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FAQ: What is West Nile virus?



In Orange County, mosquitoes have been breeding
In Orange County, mosquitoes have been breeding "at an alarming rate and volume throughout the winter months," says Jared Dever, with the county's Mosquito and Vector Control District.
Photo by Sean McCann via Flickr Creative Commons

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What is West Nile virus?

West Nile virus is almost always transmitted from mosquitoes to people. It usually causes no symptoms; some people will get headaches, body aches and other relatively mild symptoms. A very small number of those infected will develop serious complications, particularly people over 60 with other medical conditions.

In rare instances, West Nile has been spread through blood transfusions, organ transplants, and from mother to baby during pregnancy, delivery, or breastfeeding.

How do the mosquitoes get infected?

West Nile cycles between mosquitoes and birds: Mosquitoes become infected when they feed on birds that have the virus. The mosquitoes spread it to other birds, and to humans.

How great is my risk of serious illness?

The risk of serious illness is very small. An estimated 70 to 80 percent of people who contract the virus never develop any symptoms.

About one in five people who are infected will develop a rash, plus other symptoms like headache, body aches, joint pains or vomiting.

Less than 1 percent of people infected with the virus will develop a serious neurologic illness such as encephalitis or meningitis. The symptoms of neurologic illness can include headache, high fever, neck stiffness, disorientation, coma, tremors, seizures or paralysis.

Those who come down with severe disease may need several weeks or months to recover, and some of the neurologic effects may be permanent.

But even among the small fraction who develop a severe neurologic infection, only about 10 percent of them will die.

While people of any age can develop serious illness, people over 60 are at the greatest risk. Those with certain medical conditions, such as cancer, diabetes, hypertension or kidney disease, as well as people who have received organ transplants, are also at greater risk for serious illness.

You should consult your health care provider if you believe you or a relative is experiencing symptoms consistent with West Nile.

How long does it take symptoms to appear?

The incubation period is usually two to six days, but it can be as long as 14 days. This period can be longer in people with certain medical conditions that affect the immune system.

How is West Nile treated?

There are no medications to treat West Nile. Over-the-counter pain relievers can be used to reduce fever and relieve some symptoms.

People with milder symptoms typically recover on their own, although some symptoms may last for several weeks.

In more severe cases, patients often need to be hospitalized to receive supportive treatment, such as intravenous fluids, pain medication, and nursing care.

There currently is no vaccine for West Nile.

Which factors contribute to increased West Nile virus activity?

High temperatures and drought. Rainfall helps flush mosquitoes out of storm drains, where they can breed. In addition, drought tricks mosquitoes into thinking it's still summer, their prime breeding season. A lack of rain also means birds and mosquitoes seek out the same water sources. 

What can I do to prevent the spread of West Nile virus?

Where can I get more information about West Nile Virus?

You can check the state's Fight the Bite website, as well as the website of your county's mosquito and vector control district.

Sources: The Centers for Disease Control and Prevention and the California Department of Public Health.