West Nile Fever

West Nile fever is a viral infection that can result in inflammation of the brain, called encephalitis (en-seh-fuh-LYE-tis). The virus that causes it spreads to humans by way of infected mosquitoes.

What Is West Nile Fever?

West Nile fever (WNF) is caused by West Nile virus (WNV), which is part of the Flavivirus family * . First discovered in Africa, WNV can infect animals and humans, although animals (mainly birds, but also horses, cats, and bats) are the primary hosts * for the virus.

Most of the time, people with WNF become only mildly ill. In some cases, however, WNF can develop into a life-threatening disease. If the virus passes into the brain, the infection can cause serious inflammation and complications affecting the nervous system. Of those infected, people 50 years of age or older have the greatest risk of developing severe disease.

What Is West Nile Fever?

West Nile fever (WNF) is caused by West Nile virus (WNV), which is part of the Flavivirus family * . First discovered in Africa, WNV can infect animals and humans, although animals (mainly birds, but also horses, cats, and bats) are the primary hosts * for the virus. Most of the time, people with WNF become only mildly ill. In some cases, however, WNF can develop into a life-threatening disease. If the virus passes into the brain, the infection can cause serious inflammation and complications affecting the nervous system.

Of those infected, people 50 years of age or older have the greatest risk of developing severe disease.




West Nile virus neuroinvasive disease incidence in the United States reported to CDC by year, all ages,





West Nile virus neuroinvasive disease incidence in the United States reported to CDC by year, all ages, 1999–2014 The incidence of WNV neuroinvasive disease declined substantially in 2013 compared with 2012 when a large multistate outbreak occurred, with incidence nearing the levels reported in 2002 and 2003. West Nile virus neuroinvasive disease is characterized by the acute onset of fever with headache, myalgia, stiff neck, altered mental status, seizures, limb weakness, or an increased white blood count in cerebrospinal fluid (CSF). It includes illnesses such as aseptic meningitis, encephalitis, or acute flaccid paralysis (AFP).
Table by Lumina Datamatics Ltd. © 2016 Cengage Learning®. SOURCE: West Nile Virus Neuroinvasive Disease Incidence Reported to CDC by Year, 1999–2014, 2014, ArboNET, Arboviral Diseases Branch, Centers for Disease Control and Prevention. Available online at: http://www.cdc.gov/westnile/resources/pdfs/data/5-wnv-neuro-incidence-by-year_1999-2014_06042015.pdf (accessed August 18, 2015).

Is West Nile Fever Contagious?

Generally, a person cannot contract WNF from another infected person or from an infected animal, although transmission of the virus through a blood transfusion * has been confirmed in some cases. Likewise, infected people cannot spread the virus to animals. Scientists think that the virus is transmitted almost exclusively by the bite of an infected mosquito. The chances of becoming ill with WNF actually are very small. Of all the mosquitoes in any area where infected mosquitoes have been found, less than 1 percent carry the virus.

The transmission cycle begins when a mosquito bites an infected bird and takes in blood that contains WNV. If the mosquito then bites a human, it can transmit the virus to that person. Scientists have found no evidence that humans can contract the disease by handling live or dead birds or any other animal that has been infected with the virus. Still, experts recommend that people never handle a dead animal with bare hands; instead, they should always use disposable gloves and place the dead animal in a plastic bag when discarding it.

How Do People Know They Have West Nile Fever?

The first symptoms of WVF are usually fever, headache, and body aches, sometimes accompanied by a rash and swollen lymph nodes * . Serious cases of the disease may cause more severe symptoms, including high fever, stiff neck, muscle weakness, convulsions * , confusion, paralysis * , and coma * . Very severe cases can result in death, but this is rare. Symptoms usually begin 3 to 15 days after infection.

How Is West Nile Fever Diagnosed and Treated?

Diagnosis

If WNF is suspected, the first step for the physician is take a history, which means asking a person about prior health, when symptoms began, and recent travels and activities. This may help determine if the person might have been exposed to an infected mosquito. A blood test can confirm the presence of the virus.

Treatment
WEST NILE INVADES NEW YORK

In the summer of 1999, dead birds began appearing all over the New York City metropolitan area. Public health officials were called in to find out why. They soon learned that the deaths were linked to the virus that causes West Nile fever, an infection that is spread by mosquitoes. Before 1999, West Nile fever had never been seen in the Western Hemisphere.

For mild cases of WNF, no specific treatment exists. A doctor usually recommends rest and over-the-counter medications, such as acetaminophen * * in someone who is too sick to drink or who is vomiting. A person who is having trouble breathing may be put on a ventilator * .




