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Culex tarsalis mosquito, which transmits West Nile virus
Close-up anterior view of a Culex tarsalis mosquito about to begin feeding on a human host.
Credit: CDC

West Nile Virus

West Nile virus (WNV) first emerged in the Western Hemisphere in 1999 in the New York City area and has since spread across the United States. The virus is transmitted to humans by mosquitoes.

Most human infections are mild, causing fever, headache, and body aches, often accompanied by a skin rash and swollen lymph glands. If the virus crosses the blood-brain barrier, however, it can cause life-threatening conditions that include inflammation of the brain and spinal cord.

NIAID supports research on WNV through its comprehensive emerging infectious disease program. This program supports research on bacterial, viral, and other types of disease-causing microbes.

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West Nile fever is caused by a virus that is transmitted to humans by infected mosquitoes. WNV is a member of the Flaviviridae family. Other diseases caused by flaviviruses include yellow fever, Japanese encephalitis, dengue, Saint Louis encephalitis, and tickborne encephalitis.​

Transmission electron micrograph of West Nile virus
Transmission electron micrograph of West Nile virus.
Credit: CDC

WNV was first isolated in Uganda in 1937. Today, it is most commonly found in Africa, West Asia, Europe, and the Middle East. In 1999, it was found in the Western Hemisphere for the first time in the New York City area. In 2000, it appeared again in birds and mosquitoes and then spread to other parts of the eastern United States. By 2004, the virus had been found in birds and mosquitoes in every state except Alaska and Hawaii.

In 2012, the United States experienced the highest number of human cases of West Nile disease since 2003, with​ a confirmed 5,674 cases, according to the Centers for Disease Control and Prevention (CDC).

Visit the CDC WNV website for the most recent numbers of reported human cases of WNV infection in the United States.


The first step in the transmission of WNV happens when a mosquito bites an infected bird or animal and gets the virus while feeding on the animal's blood. The infected mosquito can then transmit the virus to another bird or animal when it feeds again.

Crows are highly susceptible to lethal WNV infection, as are robins, blue jays, and other birds. Scientists have identified more than 138 bird species that can be infected with WNV and more than 43 mosquito species that can transmit the virus.

Although the virus usually cycles between mosquitoes and birds, infected female mosquitoes also can transmit WNV to humans and other "incidental hosts," such as horses. With so many susceptible hosts to amplify the virus and so many mosquito species capable of transmission, WNV has spread rapidly across the United States.

Most cases of human disease occur in people over 50 years of age and in people with impaired immune systems. In a very small number of cases, WNV has been spread through blood transfusions, organ transplants, and breastfeeding, according to CDC. The virus does not spread through casual contact, such as touching or kissing.


Approximately 80 percent of people (about 4 out of 5) who are infected with WNV will not show any symptoms at all, according to CDC.

Most people who do develop illness (up to 20 percent of people infected) have relatively mild disease with symptoms that include fever, headache, body aches, nausea, and vomiting. There may be swollen lymph glands or a skin rash on the chest, stomach, and back. The illness often ends after a few days, but some people may experience symptoms for several weeks.

About 1 in 150 people infected with WNV will develop severe illness. Symptoms of serious illness include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis. Serious illness can occur in people of any age; however, people over age 50 and those with weakened immune function are at the highest risk for getting severely ill when infected with WNV. These symptoms may last several weeks to years, and neurological effects may be permanent, according to CDC.

People typically develop symptoms between 3 and 14 days after they are bitten by an infected mosquito.

Diagnosis and Treatment

According to CDC, preliminary diagnosis of WNV infection is often based on the patient's symptoms, recent activities, places and dates of travel, and whether the virus has been detected in the location where the patient was bitten by a mosquito.

Laboratory diagnosis of WNV infections generally involves testing blood serum or cerebrospinal fluid for the presence of WNV-specific antibodies, or in some cases, the viral nucleic acid. The Food and Drug Administration has approved four commercially produced diagnostic tests for WNV. None of these tests can yet be performed in doctors’ offices; rather, the blood sample must be sent to state health labs or other testing facilities for analysis.

No specific drug treatment is available for WNV infections. Milder WNV illness improves on its own and does not necessarily require medical attention. Severe cases, however, usually require hospitalization to provide intravenous fluids and respiratory support and to prevent other infections. If you develop symptoms of severe WNV illness, such as unusually severe headaches or confusion, seek immediate medical attention. Pregnant women and nursing mothers should talk to their physicians if they develop symptoms that could be WNV.


NIAID conducts and supports the research and development of new diagnostic technologies and treatments for WNV. Researchers at several institutions are working to develop simple devices for rapid, point-of-care diagnosis of arboviruses (arthropod-borne viruses), including WNV. NIAID-funded scientists are also working to develop rapid diagnostics to detect multiple viruses known to cause hemorrhagic fevers and encephalitis. It is hoped that the different approaches being investigated will lead to cost-effective, rapid diagnostics that could eventually be performed in physician’s offices or even at home, so that treatments could be administered very early in infection to reduce severity of disease.

Through the NIAID Preclinical Services Program, researchers are evaluating compounds for antiviral activity against a panel of viruses, including WNV. In 2011 and 2012, nine compounds were tested for activity against WNV in rodent models of disease, and 220 compounds were tested in vitro (in cell culture). Promising compounds will be further analyzed for safety and efficacy.

