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Patient education: West Nile virus infection (Beyond the Basics)

Lyle R Petersen, MD, MPH
Section Editor
Martin S Hirsch, MD
Deputy Editor
Jennifer Mitty, MD, MPH
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West Nile (WN) virus is an infection that primarily affects birds. Humans and other animals can also become infected, usually after being bitten by a mosquito that previously bit an infected bird.

WN virus was first discovered in Uganda in 1937, but it became a household term in the United States in 1999 when the first cases were reported in New York City [1]. Human cases of WN virus disease have occurred in all 48 contiguous states and the District of Columbia [2]. Outbreaks have also been reported in a number of other regions, including Canada, Europe, Israel, Greece, and Russia. Although there is evidence that the virus has spread throughout Latin America and the Caribbean, few human cases have been reported from these areas.

WN virus is a seasonal epidemic in the United States, lasting from summer through fall, with a peak in activity around mid-August to early September. The duration is longer in milder climates, where mosquitoes can emerge earlier in the year and survive longer.


Mosquito bites increase the risk of contracting West Nile (WN) virus, particularly during the summer in an area where WN virus activity has been reported. However, the chances of contracting WN virus after a mosquito bite vary, depending upon the time of year and geographic location. About 1 in 4 people who become infected will become ill. However, only about 1 in 230 (less than 1 percent) who become infected develops severe symptoms due to involvement of the brain or spinal cord. (See 'Severe disease' below.)

Simple measures to avoid being bitten can reduce the risk of acquiring WN virus (see 'West Nile virus prevention' below). People over the age of 50 are encouraged to take extra precautions because they are at higher risk of developing severe symptoms of WN virus if they become infected.

WN virus can also be acquired from a blood transfusion or organ transplant. However, the risk from blood transfusion is very low because testing for WN virus is routinely done. (See 'Blood transfusion and organ transplant recipients' below.)


Nearly all cases of West Nile (WN) virus are contracted through the bite of a mosquito carrying the infection.

Bird-mosquito-human cycle — When a bird is infected with WN virus, the virus can circulate in its blood for several days; a mosquito that feeds on it during this period may become a carrier of the virus. If the mosquito later bites a person, the virus can be injected from the insect's salivary glands into the person's blood. Mosquitos who are carriers also spread WN virus to other uninfected birds via bites.

Because birds are the primary hosts of the disease, they have high levels of virus. Therefore, contact with birds that are dead or ill should be avoided, and the local health department should be contacted for instructions.

WN virus is not spread through touching, kissing, or sharing utensils with a person with the infection, or through contact with a healthcare provider who has treated patients with the virus.

Pregnant and nursing women — A nationwide study has indicated that fetal abnormalities are uncommon following maternal infection [3]. Pregnant women who believe they have been infected with the virus should contact their doctor for further evaluation and advice. Testing for WN virus is not recommended for pregnant women who have no symptoms of infection. (See "Patient education: Avoiding infections in pregnancy (Beyond the Basics)".)

It is believed that WN virus may be passed from an infected mother to her baby through breast milk. This is based on only one case, in which a baby tested positive for the virus but did not develop any symptoms. There are many known benefits to breastfeeding and experts continue to recommend breastfeeding for women with known or suspected WN virus infection. (See "Patient education: Deciding to breastfeed (Beyond the Basics)".)

Pregnant and breastfeeding women should take extra precautions to avoid mosquito bites, especially in areas where WN virus has been reported. A discussion of the safety and use of insect repellents in pregnant and breastfeeding women can be found on the Centers for Disease Control and Prevention website. (See 'West Nile virus prevention' below.)

Blood transfusion and organ transplant recipients — A limited number of people have acquired WN virus after receiving donated blood or organs. Since 2003, all blood donations are screened for WN virus to identify infected blood. It is not possible to contract the virus by donating blood. (See "Patient education: Blood donation and transfusion (Beyond the Basics)".)


Most people infected with West Nile (WN) virus never develop symptoms and will not know that they have the virus. The virus remains in the body for a few days before the immune system begins to fight the infection and eliminate it. Individuals that become ill generally exhibit symptoms within two to 15 days of acquiring the virus.

West Nile fever — Approximately 25 percent of people who contract WN virus develop West Nile fever. This is a self-limited illness, usually characterized by fever, headache, tiredness, back pain, muscle aches, decreased appetite, and rash. Other symptoms can include nausea, vomiting, diarrhea, and abdominal pain.

These symptoms typically last from a few days to several weeks. However, several studies suggest that it may take one month or longer for all symptoms to disappear [4]. Prolonged fatigue is common.

Severe disease — A small number of people infected with WN virus (less than 1 percent) develop severe neuroinvasive disease, in which the virus travels through the blood and infects the brain and spinal cord. Although people of all ages may acquire severe disease, people over age 50 are at higher risk for severe disease and should take extra precautions to avoid mosquitoes. Severe disease in children is uncommon. People with certain types of advanced cancer and those taking immunosuppressive medication after organ transplantation are at very high risk of severe disease after WN virus infection.

Signs and symptoms of severe disease include headache and fever, as well as disorientation, convulsions, muscle weakness, coma, and paralysis. These symptoms can last for several weeks and long-lasting neurological abnormalities can develop.

