West Nile virus is a type of virus that is spread by mosquitoes. The infection it causes may be so mild that people don't even know they have it. But in rare cases, West Nile leads to severe illness that affects the brain or spinal cord. People older than 50 are at the highest risk for serious problems from West Nile.
Most people fully recover from West Nile. But some people who get a severe infection have permanent problems
such as seizures, memory loss, and brain damage. A few people die from it.
Most often, mosquitoes spread the virus by biting birds infected with the virus and then biting people.
Mosquitoes can also spread the virus to other animals, such as horses. But you can't get West Nile from these
animals or from touching or kissing an infected person.
West Nile can spread through an organ transplant or a blood transfusion.
That rarely happens in the United States, though, because all donated blood is screened to see if the virus is
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 still
recommends that women breast-feed. Breast-feeding has many benefits, and the risk of spreading the virus to
babies seems to be very low.
About 80 out of 100 people
who have West Nile have no symptoms.1 When symptoms do
appear, they start 2 to 15 days after the mosquito bite. Mild symptoms may
Most people who have the mild form of West Nile have a fever for 5 days, have a headache for 10 days, and feel tired for more than a month.
West Nile causes serious illness in about 1 out of 150 people who get infected.1 It can lead to swelling of the brain (encephalitis), the spinal cord (myelitis), or the tissues around the brain and spinal cord (meningitis). Symptoms of these diseases may include:
Call your doctor right away if you or someone you know has symptoms like these.
If you have a severe case of West Nile, symptoms can last for weeks or months, especially if the infection has spread to your brain.
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.
The doctor may also test your blood for
antibodies to the virus. The antibodies
can show if you have had a recent West Nile infection. The antibodies don't always appear right away, so
your doctor may test your blood again in a couple of weeks.
You may also have other tests, such as:
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. Be sure to drink enough fluids and get lots of rest. You may also want to take medicine to reduce pain or fever. You may feel well enough to keep doing your normal activities. Ask your doctor if you need to stay home.
If you have severe West Nile, you may need to stay
in a hospital so you can get treatment to help your body fight the illness. You
may get fluids given through a vein (intravenous, or IV) and get help preventing other illnesses such as
pneumonia. If you have severe trouble breathing, a machine called a ventilator may be used to help you breathe. You also may be given medicine to help with pain or fever.
You can contact
your local health department for the latest information on West Nile virus in your
area. It's also a good idea to take steps to lower your risk of getting mosquito bites:
There is no vaccine to prevent West Nile virus in humans, but researchers are working to develop one.
Learning about West Nile virus:
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/.
The National Institute of Allergy and Infectious
Diseases conducts research and provides consumer information on infectious and
CitationsCenters 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.Other Works ConsultedAmerican Public Health Association (2004). 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.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.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.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.Watson JT, et al. (2004). Clinical characteristics and
functional outcomes of West Nile fever. Annals of Internal Medicine, 141(5): 360–365.
June 25, 2013
E. Gregory Thompson, MD - Internal Medicine & W. David Colby IV, MSc, MD, FRCPC - Infectious Disease
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Last modified on: 2 April 2014