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West Nile Virus: Just The Facts

West Nile Virus: Just The Facts

West Nile Virus (WNV) was initially isolated in the West Nile district of northern Uganda in 1937. It was first detected in North America in 1999, causing 62 cases of encephalitis (brain swelling) and 7 deaths in New York. It is the leading cause of mosquito-borne disease in the U.S.

In 2018, there were 2,647 cases of WNV and 167 deaths. In addition, there were 372 blood donors who tested positive for the virus but had no symptoms at the time of their donation.

September 17, 2019:

  • 46 states and the District of Columbia have reported WNV infections in people, birds, or mosquitoes

  • 468 cases of WNV disease in people have been reported to CDC

  • Of these, 302 (65%) were classified as neuro-invasive disease (such as meningitis or encephalitis) and 166 (35%) were classified as non-neuroinvasive disease

  • There have been 21 deaths

  • 69 blood donors have tested positive

 WNV cycles between mosquitoes and birds. Infected birds can develop high levels of the virus in their blood. (After the virus infects a cell, it can program the cell to make more and more virus.) Mosquitoes become infected by biting these infected birds. Mosquitoes, in turn, transfer WNV to people by biting them. Less common forms of transfer include:

  • Exposure in a laboratory setting

  • Blood transfusion and organ donation

  • Mother to baby: during pregnancy, delivery, or breast feeding

 West Nile virus is not spread:

  • Through coughing, sneezing, or touching

  • By touching live animals

  • Through eating infected birds or animals. (Always follow instructions for fully cooking meat from either birds or mammals)


Most people develop no symptoms.

About 20% develop:

  • Fever

  • Headache

  • Body aches, joint pain

  • Vomiting, diarrhea

  • Rash

Most people with this type of WNV recover completely, but fatigue and weakness can last for weeks or months.

About 1 in 150 people develop a severe illness affecting the nervous system: encephalitis (swelling of the brain) or meningitis (swelling of the membranes that surround the brain and spinal cord):

  • High fever

  • Headache

  • Neck stiffness

  • Confusion

  • Coma

  • Tremors or convulsions

  • Muscle weakness or paralysis

  • Vision loss

  • Numbness

Recovery may take several weeks or months, leaving some with permanent disability. About 10% people who develop severe illness affecting the nervous system die.

 Diagnosis is made by testing the blood or spinal fluid.

 There are no medications to fight the virus or vaccine to prevent it from causing illness.

 WNV and Dead Birds

  • WNV has been detected in over 300 bird species

  • Birds that are predators (hawks and owls) or scavengers (crows) may become infected after eating sick or dead birds that were infected

  • Although some infected birds frequently die of infection, most birds survive

  • There is no evidence that a person can get infected from handling live or dead infected birds. However, you should avoid bare-handed contact when handling any dead animal. If you are disposing of a dead bird, use gloves or double plastic bags to place the carcass in a garbage can

Testing and Reporting

  • Reporting and testing of dead birds is one way to check for the presence of WNV in the environment. Some surveillance programs rely on citizens to report dead bird sightings to local authorities

  • State and local agencies have different policies for collecting and testing birds, so check with your state health department or state wildlife agency for information about reporting dead birds in your area

  • Wildlife agencies routinely investigate sick or dead bird events if large numbers are impacted

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