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West Nile Virus (WNV)

What is West Nile Virus (WNV)?

West Nile Virus (WNV) is a viral disease in the Flavivirus family that can result in fever and neurologic disease. The virus can infect horses, humans, and many different species of birds. Clinical signs of the disease usually present within about 15 days after a bite from an infected mosquito. The virus was introduced to the United States in 1999, since then over 27,000 horses have been infected and WNV is considered an endemic disease.

West Nile Virus Transmission

WNV is transmitted by infected mosquitoes. Wild birds serve as the host reservoir. When mosquitoes feed on infected birds they become infected and can transmit the virus to other birds, horses, and even humans during subsequent blood meals.

West Nile Virus Clinical Signs

  • Fever
  • Incoordination
  • Hind-end weakness
  • Depression
  • Anorexia
  • Muscle tremors
  • Teeth grinding
  • Inability to swallow
  • Head pressing
  • Excessive sweating
  • Behavior changes
  • Down and unable to rise

West Nile Virus Treatment

Treatment consists of supportive care and is based on the severity of clinical signs.  Prognosis may be poor for horses with severe neurologic signs.  WNV mortality rate is about 35%.

West Nile Virus Prevention

WNV vaccination is very effective at preventing clinical disease.  Vaccination schedule depends on many factors and should be discussed with your veterinarian. Vaccination needs to be repeated at least annually after the initial vaccination protocol is followed for unvaccinated horses. Horses residing in locations with heavy mosquito infestation or year-round presence of mosquitos may require more frequent booster vaccination to maintain adequate protection from the disease.  Minimize mosquito habitat by removing standing/stagnant water, using fans in stalls, maintaining clean stalls and using equine approved mosquito repellant.

Actions to take if you Suspect Infection

  1. Call your veterinarian
  2. Report any dead wild birds (especially crows, jays, magpies and ravens) in your area to your local Department of Health.
  3. Use topical insecticides on your horses (such as pyrethroid or permethrin products) to prevent continued transmission by infected mosquitos in the area.
  4. Identify and remove areas of standing water on the property where mosquitos may be hatching.
  5. Discuss WNV booster vaccination with your veterinarian.
  6. If WNV infection is confirmed by your veterinarian, take steps to protect yourself and your family from potentially infected mosquitoes.

Information for the Veterinarian

Further Information

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