Vaccinations are a vital part of proper equine health management. Few things will protect your horse from the devastation of disease as effectively as immunizations. Vaccinations provide a protective barrier between your horse and a number of infectious diseases. A good immunization program is essential to responsible horse ownership; however, vaccination does not guarantee 100% protection. There are situations where immunizations may decrease the severity of disease but not prevent it completely. Vaccination involves the injection of bacteria or viruses that are inactivated or modified to avoid causing actual disease in the horse. There have been developments of intranasal vaccines as well. Two or more doses are usually needed to initiate an adequate immune response. Once immunization is complete, protective antibodies in the blood guard against invasion of disease. Unfortunately, over time these protective antibodies gradually decline, requiring a “booster” to return immunity to an acceptable level. The specific immunizations needed by a particular horse depend upon several factors – environment, age, use, exposure risk, and general management.

Core Vaccinations

The AVMA defines core vaccinations as those “that protect from diseases that are endemic to a region, those with potential public health significance, required by law, virulent/highly infectious, and/or those posing a risk of severe disease. Core vaccines have clearly demonstrated efficacy and safety, and thus exhibit a high enough level of patient benefit and low enough level of risk to justify their use in the majority of patients.” The following equine vaccines meet these criteria and are identified as ‘core’ in these guidelines.

Rabies

Rabies is an infrequently encountered neurologic disease of equids. While the incidence of rabies in horses is low, the disease is invariably fatal and has considerable public health significance. It is recommended that rabies vaccine be a core vaccine for all equids. Exposure occurs through the bite of an infected (rabid) animal, typically a wildlife source such as raccoon, fox, skunk, or bat. Bites to horses occur most often on the muzzle, face, and lower limbs. The virus migrates via nerves to the brain where it initiates rapidly progressive, invariably fatal encephalitis.

Tetanus

Also known as “lockjaw,” tetanus is produced by toxin-producing bacteria that are present in the intestinal tract of many animals. It is found in abundance in the soil, where its spores can exist for years. Spores can enter the body via wounds, lacerations, or umbilicus of newborn foals. Although not directly contagious from horse to horse, tetanus does indeed pose a constant threat to horses. Symptoms include muscle stiffness and rigidity, flared nostrils, hypersensitivity, prolapsed 3rd eyelid, and legs held stiffly in a “sawhorse stance.” As disease progress, the muscles of the jaw and face stiffen, preventing the animal from eating/drinking. More than 80% of affected horses die. All horses should be annually immunized against tetanus.

West Nile Virus

West Nile is a neurological disease that affects horses throughout the continental United States. It is transmitted through the bite of an infected mosquito. Areas with more persistent mosquito populations may require more aggressive vaccination along with aggressive mosquito control techniques. Horses represent 96.9% of all reported non-human mammalian cases of WNV disease. The case fatality rate for horses exhibiting clinical signs of WNV infection is approximately 33%. Horses and humans are considered to be dead-end hosts for WNV; the virus is not directly contagious from horse to horse or horse to human. Indirect transmission via mosquitoes from infected horses is highly unlikely as these horses do not circulate a significant amount of virus in their blood.

Eastern/Western Encephalomyelitis

More commonly known as “sleeping sickness,” this disease is caused by Eastern or Western Equine Encephalomyelitis (EEE & WEE). WEE has been noted throughout North America, while EEE appears only in the East and Southeast. The disease is most commonly transmitted by mosquitoes, which have acquired the virus from birds and rodents. Humans are also susceptible when bitten by an infected mosquito; however, direct horse-to-human or even horse-to-horse transmission is very rare. Early signs include fever, depression, and appetite loss. As the disease progresses, infected horses may develop paralysis. All symptoms result from degeneration of the brain. The death rate of animals infected with EEE is 75-100%, while the death rate of animals infected with WEE is 20-50%. All horses need to be vaccinated for EEE/WEE at least annually. Pregnant mares and foals may require additional vaccination. The best time to vaccinate is one month before mosquitoes become active. In the southeastern United States, it may be necessary to administer a booster shot every 4-6 months to ensure extra protection year-round.

Risk-Based Vaccinations

These are vaccinations included in a vaccination program after the performance of a risk-benefit analysis. The use of risk-based vaccinations may vary regionally, from population to population within an area, or between individual horses within a given population.

