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West Nile Virus

What is West Nile virus?

West Nile virus (WNV) is a virus carried by mosquitoes. Most people infected with the virus do not have symptoms or have only a mild illness. Less than 1 in 100 people who are infected with the virus develop serious illness. Serious forms of illness caused by WNV include encephalitis (inflammation of the brain) and meningitis (inflammation of the membranes that cover the brain and spinal cord).

How does it occur?

Wild and domestic birds, mainly crows, carry the West Nile virus. Mosquitoes become carriers of the virus when they bite infected birds. Humans can get the virus when they are bitten by infected mosquitoes. There are no known cases of a human getting WNV directly from an infected bird.

The risk of West Nile virus is seasonal in the northern states of the US and begins in the spring. The peak time for infection is mid to late August. In milder southern climates, the infection may occur year-round. The risk of severe infection is greatest for people who are over 70 years old or who have a weakened immune system.

West Nile virus may be spread from person to person through blood transfusions and organ transplants. However, blood centers and hospitals screen for West Nile Virus in donated blood and organs. The virus might also be transmitted through breast milk. However, the risk of transmission of the virus to the baby is believed to be very low. If you are breast-feeding and you have a WNV infection, the American Academy of Pediatrics and the CDC say that it's OK to keep breast-feeding.

The infection is not spread by normal person-to-person contact like touching, kissing, or caring for someone who is infected.

What are the symptoms?

About 4 out of 5 infected people do not have symptoms. Children are more likely to have symptoms than adults. When symptoms do occur, they are usually mild and last a few days. Symptoms of WNV infection may include:

  • fever
  • headache
  • body aches
  • nausea or vomiting
  • rash
  • swollen lymph glands.

A WNV infection usually does not involve the brain. However, a few infected adults and children develop encephalitis or meningitis. Symptoms of these severe illnesses include:

  • tremors
  • stiff neck
  • high fever
  • severe headache
  • paralysis
  • muscle weakness
  • confusion
  • convulsions
  • coma.

Some people develop a polio-like syndrome with sudden weakness and paralysis.

Symptoms usually appear 3 to 14 days after you are bitten by an infected mosquito.

How is it diagnosed?

Your healthcare provider will ask about your symptoms and examine you. Tests you may have are:

  • blood tests
  • spinal tap (lumbar puncture), a procedure in which a needle is inserted between 2 bones of the spine into the spinal canal to take a sample of spinal fluid to test for meningitis
  • electroencephalogram (EEG), a test that measures the electrical activity of the brain (brain waves)
  • computerized tomography (CT) or magnetic resonance imaging (MRI) scans of the brain.

How is it treated?

There is no medicine that cures West Nile virus. If your symptoms are mild, they will go away on their own. Most people with mild symptoms can care for themselves at home. If you have a serious infection, you may need to stay at the hospital. You may be given intravenous (IV) fluids and pain relievers. For severe or life-threatening infection, you may need treatment in an intensive care unit.

How long will the effects last?

Most people infected with WNV, including nearly all children, do not get seriously ill, and they recover fully. Symptoms usually last 3 to 6 days, but they can last as long as several weeks or months.

If you have a serious infection, you may be ill for weeks or months. Your nervous system or brain may be injured. The injury is sometimes permanent.

If you get West Nile virus, you will probably be immune to future infection by the virus, but your immunity might decrease over time.

How can I take care of myself?

  • Rest.
  • Take acetaminophen (Tylenol) or an anti-inflammatory medicine such as ibuprofen for fever, headache, or muscle aches. Nonsteroidal anti-inflammatory medicines (NSAIDs) may cause stomach bleeding and other problems. These risks increase with age. Read the label and take as directed. Unless recommended by your healthcare provider, do not take for more than 10 days for any reason.
  • Contact your healthcare provider right away if you develop new or worsening symptoms, especially of the nervous system, for example:
    • a stiff neck
    • severe headache
    • a fever of 101.5°F (38.6°C) or higher that does not go down with medicine
    • tremors
    • seizures (convulsions)
    • slurred speech
    • confusion
    • muscle weakness or paralysis (inability to use an arm or leg).

If you are older or live alone, you may need someone to check to make sure you do not develop serious symptoms, such as confusion and coma.

Can West Nile virus infection be prevented?

Yes, WNV can be prevented by taking precautions to avoid exposure to mosquitoes:

  • Stay indoors at dawn, dusk, and in the early evening, when mosquitoes are most likely to be around.
  • Wear long-sleeved shirts and long pants when outdoors.
  • Use an insect repellent on skin that is not covered by clothing whenever you are outdoors. Do not use more repellent than recommended in the package directions. Don't put repellent on open wounds or rashes. Don't put it on your eyes or mouth. When using sprays, do not spray directly on your face. Spray the repellent on your hands first and then put it on your face. Wash the spray off your hands. Be careful with children because repellents can make them ill.

    Studies have shown that repellent products containing either DEET or picaridin as active ingredients give longer lasting protection than others. However, in some studies oil of lemon eucalyptus, a plant-based repellent, gave as much protection as repellents with low concentrations of DEET. Oil of lemon eucalyptus should not be used on children under age 3.

    • Adults should use products with no more than 35% DEET. Children older than 2 months can use repellents with no more than 30% DEET. DEET should be washed off your body when you go back indoors.
    • Picaridin may irritate the skin less than DEET.
  • Spray clothes with repellents because mosquitoes may bite through thin clothing. You may spray clothing with DEET or permethrin. Some products containing permethrin are recommended for use on clothing, shoes, bed nets, and camping gear. Permethrin should be reapplied to the clothing according to the label instructions. Some commercial products are available pretreated with permethrin. Do not put permethrin directly on your skin.
  • Install or repair window and door screens so it is harder for mosquitoes to get indoors.
  • Mosquitoes lay eggs in water. To reduce mosquito breeding, drain standing water. Routinely empty water from flowerpots, pet bowls, clogged rain gutters, swimming pool covers, buckets, barrels, cans, and other items that collect water.

Note: Vitamin B and ultrasonic devices do not help prevent mosquito bites.

A vaccine is available to protect horses from West Nile virus. No vaccine is available for humans yet, but several companies are working to develop a human vaccine.

Disclaimer: This content is reviewed periodically and is subject to change as new health information becomes available. The information provided is intended to be informative and educational and is not a replacement for professional medical evaluation, advice, diagnosis or treatment by a healthcare professional.

HIA File infd4542.htm Release 13/2010

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