Vaccination of Horses

Routine vaccination should be part of a comprehensive health maintenance program for all horses. The objectives of vaccination fall into three categories:

If you have only one horse, then the important aspect is disease prevention; but if you own a big farm, you will reduce the impact of an outbreak.

How Vaccines Work

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 specific defense. In this instance, the horse succumbs to the full range of symptoms. An immune reaction against the infection occurs, and substantial damage may have occurred already.

What you want is a specific immune response, which is one that is directed at the offending pathogen, that recognizes it, and prompts its destruction. Vaccination is intended to prime specific components of the horse's immune system such that, on subsequent exposure to a specific pathogen, the horse is able to mount a much more rapid and effective response to prevent or minimize the clinical symptoms.

Vaccines Currently Available

Most horses in the United States are regularly vaccinated against tetanus, influenza, eastern and western equine encephalomyelitis, rabies and rhinopneumonitis. Vaccines are also currently available to confer protection against viral arteritis, strangles, botulism and Potomac Horse Fever. Under special circumstances, other less commonly used vaccines may be considered. 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.


The route of vaccination depends on the product, but vaccines are available for intramuscular, and more recently, intranasal administration.

Intramuscular Injection

Most vaccines used in horses are administered by intramuscular injection. There are good reasons for this. First, the skin and subcutaneous tissues of horses are very sensitive to most vaccines and react with a nasty inflammatory reaction if they're given too close to the skin. Second, the blood supply in muscles is excellent, so the vaccine antigens can be picked up by cells that are in close contact with the bloodstream. Vaccines must be given in muscle away from solid structures like ligaments, tendons and bones. A common mistake is to give a vaccine too high in the neck, where it goes into the nuchal ligament rather than muscle.

Vaccines administered in the muscle stimulate the production of antibodies that circulate in the blood and prime the immune system to heighten its level of preparedness for future interactions with specific pathogens.

Intranasal Administration

Whereas most vaccines cause an immune reaction everywhere in the body, some newer vaccines have been developed to stage a defense in a particular part of the body, such as the respiratory system. These vaccines, administered via the intranasal route, may improve immunity against pathogens that target the respiratory system and are intended to provide the lining of the respiratory tract with protective ("neutralizing") antibodies at the entrance site of the pathogen.

Vaccines against both strangles and influenza have been made available for administration to horses via the intranasal route. Although these vaccines have demonstrated some efficacy in field trials, we do not know how long they are effective. To rely on them for protection for over 6 months is too much to expect, so it is important to vaccinate at key times or vaccinate more frequently.

Record Keeping

It is recommended that horse owners 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. This is especially important with respect to the respiratory pathogens.

When purchasing or selling a horse, a well-maintained vaccination record will help 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.

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:

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.

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.

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.