The number of diseases that can be controlled by vaccination in horses is relatively small compared with other species. Although the goal may be to prevent infectious disease in the individual horse, if a large enough percentage of horses in a given population can be immunized, then the disease will never reach epidemic proportions. Vaccination programmes are laid down taking into account the horse’s age, its use and the likely exposure level. Vaccination is important in pregnant horses to provide passive immunity in the colostrum. Most vaccines have a primary course of two or three doses followed by a half-yearly or annual booster. Young foals may be vaccinated quite early in life if they are born to unvaccinated dams. However, if they are born to vaccinated dams and have received a good passive immunity, then vaccination should be delayed. Passive immunity will interfere with the effectiveness of vaccination.

It is very important that practitioners follow the manufacturer’s advice, not only regarding storage but also frequency of administration, for any vaccine.

Certain manufacturers combine vaccines for ease of administration. These combinations have not been listed as they are so diverse and numerous. Practitioners are advised to thoroughly investigate the actual risk to their equine patients and vaccinate accordingly rather than follow a blanket approach, which may be more expensive and less effective. All countries, all areas and, indeed, all holdings are different. The clinician needs to be well informed of the availability of vaccines, likely diseases posing a risk and the legislation laid down by that country.

Although horses are an end host for this disease, vaccination is useful in endemic areas. Horses can be protected by a single dose of vaccine given at or after 3 months of age. Annual boosters are required. Mares can receive a booster dose before breeding or 4–6 weeks before foaling.

There are three types of encephalomyelitis – Eastern, Western and Venezuelan. It is a viral disease spread by mosquitoes, the reservoir hosts being rodents and reptiles. Vaccination should be timed to coincide with the time of maximum challenge in endemic areas. Vaccination can be started as young as 3 months. An initial three doses separated by 4–6 weeks should be followed by booster doses every 6 months. Pregnant mares should be boosted 4–6 weeks before parturition.

The tetanus bacterium is found worldwide. Vaccination should be encouraged in all animals, and can start as early as 4 months. The first injection should be followed by a second after 4–6 weeks. With some modern vaccines boosters are required only every 3 years; however, others may require more frequent boosters. Ideally, pregnant mares should receive a booster 4–6 weeks before foaling.