West Nile Virus

The facts you need on West Nile virus (WNV)

West Nile virus (WNV) is a mosquito-borne disease that can cause encephalitis, or an inflammation of the brain. Infected culex mosquitoes pass the virus onto birds, other animals, and people.

West Nile virus was first detected in the United States in New York during the fall of 1999 and in Illinois in 2001. Prior to arrival in the United States, the virus had only been found in Africa, Eastern Europe and West Asia.

West Nile virus cases occur primarily in midsummer or early fall, although mosquitoes that carry the disease may be present from mid-April through October.

During the 2005 mosquito season, McLean County experienced four cases of West Nile virus resulting in two deaths.

What are the symptoms of West Nile virus?

West Nile virus encephalitis is transmitted from infected mosquitoes to people, not from person to person or from bird to human. Mosquitoes get the virus by feeding on infected, migratory birds.

Mild cases of West Nile virus may cause a slight fever or headache. More severe infections are marked by:

  • Rapid onset of a high fever
  • Head and body aches
  • Disorientation
  • Tremors 
  • Convulsions

In the most severe cases, it can cause paralysis or death.

Symptoms typically occur from three to 14 days after the bite of an infected mosquito.

Who is at high risk?

Individuals at highest risk for serious illness are very young children, adults 50 years of age or older and people with compromised immune systems.

Anyone concerned about their risk -- or experiencing two or more symptoms -- should contact a physician immediately and inform the doctor of any recent mosquito bites.

How can people reduce the chance of getting infected?

The most effective way to avoid West Nile virus disease is to prevent mosquito bites. This video illustrates several important measures, both inside the house and outside the house, to avoid being bitten by mosquitos that transmit West Nile virus.