West Nile Virus

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What Is West Nile Virus?

west nile illustration

WNV is a virus most commonly spread to people by mosquito bites. In North America, cases of WNV occur during mosquito season, which starts in the spring and summer, continuing through fall.

WNV cases have been reported in all of the continental United States, including Los Angeles County.

How Does Infection Occur?

Mosquito Bites

West Nile Virus is most commonly spread to people by the bite of an infected mosquito.

Mosquitoes become infected when they feed on infected birds. Infected mosquitoes then spread West Nile Virus to humans and other mammals by biting them.

In nature, WNV cycles between mosquitoes (especially Culex species) and birds. Some infected birds can develop high levels of the virus in their bloodstream.

Mosquitoes can become infected by biting infected birds. After about a week, infected mosquitoes can pass the virus to more birds when they bite.

mosquito vector spread

Humans and other mammals are “dead end” hosts. This means they do not develop high levels of virus in their bloodstream and cannot pass the virus on to other biting mosquitoes.

Minimal Spread By

  • Exposure in a laboratory setting
  • Blood transfusion and organ donation
  • Mother to baby, during pregnancy, delivery, or breast feeding

Not Spread By

  • Coughing, sneezing, or touching
  • Touching live animals
  • Handling live or dead infected birds. (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.)
  • Eating infected birds or animals. (Always follow instructions for fully cooking meat from either birds or mammals.)

Who Is at Risk for Infection?

Anyone bitten by a mosquito in an area where the virus is circulating can be infected with WNV.

The risk is highest for persons who engage in outdoor work and recreational activities. Elderly persons are at increased risk of severe disease if they are infected.

mosquito on skin

When Do Symptoms Develop?

It takes 2-14 days after the bite of an infected mosquito to develop symptoms of WNV.

symptoms illustration

What Are the Symptoms?

None for Most

Most people (8 in 10) infected with West Nile virus do not develop any symptoms.

Febrile Illness for Some

Some people (1 in 5) who are infected develop a fever with other symptoms such as headache, body aches, joint pains, vomiting, diarrhea, or rash. Most people with this type of West Nile virus disease recover completely, but fatigue and weakness can last for weeks or months.

Serious for a Few

A few people (1 in 150) who are infected develop a severe illness affecting the central nervous system such as encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes that surround the brain and spinal cord).

  • Symptoms of severe illness include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis.
  • Severe illness can occur in people of any age. However, people over 60 years of age are at greater risk.
  • People with certain medical conditions such as cancer, diabetes, hypertension, kidney disease, and people who have
    received organ transplants are also at greater risk.
  • Recovery from severe illness might take several weeks or months. Some effects to the central nervous system might be permanent.
  • About 1 out of 10 people who develop severe illness affecting the central nervous system die.

What If I Suspect I Have WNV?

symptoms illustration

If you think you, a family member, or a pet might have a mosquito-borne disease, contact a healthcare provider for a proper diagnosis.

If you think you, a family member, or a pet might have a mosquito-borne disease, consult a healthcare provider for a proper diagnosis.