West Nile virus is a disease that only affects a few mammal species, primarily horses and humans. It can cause encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord).

This disease is spread by birds and transmitted to horses by mosquitos that have fed on an infected bird. Symptoms include lethargy, incoordination, paralysis and weakness.

WNV was first detected in the United States in 1999. Since then, thousands of horses have been infected. Vaccination has effectively reduced the incidence of this disease, but it is now endemic across the United States.

WNV is a reportable disease, meaning that if a horse has or is suspected of having this disease, vets are required to report it to agricultural authorities (usually the State Veterinarian). These authorities may investigate the case as part of a larger effort to monitor equine health and coordinate with other States and the USDA APHIS in preventing the spread of illness or disease on a national and international level.

WNV is a zoonotic disease, meaning that it affects both another species and man. Infected horses cannot directly pass this disease to other horses, animals or humans and it is unlikely that a mosquito that feeds on an infected horse could pass this disease onto a human. However, a mosquito that feeds on an infected bird can pass the disease onto a human.

If you have been exposed to a horse with (or suspected of having) WNV you should contact your doctor because this virus may be active in your geographic region.