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Am Fam Physician. 2003;68(4):671-672

What is West Nile virus?

Infection from West Nile virus has occurred in Africa, Europe, and the Middle East. Now the virus has spread to the United States, where it has been reported in 44 states and Washington, D.C.

West Nile virus infection usually causes no symptoms or only mild illness. However, infection can lead to a swelling of the brain called encephalitis (say: en-seff-uh-lie-tuss).

How do people get infected with West Nile virus?

Mosquitoes become infected by biting birds that carry the virus. People can get West Nile virus when an infected mosquito bites them.

During the 2002 West Nile virus epidemic in the United States, a few people got the virus through blood transfusions and organ transplants. However, the risk of getting the virus in these ways is low.

Doctors are not sure if West Nile virus can be passed from a pregnant or breastfeeding woman to her baby.

What are the symptoms of West Nile virus infection?

Most people who are infected with West Nile virus have no symptoms. Others may have only a mild illness, such as fever and a headache. In rare cases, West Nile virus causes a disease that can lead to permanent brain damage or death.

Symptoms usually occur two to 15 days after a person is bitten by an infected mosquito. The symptoms may include slight fever, headache, or rash. However, serious illness is possible, with symptoms such as sudden, severe headache, high fever, stiff neck, confusion, weakness, coma, and even death. Of 150 persons infected with West Nile virus, only one gets severely ill.

Who is at risk for infection with West Nile virus?

People who live where West Nile virus has been found in humans, birds, horses, or mosquitoes are at risk for infection. People 50 years of age and older are at greatest risk of getting severe disease.

How many people with severe West Nile virus infection die?

During the 2002 epidemic, about one in 11 patients with severe illness caused by West Nile virus died.

Is there a treatment for West Nile virus infection?

There is no specific treatment for West Nile virus infection. Patients with severe illness may be hospitalized and given intravenous fluids. To help them breathe, they may need to be on a machine called a ventilator. Their doctor also will try to keep them from getting other infections, such as pneumonia.

Is there a vaccine for West Nile virus?

There is no human vaccine for West Nile virus yet.

If I travel to an area where infected birds have been reported and I am bitten by a mosquito, is it certain that I will get sick?

No. Even in areas where some mosquitoes carry the virus, very few mosquitoes (usually less than one in 500) are infected. The chance that one mosquito bite will come from an infected mosquito is quite small. Also, remember that only one in 150 infected persons will get serious illness.

If a mosquito bites me, should I be tested for West Nile virus infection?

No. Illness from a mosquito bite is rare. However, if you get a high fever, mental confusion, weakness, or severe headaches, see your doctor right away. People with mild symptoms probably will get completely better and do not need any medicine or lab tests.

What time of year are mosquito viruses spread?

Viruses carried by mosquitoes are most likely to be spread during the warm-weather months, usually from spring until the first hard frost. Most human cases of this infection happen in late summer and fall. But human cases can happen year-round in places with tropical climates.

Are bird and wild game hunters at risk for infection with West Nile virus?

Hunters may be at risk for infection if they are in areas where West Nile virus has been found. They should spray insect repellent on their clothes and skin. They also should wear gloves when they handle and clean birds or animals.

Can a person get infected with West Nile virus by eating infected game birds?

As long as game birds have been cooked properly, there is no danger of getting infected by eating them.

Can West Nile virus infection be prevented?

The best way to keep from getting infected with West Nile virus is to reduce the number of mosquitoes around your home and neighborhood.

Here are a few things you can do:

Can insect repellents be used on children?

  • Get rid of standing water in birdbaths, ponds, flowerpots, wading pools, old tires, and other places where mosquitoes might breed. Garden supply stores carry products to keep mosquitoes from breeding in small ponds.

  • Repair any tears in window screens, and porch and patio screens.

  • Stay indoors during dawn, dusk, and early evening hours. If you go outdoors at these times, wear shoes and socks, long pants, and a long-sleeved shirt. Loose-fitting, light-colored clothing is best.

  • If you go outdoors, apply insect repellent according to the direction on the label. Because mosquitoes may bite through thin material, spray your clothes with an insect repellent that contains DEET or permethrin. The more DEET a product contains, the longer it can protect against mosquito bites. A product containing 20 to 30 percent DEET usually is strong enough. Do not spray insect repellent with DEET on skin that is covered by your clothes. Do not put an insect repellent with permethrin on your skin.

Talk to your doctor before you use insect repellents on young children. Do not use insect repellents with DEET on children younger than 2 months. Do not use repellents containing more than 10 percent DEET on children younger than 2 years.

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