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West Nile Virus

Alternative Names

Flavivirus , West Nile encephalitis , West Nile meningitis

West Nile Virus Definition

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. The infection caused by West Nile virus is one of the newest infectious diseases in the US, having been diagnosed for the first time in 1999 in New York City. It has been common in the eastern part of the world for many years, especially in Africa, Europe, and the Middle East.

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. This results in more serious symptoms that may even cause death. In these cases, the virus causes an inflammation of the brain known as West Nile encephalitis or an inflammation of the meninges known as West Nile meningitis.

West Nile Virus signs and symptoms of the infection?

Most people who become infected with West Nile virus have no symptoms. For people who do develop symptoms, most will be mild and can include:

  • fever
  • headache
  • body aches
  • skin rash
  • swollen glands

These symptoms generally last less than a week. Many people do have ongoing fatigue for a longer period.

A small number of infected people may develop a more serious illness. About 60 to 70 percent of those will have West Nile encephalitis, an inflammation of the brain. The other 30 to 40% will have West Nile meningitis, an inflammation of the meninges, which are the membranes covering the brain and spinal cord. Some people with meningitis eventually progress to encephalitis. If they do not progress to encephalitis, the people who have meningitis usually have a less severe illness, which is less likely to cause death.

Symptoms of West Nile encephalitis include:

  • headache
  • high fever
  • muscle weakness
  • stiff neck
  • mental status changes
  • nausea and vomiting
  • coma
  • tremors
  • paralysis
  • seizures

A small number of cases of encephalitis have been fatal. This has been most common in older adults.

West Nile Virus causes and risks of the infection?

Anyone who is bitten by a mosquito infected with the West Nile virus may develop symptoms, but only about 1 in 5 people actually do. It is not known why. Experts do know that the risk of being bitten by an infected mosquito is linked to living in an area where West Nile virus has spread, and spending time outdoors where infected mosquitoes can be found.

About 1 in 150 people bitten by a West Nile-infected mosquito will develop meningitis, encephalitis, or both. 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. 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 this illness.

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 yet, though researchers are working on it. So 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. Governments 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 people can take at home include:

  • Empty water from flowerpots, pet food and water dishes, birdbaths, swimming pool covers, buckets, barrels, and cans at least once or twice a week.
  • Clean out clogged rain gutters frequently.
  • Get 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.

Protect 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.

West Nile Virus infection diagnosed?

A diagnosis of West Nile virus infection can be hard to make because the mild symptoms are much like those of many other illnesses. When symptoms become more severe, diagnosis can be easier for the doctor.

Doctors rely on a physical exam, history of symptoms, and certain lab tests to make a diagnosis in most people. 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 doctor to make the diagnosis. Researchers are working on faster tests to identify the West Nile virus. 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. But as many as two-thirds of the people who survive the more serious West Nile encephalitis do have long-term effects lasting a year or more, such as:

  • fatigue
  • memory loss
  • difficulty walking
  • muscle weakness
  • depression


West Nile Virus 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 yet for this infection. Researchers are working on antiviral medicine, but none is available at this time. People who have encephalitis will need to stay in the hospital until they get better. Supportive treatments in the hospital will include:

  • intravenous fluids
  • ventilator, if needed
  • prevention of secondary infections, such as pneumonia or urinary tract infections
  • intensive nursing care


West Nile Virus side effects of the treatments?

There are no common side effects to the treatments outlined above.


What happens after treatment for the infection with West Nile Virus?

People with mild illness often recover without treatment, and do not need any follow up once symptoms go away. People who have West Nile encephalitis will usually need to visit their doctors on a regular basis until their health status becomes stable.


How is the West Nile Virus 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 doctor.


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