At one year, less than 40 percent of patients with severe disease achieve a full recovery; full recovery is most likely to occur in people who are less than 65 years of age [5]. Long-term neurologic abnormalities include muscle weakness, loss of concentration, confusion, and light-headedness, which continues to affect many people at 18 months. Recovery from severe disease is variable; some severely ill patients having complete or nearly complete recovery. Only about one-third of patients with paralysis will fully recover. In about 10 percent of cases, severe neuroinvasive disease results in death.


As with any health concern, possible symptoms of West Nile (WN) virus should be discussed with and evaluated by a healthcare provider. The provider will review the person's history to determine the likelihood that he or she has contracted the virus (eg, whether the patient lives in or has visited an area with WN virus activity; recent exposure to mosquitoes).

If a person's symptoms and history suggest that they have WN virus infection, blood is drawn for testing, particularly when severe disease is present. The most common test for WN virus detects the presence of antibodies (proteins) in the blood. Antibodies are produced in response to a foreign substance, such as a bacterium or virus. If antibodies are detected, it is a strong indication that the person has been infected with WN virus. Blood tests in patients with WN fever may be negative if blood is drawn within the first week of illness; later testing can help confirm the diagnosis.

For most people with neuroinvasive disease from WN virus, a blood test for WN virus is positive within about one week of the first symptoms. Cerebrospinal fluid (the fluid that surrounds the brain) can also be tested if neuroinvasive disease is suspected.


As noted above, West Nile fever generally improves on its own. Severe disease may require hospitalization for monitoring and support [4,5]. There is no specific treatment for West Nile virus, although research is underway to develop treatments.


Basic preventive measures are recommended during mosquito season for people living in areas where West Nile (WN) virus is prevalent. The best way to avoid contracting the virus is to prevent mosquito bites. Mosquitoes tend to be most active at dawn and dusk, so extra vigilance is required at these times. While outdoors, insect repellents containing DEET, picardin, oil of lemon eucalyptus, 2-undecanone, and IR3535 are recommended [6]. Long pants and long sleeves will also minimize skin exposure. Applying permethrin to clothing (not skin) can further reduce the risk of insect bites. Bug zappers, sonic devices, and mosquito traps have not been shown to reduce the risk of WN virus infection.

Community mosquito control programs use several methods to reduce the mosquito population [6]. When there is an outbreak or impending outbreak, large areas may be sprayed with mosquito-killing chemicals to rapidly reduce the mosquito population. Very small amounts of chemicals are required for this, so individual exposures are negligible and environmental impact is minimized. Individuals can help to reduce the number of mosquitos by draining areas of standing water (eg, buckets, bird baths, gutters, wading pools, and pet dishes), where mosquitoes frequently lay eggs. Ensuring that all windows and doors have good quality screens also helps keep mosquito exposure to a minimum.

There is currently no human vaccine for WN virus.


Birds are the primary hosts of the West Nile (WN) virus; the virus can be spread when a mosquito bites an infected bird and then bites a human or animal. The virus is not transmitted through person-to-person contact.

Most cases develop after being bitten by an infected mosquito.

Most people who contract WN virus do not develop any symptoms. Approximately 25 percent of people develop mild illness, called West Nile fever, which usually resolves spontaneously.

A small number of infected people (about 1 in 250) develop severe or neuroinvasive disease, which affects the central nervous system and can result in permanent neurologic dysfunction or even death.

There is no treatment for WN virus.

The best way to avoid becoming infected is by preventing mosquito bites with repellents containing DEET, picaridin, oil of lemon eucalyptus, 2-undecanone, or IR3535 and by wearing protective clothing while outdoors.


Your healthcare provider is the best source of information for questions and concerns related to your medical problem.

This article will be updated as needed on our web site (www.uptodate.com/patients). Related topics for patients, as well as selected articles written for healthcare professionals, are also available. Some of the most relevant are listed below.

Patient level information — UpToDate offers two types of patient education materials.

The Basics — The Basics patient education pieces answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials.

Patient education: Insect bites and stings (The Basics)
Patient education: Avoiding infections in pregnancy (The Basics)

Beyond the Basics — Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are best for patients who want in-depth information and are comfortable with some medical jargon.

Patient education: Avoiding infections in pregnancy (Beyond the Basics)
Patient education: Deciding to breastfeed (Beyond the Basics)
Patient education: Blood donation and transfusion (Beyond the Basics)

Professional level information — Professional level articles are designed to keep doctors and other health professionals up-to-date on the latest medical findings. These articles are thorough, long, and complex, and they contain multiple references to the research on which they are based. Professional level articles are best for people who are comfortable with a lot of medical terminology and who want to read the same materials their doctors are reading.

Clinical manifestations and diagnosis of West Nile virus infection
Epidemiology and pathogenesis of West Nile virus infection
Insect bites
Prevention of arthropod and insect bites: Repellents and other measures
Treatment and prevention of West Nile virus infection

The following organizations also provide reliable health information.

National Library of Medicine


Infectious Diseases Society of America


Centers for Disease Control and Prevention (CDC)

     Toll-free: (800) 311-3435


Literature review current through: Nov 2017. | This topic last updated: Thu Nov 30 00:00:00 GMT+00:00 2017.
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