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Page last updated: 2012-02-14

West Nile Virus

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What Is West Nile Virus?

Also called: West Nile Fever, West Nile Disease

West Nile virus is a mosquito-borne viral disease.  The disease is transmitted to humans and animals when they are bitten by an infected mosquito.  The mosquito acquires the virus in the first place by feeding on infected birds.  Only certain species of mosquito can carry West Nile virus.

There have been a few reported cases of West Nile virus transmission from organ transplants or blood transfusions.

Transmission to a child from a mother who has the disease also is possible during pregnancy, and through breastfeeding.  This has been documented in a very small number of cases.

West Nile virus is not transmitted through casual contact.  You cannot get West Nile virus from touching or kissing a person who is infected.

West Nile disease was first detected in the Western Hemisphere in the summer of 1999, during an outbreak in New York City.  Since then it has spread throughout the continental United States, and to some parts of Canada and the Caribbean.

Although a number of people die each year as a result of West Nile virus infection, death in humans is still relatively uncommon.  In fact, many people who become infected have very mild symptoms, or none at all.

West Nile virus can be serious for certain individuals, especially older people, the very young, and those with other medical conditions.  In serious cases, the virus causes inflammation of the brain and/or the membranes that surround the brain and spinal cord.  This invasion of the central nervous system can lead to paralysis and possibly death.

Unfortunately there is no sure way to know ahead of time whether or not you'll get sick if you become infected.

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Risk Factors for West Nile Virus:

People who spend a lot of time outdoors are more likely to be bitten by mosquitoes, including the types that carry West Nile virus.

Visiting or living in geographic regions where mosquito-borne viruses are prevalent increases your risk of being bitten by a mosquito with West Nile virus.  The U.S. Centers for Disease Control website provides a map for monitoring West Nile virus activity in the United States.

The risk of West Nile virus is seasonal, to a certain degree.  The number of cases tends to rise seasonally, beginning in late spring, and peaks in late summer and early fall.

Mosquitoes -- including the types that can carry West Nile virus -- breed in standing water.  Thus, the risk for West Nile disease is greater in areas with a lot of standing water, such as ponds and swamps. 

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Symptoms of Infection by West Nile Virus:

According to recent estimates from the U.S. National Institutes of Health, approximately 80% of humans infected with West Nile virus have no symptoms at all.

People who do get sick from West Nile virus begin to show symptoms between 3 and 14 days after they have been bitten by an infected mosquito.

Slightly fewer than 20% will develop mild symptoms.  These can include:

Fewer than 1% will develop the more serious infections of the central nervous system.  Serious symptoms can include:

Someone who develops any of these severe symptoms requires urgent medical care.  People with severe symptoms of West Nile infection usually require hospitalization.

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Treatment for West Nile Virus:

If you become ill and suspect that you may have West Nile virus, your doctor can confirm this through a blood test.

There is no specific treatment for West Nile virus, and mild cases usually run their course in a matter of days, leaving no after-effects.

Serious cases usually require hospitalization.  The patient is given supportive therapy such as intravenous fluids and pain relievers.  The most serious cases may need to be managed in an intensive care unit.  Patients seriously ill from West Nile virus may need to be put on a ventilator to aid their respiration.

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Prevention of West Nile Virus Infection:

Avoiding the West Nile virus hinges on eliminating mosquito breeding sites, and avoiding or reducing exposure to mosquitoes so that you will not be bitten.

To eliminate mosquito breeding sites:

To reduce your exposure to mosquitoes:

A Final Note:

West Nile virus is harbored in wild birds.  If you find a dead bird, do not pick it up or handle it with your bare hands.  Report it to your local health department.  They will advise you on how to dispose of it.

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Here are some trustworthy on-line sources of information related to the topic of West Nile Virus.

MAP: West Nile Virus Activity in the U.S. - U.S. Centers for Disease Control & Prevention  [regularly updated]

West Nile Virus Maps - U.S. Geological Survey  [regularly updated]

West Nile Virus - information and advice from, June, 2010

Five Common Myths about West Nile Virus - U.S. Centers for Disease Control & Prevention, August, 2007

West Nile Virus: What you need to know - Fact Sheet from the U.S. Centers for Disease Control & Prevention, June, 2003    [PDF format]

Information About Mosquito Repellents & Mosquito Bites

What you need to know about mosquito repellent - Fact Sheet from the U.S. Centers for Disease Control & Prevention, September, 2005    [PDF format]

Fight the Bite! - How to avoid mosquito bites and avoid infection, November, 2007

Mosquito bites - advice from, April, 2011

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In the interest of facilitating further research on the topic of West Nile Virus, we have compiled a list journal articles and other references related to the topic.  We also include a section of references regarding Mosquito Repellents.

Some of the items listed here were used as source material to prepare this information page about West Nile Virus.

Click here to view references.

If you know of reference material about West Nile Virus that you think should be included on this list, please click here to submit the information.

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