HorseWest Nile virus (WNv) is a mosquito-borne virus that was first detected in the United States in 1999 and reached Washington State in 2002. However, there was not another confirmed WNv case until 2005. In 2006, there were 6 confirmed horse cases with at least 2 horse deaths. In 2007, there occurred a grouping of 9 positive horse cases all in Yakima County.

Mosquitoes become infected by feeding on infected birds and pass the virus to other birds, animals and people. Both horses and people are considered 'dead-end' hosts; meaning that while infected, they do not spread the infections. Infected horses are not a risk to other horses. However, West Nile virus should be considered a serious threat. The virus normally causes mild symptoms, but it can cause encephalitis; an inflammation of the brain and central nervous system.

Signs may include:

  • Weakness of limbs, difficulty in walking, stumbling, knuckling over, falling, the inability to stand, or convulsions.
  • Muscle twitching, typically on the face, neck and forelimbs.
  • Head tilt or drooling
  • Hypersensitivity

Death occurs in about 30% of clinically affected equine. Most surviving animals recover completely, but some may show lasting neurological effects. Horses showing any suspicious signs should be evaluated by a veterinarian for an accurate diagnosis.

Equine Vaccination:

There are licensed vaccines to protect horses, mules, donkeys and other equine against West Nile virus. Vaccinations are the primary protection for this serious disease. The vaccinations require two doses, four to seven weeks apart. Depending on the vaccine used, immunity may not be achieved for at least eight weeks after the second dose. The vaccination process should begin in the spring during the early part of the mosquito season. Work with your veterinarian to develop the best plan for your horse or other equine.

Reporting Suspected Cases in Horses:

Horse owners are encouraged to contact their veterinarian if their animals display signs of illness including loss of appetite, fever, tremors, weakness, or inability to stand. Veterinarians are required to report all cases of equine encephalitis. Any suspected cases of West Nile virus in horses or other equines must be reported to WSDA; Washington State Department of Agriculture.

  • State Veterinarian's Office; Leonard Eldridge, 360.902.1881
  • Regional WSDA Veterinarian: Benton, Franklin, Klickitat, Walla Walla & Yakima County; Peter Tran, 509.786.2712

Additional Information & Contacts:

  • Benton-Franklin Health District; Environmental Health, 509.582.7761
  • Benton County Mosquito Control; Kevin Shoemaker, 509.967.2414
  • WSU Extension Office; Jean Smith, 509.735.3551
  • Washington State Department of Health; 866.788.4787
  • U.S. Department of Agriculture; 360.753.9430

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