Equine Influenza

Equine influenza is one of the most common respiratory diseases in horses. The highly contagious virus can be transmitted by aerosol from horse-to-horse over distances as far as 30 yards. Signs to watch for include a dry cough, nasal discharge, fever, depression, and loss of appetite. Influenza is not only expensive to treat, but also results in a great deal of “down time” and indirect financial loss. Horses that travel or exposed to other horses should be regularly immunized against influenza. Duration of protection is short-lived due to the constantly changing nature of influenza viruses. Therefore, it is recommended that horses be revaccinated every 3-6 months (depends on the vaccine administered).

Rhinopneumonitis/Herpesvirus

Equine herpesvirus type 1 (EHV-1) and equine herpesvirus type 4 (EHV-4) are two distinct viruses that cause two different diseases. Both cause respiratory tract issues. EHV-1 may also cause abortion, foal death, and neurological signs. Infected horses may show signs such as fever, lethargy, nasal discharge, cough and loss of appetite. Rhinopneumonitis is spread by aerosol and by direct contact with secretions, utensils, or drinking water. All pregnant mares must be immunized. Foals, weanlings, yearlings, and young horses under stress should also be vaccinated. Duration of immunity is short; therefore, pregnant mare should be vaccinated at a minimum during the 5th, 7th, and 9th months of gestation. Young horses at high risk should receive a booster at least every 3-4 months.

Botulism

Botulism has been observed in horses as a result of the action of potent toxins produced by the soil-borne, spore-forming bacteria, Clostridium botulinum:

Wound botulism results from vegetation of spores of Cl. botulinum and subsequent production of toxin in contaminated wounds.
Shaker Foal Syndrome (toxicoinfectious) results from toxin produced by vegetation of ingested spores in the intestinal tract.
Forage poisoning results from ingestion of preformed toxin produced by decaying plant material, including improperly preserved hay or haylage, or animal carcass remnants present in feed.
Equine Grass Sickness (Equine Dysautonomia) is considered a form of botulism resulting from the overgrowth of Cl. botulinum type C in the intestinal tract.

Botulinum toxin is the most potent biological toxin known and acts by blocking transmission of impulses in nerves, resulting in weakness progressing to paralysis, inability to swallow, and frequently, death. Of the 8 distinct toxins produced by sub-types of Cl. botulinum, types B and C are associated with most outbreaks of botulism in horses.

Potomac Horse Fever

Equine monocytic ehrlichiosis is caused by Neorickettsia risticii (formerly Ehrlichia risticii). The disease is seasonal, occurring between late spring and early fall in temperate areas, with most cases in July, August, and September at the onset of hot weather. Clinical signs are variable but may include: fever, mild to severe diarrhea, laminitis, mild colic, and decreased abdominal sounds. Uncommonly, pregnant mares infected with N. risticii (usually in the middle trimester between 90 and 120 days) can abort due to fetal infection at 7 months of gestation. If Potomac Horse Fever has been confirmed on a farm or in a particular geographic area, it is likely that additional cases will occur in future years. Foals appear to have a low risk of contracting the disease. Vaccination against this disease has been questioned because field evidence of benefit is lacking. Proposed explanations for this include lack of seroconversion and multiple field strains whereas only one strain is present in available vaccines.

Strangles

Streptococcus equi subspecies equi (S. equi var. equi) is the bacterium which causes the highly contagious disease strangles (also known as “distemper”). Strangles commonly affects young horses (weanlings and yearlings), but horses of any age can be infected. Vaccination against S. equi is recommended on premises where strangles is a persistent endemic problem or for horses that are expected to be at high risk of exposure. Following natural infection, a carrier state of variable duration may develop and intermittent shedding may occur. The organism is transmitted by direct contact with infected horses or sub-clinical shedders, or indirectly by contact with: water troughs, hoses, feed bunks, pastures, stalls, trailers, tack, grooming equipment, nose wipe cloths or sponges, attendants’ hands and clothing, or insects contaminated with nasal discharge or pus draining from lymph nodes of infected horses. Streptococcus equi has demonstrated environmental survivability particularly in water sources and when protected from exposure to direct sunlight and disinfectants, and can be a source of infection for new additions to the herd. Infection by S. equi induces a profound inflammatory response. Clinical signs may include fever (102-106F); dysphagia or anorexia; stridor; lymphadenopathy (+/- abscessation); and copious mucopurulent nasal discharge. Clinical signs develop within 2 to 4 weeks following natural or vaccinal exposure to streptococcal antigens. Clinical signs may include urticaria with pitting edema of the limbs, ventral abdomen and head; subcutaneous and petechial hemorrhage; and sloughing of involved tissues.