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West Nile Encephalitis

West Nile encephalitis is an infectious disease that primarily causes inflammation, or encephalitis, of the brain. The etiology stems from the West Nile virus (WNV), a flavivirus commonly found in humans, birds, and other vertebrates in Africa, West Asia, and the Middle East. This disease is part of a family of vectorborne diseases that also includes malaria, yellow fever, and Lyme disease.

The virus was first documented in the Western Hemisphere in August 1999, when a virus found in numerous dead birds in the New York, New Jersey, and Connecticut region was identified by genetic sequencing as WNY. Scientists in the United States first discovered the rare strain in and around the Bronx Zoological Park and believe imported birds may have carried the disease, which spread by mosquitoes that fed on the infected birds.

In temperate areas of the world, West Nile encephalitis cases occur mainly in late summer or early fall. In climates where temperatures are milder, West Nile encephalitis can occur year-round.

As of mid-November 1999, health officials at the Centers for Disease Control and Prevention (CDC) reported 56 cases of WNV infection (31 confirmed and 25 probable), including 7 deaths.

The risk of contracting West Nile encephalitis is greater for residents of areas where active cases have been identified. Individuals older than age 50 and those with compromised immune systems have the greatest risk. At this time, there is no documented evidence that a pregnant woman's fetus is at risk due to an infection with WNV. The mortality rate for West Nile encephalitis ranges from 3% to 15%; the mortality rate is higher in the elderly population.


This virus is transmitted to humans by the bite of an infected mosquito. Mosquitoes become infected by biting birds that harbor the virus. The virus is not spread from person to person or from infected birds to humans without a mosquito bite. The virus has now been found in 111 bird species and about a dozen mammals.

  • How West Nile virus entered New York is not entirely clear. The most likely explanation is that the virus was introduced by an imported infected bird or by an infected human returning from a country where West Nile virus is common.
  • Most cases of West Nile occur during the warm weather months. Nonetheless, the mild climate in southern states is expected to sustain the mosquitoes beyond those months.

Symptoms and Signs

Common symptoms includes:

  • Fever
  • Headache
  • Back pain
  • Muscle aches
  • Lack of appetite
  • Sore throat
  • Nausea
  • Vomiting
  • Abdominal pain
  • Diarrhea
  • Muscle weakness
  • Stiff neck
  • Confusion

Diagnostic tests

The enzyme-linked immunosorbent assay (ELISA), the MAC-ELISA, is the test of choice for obtaining a rapid definitive diagnosis. The major advantage of MAC­ELISA lab analysis is the high probability of an accurate diagnosis of WNV infection. An accurate diagnosis is possible only when serum or cerebrospinal fluid specimens are obtained while the patient is still hospitalized with acute illness.

When developing a differential diagnosis, another condition to consider is St. Louis encephalitis, which causes similar symptoms. Inflammation of the brain can be caused by numerous viral and bacterial infections, so all data must be examined to make a definitive diagnosis.


There is no specific therapy to treat West Nile encephalitis and no known cure. Treatment is generally aimed at controlling the specific symptoms. Supportive care measures such as I.V. fluids, fever control, and respiratory support are rendered when necessary.

There is no vaccine at present to prevent the transmission of West Nile encephalitis.

  • Dress to protect yourself. Wear long-sleeved shirts and long pants if you're outside between dusk and dawn.
  • Avoid mosquitoes. Refrain from unnecessary activity in places where mosquitoes are most prevalent.
  • Keep mosquitoes out of your home. Repair holes in screens on doors and windows.
  • Look for outdoor signs of viral disease. Keep an eye out for sick or dying birds and report them to your local health department.
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