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What is West Nile virus?

West Nile Virus is an arbovirus (arthropod-borne virus) transmitted primarily through the bite of infected mosquitoes. West Nile virus was first identified in the United States in 1999. The virus can infect humans, birds, mosquitoes, horses and some other mammals.

How is West Nile virus spread?

West Nile virus is most often spread by the bite of an infected mosquito and can infect many types of birds and mammals as well as people. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. Infected mosquitoes can then transmit West Nile virus to humans and animals while biting to take blood.

There has been documented transmission of West Nile virus through organ transplantation, blood transfusion, and transplacentally (mother-to-child). These modes of transmission represent a very small proportion of West Nile virus cases. The most common means of spreading West Nile virus is through the bite of infected mosquitoes.

What are the symptoms?

Most people who become infected with West Nile virus either have no symptoms or only mild symptoms such as fever, headache and body aches. Approximately 80% of people with West Nile virus infection will have no symptoms.

About 20% of people with West Nile virus infection will develop "West Nile fever", a mild disease in people, characterized by flu-like symptoms. West Nile fever typically lasts only a few days and does not appear to cause any long-term health effects.

About one percent of those infected with West Nile virus will develop more severe neurologic (brain) disease called - "West Nile encephalitis", "West Nile meningitis" or "West Nile meningoencephalitis." Encephalitis refers to an inflammation of the brain, meningitis is an inflammation of the membrane around the brain and the spinal cord, and meningoencephalitis refers to inflammation of the brain and the membrane surrounding it. The risk of severe infection is higher among people who are 50 years of age and older.

Where can I find information on the spread of the disease in Washington State and in the United States?

  • Washington Department of Health West Nile Website - Washington State Department of Health has a West Nile virus website with information on human, horse and bird cases in Washington State and a toll free information line.
  • Washington State Department of Health West Nile Virus Information Line:
    1-866-78VIRUS (1-866-788-4787)
  • CDC West Nile Virus Website - The U.S. Centers for Disease Control and Prevention (CDC) West Nile virus website has national case counts for West Nile virus and additional information.
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