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West Nile Virus Encephalitis

West Nile Virus-Another Important Spring Vaccination

Encephalitis is inflammation of the brain and or spinal cord.  It develops in horses exposed to the West Nile Virus when the virus itself invades these sensitive tissues causing inflammation.  The disease is also called West Nile Fever.  Horses can be exposed to the virus without developing sickness but in horses with West Nile Encephalitis, it is often fatal (50% of horses with clinical signs die) and horses that do recover can continue to have signs over a prolonged period of time.


West Nile Virus also infects people. It was introduced to the United States in 1999.  Both people and horses are exposed to the virus by mosquito bites.  The disease is not transmitted from horse-to-horse or horse-to-person. 


The signs for West Nile Disease can vary and are not specific for West Nile Encephalitis.  Any disease that causes encephalitis will cause similar signs including, but not limited to, Eastern and Western Equine Encephalitis, EPM, and the neurologic form of rhinopneumonitis.  Not all horses infected with the virus develop the disease, and some horses can die without showing signs of illness prior to death. The first signs of the disease may be general and include signs of discomfort or anxiety, lameness, inappetencelow-grade fever, or malaise. Neurologic signs can include muzzle twitching, impaired vision, aimless wandering, head pressing, circling, inability to swallow, irregular gait, trembling, lack of coordination, weaknessmuscle contractions, paralysis, convulsions, and death. Horses may become abnormally sensitive to sights, sounds, or touch


There is no specific treatment for West Nile Encephalitis.  Treatment consists of supportive care and treatment of clinical signs as with anti-inflammatories, fluids, bandaging, etc.  There is testing to diagnose if an affected horse has the disease.


The good news is that the vaccine is very effective against the disease.  And in the rare case a vaccinated horse shows signs of disease, the horses do not die and can recover completely.  

The best way to protect your horse is to vaccinate.  The vaccine is typically given as a series of two injections followed by yearly revaccination.  It is usually recommended to vaccinate before the start of mosquito season which starts in April in New York. In some select cases, vaccination may be recommended every 6 months.  This may be the case for horses at increased risk because of their environment or in a year when the virus is known to be prevalent or there have been reported equine cases.  This is especially true for horses less than 5 years of age or geriatric horses that may be immunocompromised.

Other preventative measures include best management practices including draining standing water from farms, only turning horses out when mosquitos are least active and proper use of fly sprays and other insect deterrents.

Information about the prevalence of the virus in your area, as well as cases in horses can be found at:


For more information about clinical signs associated with West Nile Virus, Dr. Maureen Long of the University of Florida put together a short video published by The Horse magazine: