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West Nile Virus

West Nile Virus

Alternate Names

  • flavivirus
  • West Nile encephalitis
  • West Nile meningitis


This virus belongs to a group of disease-causing viruses known as flaviviruses. Other viruses in this group include yellow fever virus, St. Louis encephalitis virus, and dengue virus.

What is going on in the body?

West Nile virus is spread between mosquitoes and certain infected birds, such as crows. Once a mosquito bites an infected bird, it can then transfer the virus to a human that it bites later. Most people do not develop enough virus in the bloodstream to become sick. For those who do, they usually become ill within 3 to 15 days after they are bitten.
Most people who do get sick have only mild symptoms, but in a few people, the virus crosses from the blood into the brain. In these cases, the virus causes an inflammation of the brain known as West Nile encephalitis, which can be fatal.


What are the causes and risks of the infection?

About one in five individuals bitten by a mosquito infected with the West Nile virus develop symptoms, most of those very mild. The risk of being bitten by an infected mosquito is linked to living in an area to which West Nile virus has spread, and to spending time outdoors where infected mosquitoes can be found.
About 1 in 150 people bitten by a West Nile-infected mosquito will develop encephalitis. Age is the greatest risk factor for severe neurologic disease, for long-term illness, and for death. In fact, people who are ages 50 to 59 have 10 times the risk of younger people for developing serious disease. Those who are age 80 or older are 43 times as likely to develop serious illness. People age 75 and older are 9 times more likely to die as a result of West Nile encephalitis.
NOTE: Anyone with severe or unusual headaches should seek medical attention right away.


What can be done to prevent the infection?

There is no vaccine for West Nile virus available as yet, though developmental research continues. Thus, prevention rests on two methods:
Reducing the number of mosquitoes in an area. Mosquito breeding sites, such as standing pools of water, should be destroyed emptied. Public health officials may also spray known or potential mosquito breeding areas. An increase in bird deaths, especially crows and ravens, can be a clue that West Nile virus may have entered an area.
Actions that people can take at home include:
  • emptying water from flowerpots, pet food and water dishes, birdbaths, swimming pool covers, buckets, barrels, and cans at least once or twice a week
  • cleaning out clogged rain gutters frequently
  • getting rid of old tires or any other items laying around in a yard that might collect water
NOTE: Home "bug zappers" and Vitamin B have not proven useful in preventing mosquito bites.
Protecting against mosquito bites. People can protect themselves against being bitten by taking the following actions.
  • Lightly spray an insect repellent containing up to 50 percent DEET on exposed skin when going outdoors. (Higher concentrations are not necessary and provide no added protection.)
  • Spray clothes, tents, sleeping bags, and screens with a repellent containing either DEET or permethrin. If spraying clothes, a person does not need to spray the skin areas under the clothes.
  • Wear long-sleeved shirts and long pants when outdoors.
  • Stay indoors at dawn, dusk, and in the early evening. These are peak mosquito biting times.
  • Place mosquito netting over baby cribs, playpens, and carriers when outdoors.
  • Repair window and door screens so mosquitoes can't get indoors.
NOTE: Be sure to follow the directions on any insect repellent used. Apply repellent sparingly to skin of children. Do not use on infants under age 2 months.


How is the infection diagnosed?

The diagnosis of West Nile virus infection can be difficult because the mild symptoms are much like those of many other illnesses. When symptoms of encephalitis become apparent, the list of possible causes becomes shorter. Any time an older adult has a sudden onset of encephalitis-like symptoms, West Nile virus should be suspected. This is especially true if West Nile virus is known to have entered a geographic area, and if it is late summer or early fall, when mosquitoes are at their peak in most areas.
When a person is infected with West Nile virus, the body makes certain antibodies to fight the infection. These antibodies can be detected in blood tests or tests of the spinal fluid. This is the best way for a healthcare professional to make the diagnosis. Researchers are working on faster tests to identify the West Nile virus.

Long Term Effects

What are the long-term effects of the infection?

People who have the mild form of West Nile virus infection usually have no long-term effects. However, as many as two-thirds of the people who survive more serious cases of West Nile encephalitis have long-term effects lasting a year or more, such as:
  • fatigue
  • memory loss
  • difficulty walking
  • muscle weakness
  • depression

Other Risks

What are the risks to others?

West Nile virus cannot be spread from person to person, so there are no risks to others.


What are the treatments for the infection?

There are no specific treatments as yet for this infection. Researchers are working on antiviral medicine, but none is available at this time.
An individual with a serious case of encephalitis needs supportive treatments in a hospital, including:
  • intravenous fluids
  • ventilator, if he or she is having difficulty breathing
  • prevention of secondary infections, such as pneumonia or urinary tract infections
  • intensive nursing care

Side Effects

What are the side effects of the treatments?

Side effects depend on the treatments used. Anyone being treated for West Nile virus infection should talk with the health care professional to learn more about the risks of his or her specific treatments.

After Treatment

What happens after treatment for the infection?

People with mild illness usually recover without treatment, and do not need any follow up once symptoms go away. An individual with West Nile encephalitis needs to visit a healthcare professional regularly until his or her health status becomes stable.


How is the infection monitored?

Ongoing monitoring of any long-term effects may be needed in some cases. Any new or worsening symptoms should be reported to the healthcare professional.

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