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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.

  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.

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.
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%.

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.


  • 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

Annual Summary Reports 2006-2018.
Number of confirmed eastern equine West Nile cases by State and County.

Surveillance Information

Reports of equine cases of arboviral (i.e., arthropod-borne virus) diseases, including West Nile Virus, are reported to ArboNET, an electronic-based surveillance and reporting system used to track and report arboviral activity. ArboNET captures laboratory-confirmed positive cases in humans, horses, other mammals, birds and mosquitoes across the United States. Equine case-finding varies by State, but primarily relies on passive reporting from private practitioners and diagnostic submissions. In some States, reporting is mandatory.

West Nile Virus, 2018 - States with Equine Cases
Total cases - 493



West Nile Virus, 2017 - States with Equine Cases
Total cases - 307



West Nile Virus, 2016 - States with Equine Cases
Total cases - 380



West Nile Virus, 2015 - States with Equine Cases
Total cases - 225


West Nile Virus, 2014 - States with Equine Cases
Total cases - 141

 


West Nile Virus, 2013 - States with Equine Cases
Total cases - 377


West Nile Virus, 2012 - States with Equine Cases
Total Cases - 627


West Nile Virus, 2011 - States with Equine Cases
Total cases - 87

US map showing the number of equine West Nile virus cases in each State


West Nile Virus, 2010 - States with Equine Cases
Total cases - 125

Equine cases of West Nile Virus for 2010 - preliminary


West Nile Virus, 2009 - States with Equine Cases
Total Cases - 276

Map of US showing 2009 distribution of equine WNV cases


West Nile Virus, 2008 - States with Equine Cases
Total Cases - 179

Map of U.S.


West Nile Virus, 2007 - States with Equine Cases
Total Cases - 468

equine West Nile virus cases for 2007 map


West Nile Virus, 2006 - States with Equine Cases
Total Cases - 1,086


West Nile Virus, 2005 - States with Equine Cases
Total Cases - 1,088

West Nile Virus, 2004 - States with Equine Cases
Total Cases - 1,406

West Nile Virus, 2003 - States with Equine Cases
Total Cases - 5,181

West Nile Virus, 2002 - States with Equine Cases
Total Cases - 15,257

West Nile Virus, 2001 - States with Equine Cases
Total Cases - 738

West Nile Virus, 2000 - States with Equine Cases
Total Cases - 60

West Nile Virus, 1999 - States with Equine Cases
Total Cases - 25

 


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