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West Nile Virus

Topic Overview

What is West Nile virus?

West Nile virus causes an infection that is spread by certain kinds of mosquitoes. Most often, mosquitoes get infected when they bite infected birds. Then the mosquitoes spread the virus when they bite people or other animals, such as horses. West Nile cannot spread from these animals to people or from person to person through casual contact.

West Nile can spread through an organ transplant or a blood transfusion. So all donated blood in the United States is screened to see if the virus is present. Some evidence suggests that West Nile can spread from a mom to her baby during pregnancy, at birth, or through breast milk. But the U.S. Centers for Disease Control and Prevention (CDC) still recommends that women breast-feed, because the risk of spreading the virus to babies is unclear and the benefits of breast-feeding are known.1

Most people who have West Nile have no symptoms. Or the symptoms may be so mild that people may not even realize that they have the virus. In rare cases, West Nile can lead to swelling of the brain (encephalitis), swelling of the spinal cord (myelitis), or swelling of the tissues around the brain and spinal cord (meningitis). But very few people with West Nile will get a severe illness that affects the brain or spinal cord.2

Anyone who is bitten by a mosquito may get West Nile. Most of the time people fully recover from it. But permanent problems such as seizures, memory loss, and brain damage can occur, especially in children and older people. As you get older, you have a higher risk for getting encephalitis and other serious problems from West Nile. In a few cases, West Nile can be fatal.

What are the symptoms?

About 80 out of 100 people who have West Nile have no symptoms and only about 1 out of 150 people infected develops serious illness.2 When symptoms do appear, they begin 3 to 14 days after the mosquito bite. Mild symptoms include:

  • A fever.
  • Headaches, body aches, or pain in your eyes.
  • A rash, usually on the chest, back, and arms.
  • Feeling very tired.
  • Not feeling hungry.
  • Feeling sick to your stomach or throwing up.
  • Swollen glands (lymph nodes), in rare cases.

In mild cases of West Nile, symptoms usually last for 3 to 6 days. If you get a more severe case of West Nile, symptoms can last for weeks or months. Severe cases that involve problems with the brain and spinal cord are rare, but they may cause:

  • Headaches.
  • A high fever.
  • A stiff neck or paralysis.
  • Confusion.
  • Reduced attention to surroundings.
  • Tremors, convulsions, or muscle weakness.
  • A coma.

In rare cases, West Nile virus can cause death.

How is West Nile virus diagnosed?

If your doctor thinks that you may have West Nile, he or she will ask questions to find out when you were bitten by a mosquito and what symptoms you have.

If you have symptoms of encephalitis, your doctor will test your blood for antibodies to the virus. If you have these antibodies in your blood, your doctor will know that you have West Nile. In that case, your doctor may test your blood 2 weeks later to see if the antibodies are increasing.

Your doctor may also do a test called a spinal tap (lumbar puncture) to look for antibodies in the fluid that surrounds your brain and spinal cord.

To help find out if you have encephalitis, you may have an MRI scan so your doctor can see pictures of your brain.

How is it treated?

There is no treatment for West Nile. Your body just has to fight the infection on its own. If you have a mild case, you can recover at home. But if West Nile is severe, you may need to stay in a hospital while you get treatment to help your body fight the illness. You may get IV (intravenous) fluids, help with breathing (using a ventilator), and help preventing other illnesses such as pneumonia. You also may get medicine to help with pain or fever.

How can you prevent infection?

You can contact your local health department for the latest information on the virus in your area. It's also a good idea to take steps to lower your risk of getting a mosquito bite:

  • Use insect repellent when you go outdoors in the late spring, summer, and early fall.
  • Wear long-sleeved shirts and long pants if you know that you will be in areas with lots of mosquitoes or where you know West Nile virus has been found.
  • Do not leave puddles or open containers of water near your house, because mosquitoes breed in standing water.
  • Stay indoors at dawn, dusk, and in the early evening when mosquitoes are the most active.

A West Nile virus vaccine is available for horses only. If you own horses, you may think about getting the vaccine for them. The death rate from West Nile is much higher in horses than in humans.

Researchers are working to create a vaccine that prevents West Nile virus in humans.

