Patient information: West Nile virus infection

INTRODUCTION

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]. During 2007, WN virus activity was detected in 47 states, the District of Columbia, and Puerto Rico [2]. Outbreaks have also been reported in a number of other regions, including Canada, Europe, and Israel. Although there is evidence that the virus has spread throughout Mexico, the Caribbean, and Central America, 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 late August or early September. The duration is longer in milder climates, where mosquitoes can emerge earlier in the year and survive longer.

WEST NILE VIRUS RISK FACTORS

Mosquito bites increase the risk of contracting West Nile 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 varies, depending upon the time of year and geographic location. About 1 in 5 people who become infected will become ill. However, only about 1 in 150 (less than 1 percent) who become infected develop severe symptoms due to involvement of the brain. (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 is very low because testing for WN virus is routinely done. (See 'Blood transfusion and organ transplant recipients' below.)

WEST NILE VIRUS TRANSMISSION

The vast majority of cases of WN virus are contracted through the bite of a mosquito carrying the infection.

Bird-mosquito-human cycle — WN virus is spread when a mosquito bites an infected bird and then bites a human or other animal. When a bird is infected, 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.

Because birds are the primary hosts of the disease, contact with birds and other animals 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 — There has been at least one probable case of fetal abnormalities in a pregnant women who was infected with WN virus [3]. However, a nationwide study indicated that fetal abnormalities are uncommon following maternal infection [4]. Nevertheless, this study showed that some newborns developed WN virus-related illness, such as rash, fever, and encephalitis when the mother was infected in the weeks before delivery.

Pregnant women who believe they have been infected with the virus should contact their doctor for advice, and babies born to mothers with confirmed or suspected WN virus should be evaluated. Testing for West Nile virus is not recommended for pregnant women who have no symptoms of infection. (See "Patient information: Avoiding infections in pregnancy".)

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 information: Deciding to breastfeed".)

Because of the risk of mother-to-child transmission, pregnant and breastfeeding women should take extra precautions to avoid mosquito bites, especially in areas where WN virus has been reported. Insect repellents containing DEET and picaridin are safe for pregnant women. (See 'West Nile virus prevention' below.)

Blood transfusion and organ transplant recipients — There have been a limited number of cases of people who acquired WN virus after receiving donated blood or organs. Since 2003, all blood donations have been screened for WN virus to identify infected blood.

Blood collection centers also avoid taking blood from people who have fever and other symptoms that might indicate the presence of WN virus. It is not possible to contract the virus by donating blood. Screening procedures and tests are also conducted on potential organ donors whenever possible, and donors and recipients are monitored for symptoms that might indicate the presence of WN virus. (See "Patient information: Blood donation and transfusion".)

WEST NILE VIRUS SYMPTOMS

Most people infected with 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 20 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, a 2004 study suggested that it may take one month or longer for all symptoms to disappear [5]. 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 [6]. 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. In rare cases, severe neuroinvasive disease results in death.

WEST NILE VIRUS DIAGNOSIS

As with any health concern, possible symptoms of 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. The most common test for WN virus detects the presence of antibodies (proteins) in the blood and cerebrospinal fluid (the fluid that surrounds the brain) of a person infected with WN virus. 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.

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 can also be tested if neuroinvasive disease is suspected. 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. Testing for other viruses may be recommended if the person has signs or symptoms of illnesses that resembles West Nile.

WEST NILE VIRUS TREATMENT

As noted above, West Nile fever generally improves on its own. Severe disease may require hospitalization for monitoring and support [5,6]. There is no specific treatment for WN virus, although research is underway to develop treatments.

WEST NILE VIRUS PREVENTION

Basic preventive measures are recommended during mosquito season for people living in areas where 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 or picardin are recommended. 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 that spray large areas with mosquito-killing chemicals may be used to reduce the mosquito population. 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, although studies to develop a vaccine are underway.

SUMMARY

• Birds are the primary hosts of the West Nile 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; less commonly, it develops after receiving donated blood or organs or by mother-to-child transmission.
• Most people who contract WN virus do not develop any symptoms. Approximately 20 percent of people develop mild illness, called West Nile fever, which usually resolves spontaneously.
• A small number of infected people (about 1 in 150) 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 West Nile virus.
• The best way to avoid becoming infected is by preventing mosquito bites with repellents containing DEET or picaridin and wearing protective clothing while outdoors. Pregnant or nursing women, people over age 50, and people who have received organ transplants are especially encouraged to take preventive measures.

WHERE TO GET MORE INFORMATION

Your healthcare provider is the best source of information for questions and concerns related to your medical problem. Because no two people are exactly alike and recommendations can vary from one person to another, it is important to seek guidance from a provider who is familiar with your individual situation.

This discussion will be updated as needed every four months on our web site (www.uptodate.com/patients). Additional topics as well as selected discussions written for healthcare professionals are also available for those who would like more detailed information.

Some of the most pertinent include:

Patient Level Information:
Patient information: Avoiding infections in pregnancy
Patient information: Deciding to breastfeed
Patient information: Blood donation and transfusion

Professional Level Information:
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

A number of web sites have information about medical problems and treatments, although it can be difficult to know which sites are reputable. Information provided by the National Institutes of Health, national medical societies and some other well-established organizations are often reliable sources of information, although the frequency with which they are updated is variable.

• National Library of Medicine

      (www.nlm.nih.gov/medlineplus/healthtopics.html)

• Infectious Diseases Society of America

      (www.idsociety.org)

• Centers for Disease Control and Prevention (CDC)

      Toll-free: (800) 311-3435
      (www.cdc.gov/)

UpToDate wishes to acknowledge Kelly Crowley for her contributions to this topic.

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