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West Nile Virus

Reviewed by: Yamini Durani, MD

Most mosquitoes are simply annoying. But a small percentage can carry diseases like West Nile virus. In recent years, West Nile virus has been found in animals, birds, and humans in all continental states in the United States.

But don't let worries about mosquitoes keep you and your family from enjoying the outdoors. The chances of anyone in your family becoming sick with the West Nile virus are extremely low, no matter what part of the country you live in.

Taking precautions against mosquito bites can help lessen the fear, but so can learning about the virus. Here's what you need to know:

  • Less than 1% of mosquitoes in any area where carrier mosquitoes have been found are actually infected with West Nile virus.
  • Fewer than 1% of people who are bitten and infected with West Nile virus will become seriously ill.
  • Kids and adults younger than 50 are less likely to become sick from the infection if they do get it.

About West Nile Virus

West Nile is a virus that is spread by the Culex species of mosquito. Mosquitoes become infected by feeding on infected birds, and then transmit the virus to humans and animals through bites. It is not spread from person to person, animal to animal, or animal to person.

Once in the bloodstream, the virus multiplies and spreads. If the virus reaches the brain, the result is encephalitis — an inflammation of the brain that can affect the entire nervous system. Although rare, encephalitis is the most serious complication associated with the virus; those at greatest risk are people older than 50.

West Nile virus has been around a long time. Common in Africa, Eastern Europe, West Asia, and the Middle East, it had not been found in the Western Hemisphere until 1999. It's unknown how the virus entered the United States, but experts say it's most similar to the strains found in the Middle East.

Since that time, the Centers for Disease Control and Prevention (CDC) has verified reports of West Nile virus infection in birds, mosquitoes, or humans throughout most of the country. There is no evidence to suggest that West Nile virus is transmitted by ticks, birds, or any other insect besides mosquitoes. It is possible for your pet to become infected, but your pet cannot transmit the virus to you.

Although the risk of infection is very low due to routine screening of donated blood, the infection has been transmitted through blood transfusion in a few cases. There have also been a few reports of fetuses of pregnant women becoming infected when the mother developed West Nile virus infection.

Signs and Symptoms

Most of the time, a person bitten by an infected mosquito does not get sick. Most people who do get sick from  the virus become only mildly ill. A person may have flu-like symptoms such as fever, headache, body aches, and sometimes skin rash and swollen glands.

About 1 person in 150 infected with the West Nile virus will get severely ill. Severe infection can result in encephalitis — symptoms can include severe headache, high fever, neck stiffness, disorientation, and sometimes convulsions. Although very rare, severe West Nile infection in healthy, younger people can be fatal, but the elderly are at greatest risk for severe complications.

The incubation period for West Nile virus is usually 3 to 14 days. In the northern United States, infections happen mainly in the summer and early fall, but in the warmer southern regions, the virus can be transmitted all year.


What can you do to protect yourself from West Nile virus? The CDC recommends these precautions:

  • Try to stay indoors at dusk and dawn, when mosquitoes are most active.
  • Use screens on doors and windows to keep mosquitoes from entering your home. Repair any broken or damaged screens.
  • Wear long-sleeved shirts and long pants whenever you are outdoors.
  • Don't rely on vitamin B or ultrasonic devices to prevent mosquito bites — neither has been proven effective.
  • Get rid of mosquito-breeding areas around your home — empty standing water from gutters, old tires, wading pools, tarps, potted plants, and outside buckets and pails. Also, be sure the water in pet dishes and birdbaths is changed regularly.

If you choose to use an insect repellent, the Environmental Protection Agency (EPA) offers these safety tips:

  • Don't use insect repellent on skin that is covered by clothing.
  • Don't spray products directly onto your face (spray the repellent onto your hands first and then rub it onto your face) and don't use repellent on your eyes or mouth.
  • Wash repellent off with soap and water once you get back indoors.
  • Don't use repellent on areas of the skin that are irritated or cut.
  • Stop using a repellent if it gives you a rash or other skin reaction. Wash it off and call your doctor.
  • Don't let little kids touch the repellent.
  • Mosquito repellents that include one of the following ingredients are most effective: DEET (N,N-diethyl-meta-toluamide) — those that contain between 10% and 30% of DEET are best, picaridin (KBR 3023), some types of oil of lemon eucalyptus (p-menthane 3,8-diol or PMD).
  • Reapply mosquito repellent according to the directions on the product label. How often to reapply depends on how much DEET is in the product. For children, choose a repellent that contains no more than a 30% concentration of DEET; in higher concentrations, the chemical, which is absorbed through the skin, can be toxic.
  • Don't use a sunscreen that includes an insect repellent — sunscreen should be reapplied frequently but insect repellents should not.
  • If you apply insect repellent to exposed skin, do so sparingly. Do not apply repellent to your child's hands, as it can cause irritation if transmitted to the eyes or mouth.


If anyone in your family has any symptoms of West Nile virus, contact your doctor right away. The doctor will determine your risk based on where you live or whether you've traveled to an area where infected mosquitoes have been found. If the doctor suspects West Nile virus, a blood sample will be drawn and sent to a lab to be tested for infection.


There is no specific treatment for West Nile encephalitis other than supportive therapy (such as hospitalization, intravenous fluids, and respiratory support) for severe cases. Antibiotics will not work because a virus, not bacteria, causes West Nile disease. No vaccine for the virus is currently available.

The public has played a large role in helping track West Nile infection patterns by reporting the appearance of dead birds to the authorities. Continued surveillance is essential to keep the virus in check.

If you see a dead bird in your area, do not handle the body with your bare hands. Instead, contact your state or local health department immediately. Once virus activity is discovered in an area — even before the first case of human disease is reported — mosquito control measures can go into effect, including the widespread spraying of pesticides against adult mosquitoes and larvae.

Reviewed by: Yamini Durani, MD
Date reviewed: May 2015