Frequently Asked Questions

Learning about West Nile virus:

Being diagnosed:

Getting treatment:

Symptoms

Most people infected with West Nile virus do not have symptoms. Others have mild symptoms. In rare cases, infection can lead to inflammation of the brain (encephalitis), the spinal cord (myelitis), or the tissues surrounding the brain and the spinal cord (meningitis). The time from infection to the appearance of symptoms (incubation period) usually is 3 to 14 days. Symptoms of the mild form of West Nile virus include:

  • Fever.
  • Headache, body aches, or pain in your eyes.
  • Skin rash.
  • Feeling tired.
  • Not feeling hungry.
  • Feeling sick to your stomach or throwing up.
  • Swollen glands (lymph nodes).

Most people who have the mild form of West Nile virus have a fever for 5 days, a headache for 10 days, and feel tired for more than a month.3

More severe infections involving the brain and spinal cord may cause:

  • Headache.
  • High fever.
  • Stiff neck.
  • Confusion or reduced attention to surroundings.
  • Tremors and convulsions.
  • Muscle weakness or paralysis.
  • Coma.

If you or a person in your care has symptoms like these, contact your doctor immediately.

You may have tremors, muscle aches, and fatigue for months after the illness, especially if your brain became infected. Other possible effects include seizures, memory loss, personality changes, paralysis, and symptoms similar to Parkinson's disease. Some of these problems may last a long time.

Exams and Tests

West Nile virus causes an infection that can lead to inflammation of the brain (encephalitis), the spinal cord (myelitis), or the tissues surrounding it and the spinal cord (meningitis). If your doctor suspects a mosquito-borne illness, he or she will take a medical history to assess your risk of West Nile virus infection. People who live in or travel to areas where the virus has been found are at risk of West Nile virus infection.

If you have symptoms that West Nile is affecting the brain and spinal cord, your blood will be tested for antibodies to the virus, a sign that you have been infected. The initial blood test screens for immunoglobulin M (IgM) and immunoglobulin G (IgG) antibodies to the virus. If the initial test shows West Nile virus infection, you may have a follow-up test 2 weeks later to see if antibodies are increasing. For more information on the immunoglobulins test, see the topic Immunoglobulins.

False-positive results, which show that you have the virus when in fact you do not, can occur if you have been infected with a similar virus, such as the St. Louis encephalitis virus. But this does not affect treatment, because the treatment for all forms of encephalitis is similar. A false-positive result may also occur if you have recently received certain vaccinations, such as those for yellow fever and Japanese encephalitis.

If you have symptoms of meningitis or encephalitis, a lumbar puncture, also called a spinal tap, may be done to look for antibodies and signs of infection in the cerebral spinal fluid, which surrounds the brain and spinal cord. If antibodies are found, another test will be done to look at the virus's genetic material.

You may have a magnetic resonance imaging (MRI) test to identify encephalitis or inflammation of the brain and the membranes surrounding the brain and the spinal cord (meningoencephalitis).

Donated blood may be screened with a West Nile virus blood test called Procleix. This blood test detects RNA in donated blood and plasma. Screening donated blood for West Nile virus helps to keep it safe so that people who are receiving the blood do not get the virus.

Treatment Overview

West Nile virus causes an infection that can lead to inflammation of the brain (encephalitis), the spinal cord (myelitis), or the tissues surrounding it and the spinal cord (meningitis). No specific treatment is available. Mild infections go away on their own. Severe cases of encephalitis are treated with supportive care in a hospital. Supportive care involves helping the body fight illness on its own. It often is used when no specific treatment exists for an illness, as is the case with some viruses.

Supportive treatment for West Nile virus can include receiving fluids through a vein (intravenous, or IV), help with breathing (using a ventilator), and prevention of secondary infections, such as pneumonia. Medicine may also be used to relieve pain or a fever. For more information, see the topic Encephalitis or Meningitis.

Home Treatment

West Nile virus causes an infection that can lead to inflammation of the brain (encephalitis), the spinal cord (myelitis), or the tissues surrounding the brain and the spinal cord (meningitis). If you have a fever and headache that continue for more than 2 or 3 days during West Nile virus season, or if you have any of the more severe symptoms of West Nile encephalitis, call your doctor immediately.

If your doctor determines that you have a mild infection, make sure to drink plenty of fluids and get lots of rest. You may also want to take medicine to reduce pain or a fever. You may feel well enough to continue your normal activities. Talk to your doctor about whether you need to stay home.