Average annual incidence of West Nile virus neuroinvasive disease in the United States reported to CDC by age group, 1999–2014 West Nile virus neuroinvasive disease is characterized by the acute onset of fever with headache,





Average annual incidence of West Nile virus neuroinvasive disease in the United States reported to CDC by age group, 1999–2014 West Nile virus neuroinvasive disease is characterized by the acute onset of fever with headache, myalgia, stiff neck, altered mental status, seizures, limb weakness, or an increased white blood count in cerebrospinal fluid (CSF). It includes illnesses such as aseptic meningitis, encephalitis, or acute flaccid paralysis (AFP).
Table by Lumina Datamatics Ltd. © 2016 Cengage Learning®. SOURCE: Average Annual Incidence of West Nile Virus Neuroinvasive Disease Reported to CDC by Age Group, 1999–2014, 2014, ArboNET, Arboviral Diseases Branch, Centers for Disease Control and Prevention. Available online at: http://www.cdc.gov/westnile/resources/pdfs/data/8-wnv-neuro-incidence-by-age-group_1999-2014_06042015.pdf (accessed August 18, 2015).

How long WNF illness lasts depends on the severity of the infection. If a person has a mild infection, symptoms often go away in about a week. Recovery from serious infection may take several weeks to months. Most people who are infected with WNV do not become very sick. Only about 1 percent of all infected people become severely ill. Of these severe cases, up to 15 percent are fatal. Elderly people have the highest risk of developing serious complications from the disease.

Can West Nile Fever Be Prevented?

No vaccine is available for WNF, so the best way to prevent the spread of the virus is to prevent mosquito bites. To do so, experts recommend that people avoid being outside at times when mosquitoes are most active (dawn, dusk, and early evening), that people wear long sleeves and long pants, and that they use insect repellent when outside. When using repellent, everyone, especially children, should carefully follow the instructions on the package.

In the United States, health officials have often traced WNV to areas where dead birds have been found. By tracking the disease and looking for patterns of infection, public health officials are better able to prevent future outbreaks. Experts advise people to contact the local or state health department if a dead bird is found in an area where WNV has been reported; a representative will collect the bird for testing.

See also Encephalitis • Meningitis • Zoonoses: Overview

Resources

Books and Articles

Kim, Meeri. “West Nile Virus Outbreaks Will Flare Up, Experts Say.” Washington Post, July 16, 2013. http://www.washingtonpost.com/national/health-science/west-nile-virus-outbreaks-will-flare-upexperts-say/2013/07/16/115dd0c8-ed99-11e2-a1f9-ea873b7e0424_story.html (accessed November 3, 2015).

Salinas, Jess D. “West Nile Virus.” Medscape, July 14, 2015. http://emedicine.medscape.com/article/312210-overview (accessed November 3, 2015).

Websites

MedlinePlus. “West Nile Virus.” U.S. National Library of Medicine. https://www.nlm.nih.gov/medlineplus/westnilevirus.html (accessed November 3, 2015).

U.S. Geological Survey. “West Nile Virus (WNV).” National Wildlife Health Center. http://www.nwhc.usgs.gov/disease_information/west_nile_virus/ (accessed November 3, 2015).

Organizations

Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30329-4027. Toll-free: 800-232-4636. Website: http://www.cdc.gov (accessed November 3, 2015).

World Health Organization. Ave. Appia 20, 1211 Geneva 27, Switzerland. Telephone: 41-22-791-21-11. Website: http://www.who.int/en/ (accessed November 3, 2015).

* Flavivirus family (FLAY-vih-vy-rus) is a group of viruses that includes those that cause dengue fever and yellow fever.

* hosts are organisms that provide another organism (such as a parasite or virus) with a place to live and grow.

* blood transfusion is the process of giving blood (or certain cells or chemicals found in the blood) to a person who needs it due to illness or blood loss.

* lymph nodes (LIMF) are small, bean-shaped masses of tissue containing immune system cells that fight harmful microorganisms. Lymph nodes may swell during infections.

* convulsions (kon-VUL-shuns), also called seizures, are involuntary muscle contractions caused by electrical discharges within the brain and are usually accompanied by changes in consciousness.

* paralysis (pah-RAH-luh-sis) is the loss or impairment of the ability to move some part of the body.

* coma (KO-ma) is an unconscious state, like a very deep sleep. A person in a coma cannot be awakened, and cannot move, see, speak, or hear.

* acetaminophen (uh-see-tehMIH-noh-fen) is a medication commonly used to reduce fever and relieve pain.

* dehydration (dee-hi-DRAY-shun) is a condition in which the body is depleted of water, usually caused by excessive and unreplaced loss of body fluids, such as through sweating, vomiting, or diarrhea.

* ventilator (VEN-tuh-lay-ter) is a machine used to support or control a person's breathing.

  This information is not a tool for self-diagnosis or a substitute for professional care.