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West Nile Virus

West Nile Virus is an arbo-virus (arthropod-borne) that is commonly  found in Africa, Asia, and the Middle East and was first isolated from an adult woman in the West Nile District of Uganda in 1937, hence its name.  It is closely related to St. Louis encephalitis virus, which is also found in the United States, including California.  The virus is transmitted by mosquitoes and can infect humans, birds, horses, and other animals.

Symptoms of West Nile Virus

No symptoms in most people:  Approximately 80 percent of people (about 4 out of 5) who are infected with WNV will not show any symptoms at all.

Milder symptoms in some people:  Up to 20 percent of the people who become infected have symptoms such as fever, headache, and body aches, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back.  Symptoms can last for as short as a few days, though even healthy people have become sick for several weeks and may even have lingering problems for years.

Serious symptoms in a few people:  About one in 150 people infected with WNV will develop severe illness.  The severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis.  These symptoms may last several weeks, and neurological effects may be permanent.

To see how WNV can affect a person's life, watch this short video of some people in California that were infected by West Nile Virus. View Now


Follow the 3-D's to protect yourself from mosquito bites and WNV


...all standing water at least once a week - even small amounts of water can harbor hundreds of mosquito larvae.

Dusk and Dawn the time when mosquitoes are most active, so that's when you need to avoid contact with them by staying indoors or following the guidelines below.

Defend yourself

...Wear repellent containing DEET, Picaridin, Oil of Lemon Eucalyptus, or IR3535 (see CDC recommendations), wear long sleeves and pants, install or repair window & door screens

Centers for Disease Control and Prevention -

California Department of Public Health -

American Mosquito Control Association -

West Nile Virus Myths

Myth #1: Kids are at the most danger of getting sick from West Nile Virus.
Truth: People over 50 and those with other health issues (e.g. diabetes, high blood pressure) are at the highest risk for developing severe West Nile disease.

Myth #2: It's only people who are already in poor health that have to worry about West Nile Virus.
Truth: Healthy active older adults who spend time working and exercising outdoors have also been affected by severe West Nile Virus infections.

Myth #3: There's not much I can do about West Nile Virus.
Truth: There is a lot that you, personally, can do to reduce your chance of West Nile virus - see above

Myth #4: Repellents containing DEET are not safe.
Truth: Repellents containing DEET are very safe when used according to directions.

Myth #5: As long as my area has a mosquito control program, I don't have to worry about using repellent.
Truth: Mosquito control activities don't eliminate every mosquito, so personal protection is still important.

Transmission Cycle

The virus is generally passed between mosquitoes and birds.  Some birds will die from the infection, while others will survive and amplify the virus, hence infecting more mosquitoes and also spreading it to other areas through travel and migration.  Some species of mosquitoes will bite birds as well as humans and other animals, and can thereby transmit the virus to them.  We don't usually produce enough of the virus in our system for mosquitoes to pick it up from us, that's why we are considered "dead-end hosts".

The most current map of WNV infections in California can be found at

For a current breakdown of cases/deaths by state, please refer to the Center for Disease Control and Preventions WNV website.

For current maps of WNV transmission in humans and animals, check out:

West Nile Virus in the United States

WNV was first identified in New York City in September 1999.  From 1999 through 2004, WNV has quickly spread across the nation and has now been documented in every state and the District of Columbia except for Alaska and Hawaii.

Since 1999 over 48,000 confirmed human cases and 2,163 deaths have been reported across the United States.  Since its arrival in 2002 there have been almost 7,000 confirmed human cases and over 300 deaths in California. 

The Antelope Valley has first identified human cases of West Nile Virus in 2007, and has recorded 44 cases and two fatalities since then.  For current WNV numbers check our News and Updates page.