You can take steps to lower your risk of mosquito bites:

  • Stay indoors at dawn, at dusk, and in the early evening, when mosquitoes are most active.
  • Wear long-sleeved shirts and long pants whenever you are outdoors and are likely to be where mosquitoes are.
  • Avoid wearing floral fragrances from perfumes, soaps, hair care products, and lotions. These may attract mosquitoes.
  • Spray clothing with an insect repellent containing permethrin or DEET (N,N-diethyl-meta-toluamide), because mosquitoes may bite through thin clothing. DEET can damage plastic items, such as watch crystals or eyeglass frames, and some synthetic fabrics. You also can use natural products such as soybean-based Bite Blocker.
  • Apply insect repellent sparingly to exposed skin. An effective repellent will contain 24% DEET. DEET in concentrations greater than 50% does not provide any additional protection.
  • Avoid applying repellent to the hands of children. Repellents may irritate the eyes and mouth.
  • Whenever you use an insecticide or insect repellent, be sure to read and follow the directions for use.
  • Do not keep open containers of water near your house. Standing water is a breeding place for mosquitoes.

Vitamin B and ultrasonic or ultraviolet (UV) devices such as "bug zappers" are not effective for preventing mosquito bites.

Mosquito protection time of DEET
Concentration of DEET Protection time (approximate)
30% 6 hours
15% 5 hours
10% 3 hours
5% 2 hours

Other Places To Get Help

Organizations

Division of Vector-Borne Infectious Diseases, U.S. Centers for Disease Control and Prevention (CDC)
Web Address: www.cdc.gov/ncidod/dvbid/westnile
 

This Web site, maintained by the CDC Division of Vector-Borne Infectious Diseases, contains information and the latest statistics about West Nile virus. It includes a link to state health departments for local information and to report possible cases of insect-borne (arboviral) illness. The Web site also has information on other insect-borne illnesses. The site includes a link to the latest U.S. Geological Survey maps showing where West Nile virus has been found. You can find the maps at the following Web site: http://westnilemaps.usgs.gov/2002/.


National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health
NIAID Office of Communications and Government Relations
6610 Rockledge Drive, MSC 6612
Bethesda, MD  20892-6612
Phone: 1-866-284-4107 toll-free
Phone: (301) 496-5717
Fax: (301) 402-3573
TDD: 1-800-877-8339
Web Address: www.niaid.nih.gov
 

The National Institute of Allergy and Infectious Diseases conducts research and provides consumer information on infectious and immune-system-related diseases.


Public Health Agency of Canada
Web Address: www.phac-aspc.gc.ca/wn-no/index-eng.php
 

The Public Health Agency of Canada's West Nile virus information Website provides an overview of the virus, its effects, and how it is spread; information on how you can protect yourself and your family from infection; and the latest statistics and maps of reported West Nile virus cases in Canada.


Related Information

References

Citations

  1. Centers for Disease Control and Prevention (2012). West Nile virus, pregnancy and breastfeeding. Available online: http://www.cdc.gov/ncidod/dvbid/westnile/qa/breastfeeding.htm.
  2. Centers for Disease Control and Prevention (2012). West Nile virus: What you need to know. Available online: http://www.cdc.gov/ncidod/dvbid/westnile/wnv_factsheet.htm.
  3. Watson JT, et al. (2004). Clinical characteristics and functional outcomes of West Nile fever. Annals of Internal Medicine, 141(5): 360–365.

Other Works Consulted

  • American Public Health Association (2008). Other mosquito-borne and culicoides-borne fevers: Bunyamwera viral fever, Rift Valley fever, West Nile fever (including Kunjin viral fever), Group C virus disease, Oropouche virus disease, Zika virus disease. In DL Heymann, ed., Control of Communicable Diseases Manual, 18th ed., pp. 45–48. Washington, DC: American Public Health Association.
  • Kanzaria HK, Hsia RY (2012). Mosquitoes and mosquito-borne diseases. In PS Auerbach, ed., Wilderness Medicine, 6th ed., pp. 883–900. Philadelphia: Mosby Elsevier.
  • Vaughn DW, et al. (2010). Flaviviruses (yellow fever, dengue, dengue hemorrhagic fever, Japanese encephalitis, West Nile encephalitis, St. Louis encephalitis, tick-borne encephalitis). In GL Mandell et al., eds., Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases, 7th ed., vol. 2, pp. 2133–2156. Philadelphia: Churchill Livingstone Elsevier.

Credits

By Healthwise Staff
Primary Medical Reviewer E. Gregory Thompson, MD - Internal Medicine
Specialist Medical Reviewer W. David Colby IV, MSc, MD, FRCPC - Infectious Disease
Last Revised August 10, 2012

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