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West Nile Virus Overview


What are the Symptoms of West Nile Virus?
Who's at Risk?
Although anyone can get West Nile, people ages 50 and over and anyone with a compromised immune system face the greatest risk of developing severe illness from it. Also, people who work outside and who live in or visit parts of the world where West Nile is currently active are at greater risk of contracting the virus.

Although West Nile gets a lot of press attention each summer, it is actually difficult to contract. The odds of being bitten by an infected mosquito are extremely slim, and the odds of becoming seriously ill following a bite are even slimmer. Approximately 80 percent of those who are bitten by an infected mosquito will have no symptoms at all. Approximately 20 percent will have mild, flu-like symptoms (such as fever, muscle aches, headache and swollen glands). Symptoms usually emerge within three to 14 days after the bite.

Less than 1 percent of people bitten by an infected mosquito come down with the most serious nervous-system conditions related to West Nile:

  • West Nile encephalitis - inflammation of the brain
  • West Nile meningitis - inflammation of the membrane surrounding the brain and the spinal cord
  • West Nile meningoencephalitis - inflammation of the brain and surrounding membrane
  • West Nile poliomyelitis - inflammation of the spinal cord
These conditions occur when the virus is able to penetrate the blood-brain barrier and attack the brain and/or spinal cord. The immune system, in trying to fight off the infection, sends out chemical signals that cause inflammation. This inflammation can interrupt normal functioning of the brain and/or spinal cord.


West Nile encephalitis is a condition that causes inflammation of brain tissue (right).

Symptoms of the most serious forms of West Nile virus can include:

  • high fever
  • stiff neck
  • disorientation
  • convulsions
  • coma
  • numbness
  • paralysis
The neurological effects of these conditions can be permanent. Anywhere from 5 to 15 percent of people with the most severe form of West Nile virus die. West Nile fatality rates are highest among the elderly.

Once someone has had West Nile virus (even the mild version), he or she becomes immune to it. Doctors can identify whether a person has West Nile virus by doing a blood test that detects the presence of antibodies to the virus. Computerized tomography (CT) or magnetic resonance imaging (MRI) can detect swelling in the brain associated with encephalitis.