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AT the LAb

West Nile Virus

West Nile virus (WNV) is an enveloped single-stranded RNA virus that received its name in 1937 when it was first isolated from the blood of woman living in the West Nile district of Uganda. Since 1999, the West Nile virus has been reported in 48 states of the US. Now the virus has also spread to Canada and South America. WNV is transmitted by mosquitoes and has been identified in 37 mosquito species in North America. The virus is maintained in a mosquito – bird – mosquito cycle. In the USA, West Nile virus has been detected in more than 162 bird species. The most significant effect of WNV in veterinary medicine is the infection of equines.

The first symptoms of WNV infection are generally unspecific and include:
  • fever;

  • anxiety;

  • depression;

  • loss of appetite.

Usually, horses become ill 3 to 15 days after exposure to an infected mosquito. Some horses with past clinical infection express long-term effects such as weight loss, lethargy, and signs of neurological disorders.

These neurological disorders include:

encephalitis that leads to ataxia (stumbling, staggering, inability to stand, incoordination);

  • lameness (multiple limb paresis);

  • blindness;

  • muzzle twitching;

  • trembling;

  • circling;

  • inability to swallow;

  • hypersensitivity;

  • seizures.

​WNV is the most common cause of equine encephalitis. However, not all horses show clinical symptoms. For approximately 35% of horses the WNV encephalitis and its complications are fatal. However, the prognosis for an individual horse is dependent upon the severity of clinical signs.


Thus, we recommend that horse owners monitor their animals for encephalitic signs and quickly contact their veterinarian, who should order a clinical diagnostic test for the West Nile virus infection. The most important and convenient tool for a WNV diagnostic is the detection of specific anti-WNV antibodies in equine serum. These serological tests identify a very early antibody response (7 – 10 days after WNV infection) by detection of IgM antibodies for diagnosis of acute WNV infection and IgG antibodies that are generated after 10 days and can be detected during at least 15 months after WNV infection.

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