CORE VACCINES: Needed by all horses

• Tetanus: a paralytic disease caused by the neurotoxin of Clostridium tetani, a spore-forming anaerobic bacterium. C. tetani is ubiquitous in the soil, putting all horses at risk of exposure. Horses typically contract tetanus when the bacteria enter through a puncture wound or deep laceration, but foals can be infected through their umbilical stump, and mares via their reproductive tracts following a difficult foaling. Signs include extreme sensitivity to light, sound and touch, followed by rigid paralysis that causes the horse to adopt a classic “sawhorse” stance and protrusion of the third eyelid.

• West Nile virus (WNV): a flavivirus transmitted by mosquitoes. In horses, infection usually causes little or no illness. However, West Nile infection sometimes triggers swelling of the brain (encephalitis) that produces limb weakness, muscle twitching, incoordination, behavioral changes and paralysis.

• Equine viral encephalomyelitis (EEE/WEE): brain and spinal cord inflammation caused by several species of alphaviruses that are usually transmitted by mosquitoes. Two forms that most commonly affect horses are eastern equine encephalomyelitis (EEE) and western equine encephalomyelitis (WEE). Both are characterized by fever, erratic behavior and lethargy as the brain begins to swell. WEE has a mortality rate of about 50 percent, while EEE is fatal in about 90 percent of cases.

• Rabies: a fatal viral disease of the central nervous system. During the incubation period, the virus migrates from the site where it entered the body to the brain. Signs include low-grade fever, convulsions, erratic behavior and recumbency. Because rabies can be transmitted to people, it is considered a public health threat, and veterinarians are required to report all cases.

RISK-BASED VACCINES: Administered depending on a horse’s situation

• Anthrax: systemic infection caused by the spore-forming bacterium Bacillus anthracis, which is found in the soil in some regions. Spores are typically picked up by horses as they graze or through bites from flies carrying the bacteria. Once inside the body, the spores multiply and produce a toxin. Signs include fever and colic, with swelling around a bite site if that was the mode of transmission. Even with prompt antibiotic treatment, anthrax can be fatal.

• Botulism: neuropoisoning caused by the toxin secreted by Clostridium botulinum bacteria, which can contaminate feed, water or wounds. Botulism is characterized by progressive paralysis, beginning with the muscles of swallowing. Horses may die from respiratory failure or complications of recumbency. Shaker foal syndrome is another form of the disease.

• Equine influenza: a common, acute viral infection involving the respiratory tract. Influenza is marked by inflammation of the nasal mucous membrane, the pharynx, the conjunctiva, the lungs and sometimes the heart muscle. The virus mutates easily and immunity is short-lived, making horses susceptible to influenza even after they’ve had the disease.

• Equine viral arteritis (EVA): a highly contagious disease that causes mild respiratory signs in most horses. EVA can trigger abortion in broodmares. In fact, abortion “storms” may race through an entire herd. Breeding stallions can be silent carriers, passing the disease along to mares without showing any signs of illness themselves.

• Potomac horse fever (PHF): a disease caused by the organism Neorickettsia risticii, contracted when the horse eats infected insects. Cases typically begin to appear with the arrival of warm weather and persist through the summer months. The disease is characterized by severe fever, diarrhea and laminitis.

• Rhinopneumonitis (equine herpesvirus): a contagious disease caused by two forms of the same virus, EHV-1 or EHV-4. The virus causes coughing, nasal discharge and lethargy, with cases anywhere from mild to severe. EHV-1 can cause respiratory issues, abortions in broodmares or neurological problems. EHV-4, known as the respiratory EHV, can also cause neurological signs, including progressive weakness and incoordination.

• Rotavirus A: primary cause of excessive diarrhea in foals younger than 3 months old. Mortality is less than 1 percent, but the disease is highly contagious. In addition to diarrhea, signs of the virus include depression, failure to nurse and difficulty standing.

• Strangles (distemper): a highly contagious infection of the lymph nodes, usually of the throat, caused by Streptococcus equi bacteria. Resulting abscesses may become so large they obstruct the airway. If they break internally, a thick, yellow pus drains through the nose. The abscesses may also rupture externally, draining through a spontaneous or surgical opening in the skin. Horses can become carriers of this disease and may shed the bacteria even if they look healthy.

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