body of work has been undertaken to develop modern adjuvants, with the theory that vaccines will work much better in their presence. Without the adjuvant, the vaccine titers you get are smaller. Some adjuvants work better in one species than another.

    The practice of vaccinating animals with multiple antigens in a single treatment has been challenged recently. Using several antigens (for different diseases) in one vaccine might not yield the optimal response by the immune system. This may be due to the fact that only one or two adjuvants can be used for the vaccine, but an adjuvant cannot be picked that is highly suitable for all the components of the vaccine. One or another is bound to be short-changed.

    Interestingly, manufacturers are allowed to use some very modern and potent adjuvants in animal vaccines (including horse vaccines) that are not allowed to be used in vaccines produced for use in people. This has the advantage of stimulating a greater response, but the disadvantage perhaps of inciting more frequent side-effects.

    Vaccinating the Sick Horse

    You should not use vaccines in sick animals for protection against the disease it has or against other pathogens. First of all, all vaccines require time (several weeks) for the generation of a meaningful immune response. Secondly, the immune system of sick or stressed horses is not able to respond particularly well to vaccines. Sick horses are not able to deal with a reaction to the vaccine either. Therefore, the use of vaccine in the face of disease is illogical, because the affected horse will not be able to mount a satisfactory immune response to the vaccine, and the time delay prior to onset of vaccine-mediated immunity would not confer any immediate benefit.

    In The Face of an Outbreak

    This is a special situation that arises commonly. The classic example is the appearance of an animal on the farm with strangles. Isolation of that animal is crucial, but it will be difficult to guarantee completely that the other horses have not and will not be exposed. If the other horses are healthy, it is not unreasonable to embark on vaccination, assuming they've had the vaccine before and only require a booster.

    This theory was tested in a large field trial using an intramusclar strangles vaccine. Horses in the early phase of an outbreak, when less than 5 percent horses were infected, received vaccination, followed by biweekly boosters. The rate of infection was 20 percent for the vaccinated, and 70 percent for the unvaccinated. The live intranasal vaccine will give a local immune response in the nose quickly as well, and may be another option for vaccination during the earliest stage of an outbreak of strangles.

    This practice will probably not work for influenza because the antigens change so quickly that there is a higher likelihood that an ineffective antigen will be introduced. This is a controversial area because there is no research to support the practice of vaccinating during an outbreak either way. Many veterinarians are reasonably uncomfortable vaccinating horses when a few are sick on the farm, because the next day, a horse that was vaccinated might also be due to contract the infection, and feel even sicker because of the vaccine. More studies are needed in this area.

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