Vaccination Program for Horses

The pathogen that causes tetanus, Clostridium tetani, is present in all parts of the world. Tetanus occurs most commonly when wounds become contaminated with the organism from the soil. Vaccination is recommended for all horses and ponies annually. A horse with an unknown vaccination status that sustains an injury should receive a dose of tetanus antitoxin along with a dose of tetanus toxoid. A second dose of toxoid should be administered 4 weeks later.

Foals from vaccinated mares should be administered a three-dose series at 6, 7, and 9 months old. Foals from unvaccinated mares should receive tetanus toxoid at 3, 4, and 6 months. Whenever a vaccinated horse experiences a serious wound, a tetanus booster may be indicated.

Two types of equine herpesviruses vaccine are available, EHV-1 and EHV-4. The EHV-1 virus is the primary cause of abortion and also neurologic disease in horses. The EHV-4 virus is the primary cause of respiratory infections, especially in young horses. The control of rhinopneumonitis is by a combination of vaccination and good management practices.

To prevent abortion, a killed EHV-1 vaccine should be administered to pregnant mares at 3, 5, 7, and 9 months of gestation. Respiratory disease is best prevented using a combination vaccine of EHV-1 and EHV-4. There is no evidence that vaccination protects against the neurologic form of the disease.

Vaccine recommendations for foals are three doses every 4 weeks starting at 6 months, and a booster at 1 year. Pleasure and performance horses should be vaccinated every 3–6 months, depending on the risk of exposure. Broodmares should have an EHV-4 vaccination 2–4 weeks before foaling to ensure the availability of colostral immunity.

Eastern equine encephalitis (EEE) virus has a wide distribution. This includes the eastern US, Central and South America, and eastern Canada. In the US, EEE occurs primarily in the southeastern states but has been reported in all states east of the Mississippi River. EEE in horses is nearly always fatal regardless of treatment. Infected horses usually become comatose, seizure, and die in 36–48 hours.

The Western equine encephalitis (WEE) virus is primarily located in the western US. In recent years, reports of WEE have not been common, probably as a result of adequate vaccination programs. The virus is not as pathogenic as EEE, and animals infected with WEE have a greater chance of survival. Mortality is usually 50%.

Venezuelan equine encephalitis (VEE) virus causes outbreaks of disease in horses in Mexico, Central and South America, and occasionally the southern US. The VEE vaccine is a single vaccine or in combination with EEE and WEE. Its need is limited to those horses traveling to or located in endemic areas.

West Nile virus (WNV) disease is considered a zoonotic disease similar to the encephalitis viruses. The bird population allows for maintenance of the virus, which is then transmitted by mosquitoes to both people and horses. There is very little risk of contracting the disease from any infected horses. Vaccination protocols for broodmares, foals, and adult horses are the same as those for Eastern and Western encephalitis. Combination vaccines that include EEE, WEE, and WNV are available.

Influenza is one of the most frequent causes of viral respiratory disease in horses and is highly infectious among susceptible horses. Therefore, vaccination is recommended for all foals, broodmares, and horses at risk of exposure, usually as a result of showing, racing, or shipping. Young foals from vaccinated mares, because of maternal antibodies, should be vaccinated at 9, 10, and 12 months. Foals from unvaccinated mares should be vaccinated earlier. The intranasal vaccine can be administered every 6 months in young, susceptible performance horses 2–4 years old. Vaccination is recommended every 6 months for horses exposed to other horses at equine events.

Clinical signs of rabies include the inability to eat or drink, disorientation, and incoordination. Rabies is fatal and also poses a health risk to those who handle infected horses. Prevention in horses is primarily via vaccination. Broodmares should be vaccinated 4–6 weeks before foaling. Foals from vaccinated mares should be vaccinated at 6 and 7 months old and again at 12 months. Foals from unvaccinated mares should be vaccinated at 3, 4, and 12 months. All adult horses should be vaccinated annually.

Potomac horse fever (equine monocytic ehrlichiosis, equine ehrlichial colitis) is due to Neorickettsia risticii. Vaccination is recommended in endemic areas in the US, such as near freshwater streams, rivers, ponds, and heavily irrigated pastures. Annual boosters in the spring are recommended. Pregnant mares should receive a booster before foaling. Although the vaccine may lessen the impact of the disease, it appears not to prevent abortion in pregnant mares.

Equine botulism is mostly seen in the Mid-Atlantic area of the United States, but the disease has also been reported worldwide. The spores of Clostridium botulinum are found in the soil and are resistant to light, drying, and heat. In the US, the most common types are B and C botulism, although type A has been reported in the western US.

The type B vaccine gives good protection in adult horses. Adult horses and broodmares in areas where botulism is a potential hazard should receive three doses at 30-day intervals initially and then annual boosters. Foals from vaccinated mares in endemic areas should be vaccinated at 2, 3, and 4 months old, because colostral immunity does not always occur. Foals from unvaccinated mares should be vaccinated at 2, 4, and 8 weeks. The use of type B vaccine in areas where type C occurs has uncertain results.

S equi infection (equine strangles, equine distemper) is highly infectious. Use of the IM or intranasal vaccine is recommended only in those situations or on premises where the disease has been a problem or for horses at risk (sharing water or nose-to-nose contact at shows/sales, etc). The IM vaccine involves three injections administered 2–4 weeks apart, starting at 4 months old in foals, with a booster at 12 months. If the intranasal vaccine is used, vaccination can begin at 6–9 months, with a second dose administered 3–4 weeks later and a third dose at 12 months.

Adult horses, if in potential disease situations, should receive an annual vaccination. Broodmares on endemic farms should receive an annual booster using the IM vaccine 4–6 weeks before foaling. Because of the increased risk of inducing immune-mediated purpura hemorrhagica, horses with titers to the SeM surface protein of S equi in excess of 1:1,600 may not need to be vaccinated.

Foal diarrhea as a result of rotavirus infection can be a severe problem on some breeding farms. Single cases or a farm outbreak can occur. Therefore, pregnant mares should be vaccinated IM at 8, 9, and 10 months of gestation. This will increase the amount of colostral immunoglobulins. This vaccine is for use in particular situations or areas of high endemicity.

In the acute state, equine infectious anemia (swamp fever) causes severe RBC destruction and anemia, and no vaccine is available. Once infected, a horse can become a carrier for life and is a threat to other horses. The virus is transmitted by bloodsucking insects. Currently, the disease is uncommon, and most horse owners are aware of its dire consequences.

A simple blood test, the Coggins test, is available to detect infected horses. A horse that tests positive cannot cross state lines and is required to be maintained in strict isolation for life. For horses participating in equine activities or being transported across state lines, proof of a negative Coggins test is required. Annual testing of every horse is recommended.