In 2011 and 2012, NIAID awarded six research grants to scientists studying small-molecular-weight compounds as potential antivirals to treat flaviviruses, including WNV. Other therapeutic approaches that are being investigated include

  • Monoclonal antibodies that target WNV particles to inhibit spread of infection
  • Monoclonal antibodies to target and destroy WNV-infected cells
  • Broad-spectrum therapeutics for flaviviruses, including WNV
  • Therapeutics that are able to cross the blood-brain barrier
  • Identification of drug targets for viruses that infect the nervous system
  • Development of broad-spectrum immunotherapeutics, such as monoclonal antibodies or bi-specific antibodies, that target specific viral pathways


Man applies mosquito repellent
A man applies a DEET repellent to his long-sleeved shirt to repel mosquitoes.
Credit: CDC

Currently, there are no vaccines to prevent WNV infection in people. The best way to prevent infection is to limit exposure to mosquitoes. For example

  • Use insect repellent containing a U.S. Environmental Protection Agency-registered active ingredient when you are outside.
  • Wear long sleeves and pants at dusk and dawn, when many mosquitoes are most active, or stay indoors during these hours.
  • Screen windows and doors to keep mosquitoes out.
  • Get rid of mosquito breeding sites by emptying standing water. Water should be dumped from flower pots, buckets and barrels. Drill holes in tire swings so water drains out. Keep children's wading pools empty and on their sides when they are not being used. Change the water in pet dishes, and replace the water in bird baths weekly.


NIAID supports research on a variety of vaccine approaches that could potentially lead to a safe and effective preventive vaccine for WNV. These approaches include vaccines containing cocktails of individual WNV proteins and chimeric vaccines, which combine genes from more than one virus into a single vaccine. A third approach involves DNA vaccines, in which DNA that codes for a particular virus protein is combined with bacterial DNA, and the combined product is injected directly into the skin of the person or animal being vaccinated.

Currently, there is no licensed WNV vaccine for people. In 2005, the U.S. Department of Agriculture licensed a DNA vaccine to prevent WNV in horses, and since then, at least four other types of WNV vaccines have been approved for use in horses. Because federal regulations for veterinary products are less stringent than those intended for human use, products developed for animals can proceed at a faster pace.

WNV vaccine research conducted and supported by NIAID includes

  • Early-stage research by scientists at Oregon Health and Science University who used hydrogen peroxide treatment as a way to develop inactivated vaccines. This method is being investigated to develop vaccines for a number of illnesses, including West Nile fever and neuroinvasive disease, yellow fever, and dengue.
  • A vaccine grown in insect cells that has been shown to produce protective antiviral antibodies in mice infected with WNV and has been shown to prevent WNV disease in horses. This research was conducted by the biotech company L2 Diagnostics, LLD, of New Haven, Connecticut. It remains to be seen if the product will be advanced for use in humans.
  • A candidate vaccine made with portions of two WNV proteins is being developed by Hawaii Biotech, Inc., in Aiea, Hawaii. Early research on this product was funded by NIAID. The company has completed a Phase I trial of the vaccine, which successfully demonstrated safety and immunogenicity.
  • NIAID-supported researchers at Duke University are working on a WNV vaccine made of immune proteins (called mast cell-activating peptides, or MCAPs) that would be formulated for delivery as a dry nasal powder. The use of MCAPs to create vaccines that can be administered through the nose might be applicable to other diseases as well.
  • NIAID provided initial support to the biotech firm Acambis to develop a live attenuated recombinant vaccine for WNV called ChimeriVax. The chimeric vaccine is derived from the well-established yellow fever 17D vaccine, in which two genes from the yellow fever vaccine virus, including the gene for the viral envelope protein, are replaced with comparable genes from WNV. Several successful Phase I and Phase II trials were conducted in various populations of healthy volunteers. ChimeriVax was acquired by Sanofi Pasteur in 2008.
  • NIAID scientists are developing a molecularly engineered, live attenuated chimeric West Nile/dengue vaccine. In this candidate vaccine, genes coding for two proteins from dengue-type 4 virus (a related flavivirus) were replaced with the corresponding WNV genes. The candidate vaccines have been further weakened by deleting additional portions of dengue viral genetic material. Several vaccine candidates have been tested in Phase I clinical trials; further trials are planned with varying doses of vaccine and in older healthy individuals.
  • In collaboration with the San Diego biotech firm Vical and CDC, scientists from the NIAID Vaccine Research Center developed a DNA-based investigational WNV virus vaccine. This vaccine candidate was tested in Phase I and II clinical trials, and it has been licensed by CDC to Vical.

NIAID Basic and Clinical Research

NIAID conducts and funds basic and clinical research on

  • WNV biology and viral structure
  • Ways the virus causes human disease and persists in the body
  • Viral interaction with and transmission among humans, birds, and mosquitoes
  • Processes underlying WNV emergence and ecological patterns in the United States
  • Insecticide resistance

With NIAID support in 2001 and 2002, researchers found that hamsters and mice are good models for WNV in humans. As a result, golden hamsters and mice are now used to test the efficacy of WNV vaccine candidates and antiviral treatments. NIAID-supported researchers are studying the different ways the immune system responds to WNV and how these responses interact to affect infection. Scientists are also examining how WNV evolves and adapts to changing environments and the structural changes that occur to the virus when it replicates.

NIAID has long supported the World Reference Center for Emerging Viruses and Arboviruses (WRCEVA), located at the University of Texas Medical Branch at Galveston, which helps support research and outbreak investigations throughout the world. In response to the 2012 WNV outbreak in the United States, WRCEVA scientists sequenced several newly isolated WNV strains from Texas. Using genetic analysis, they compared different WNV strains to see if the virus has changed over time. In addition, the center has long provided research resources, such as viral isolates and reagents, to the scientific community. The center’s repository currently includes more than 500 WNV strains and isolates from across the globe.

In July 2012, in a NIAID-supported observational study of WNV patients in the Houston area, researchers at the Baylor College of Medicine discovered chronic kidney disease among 40 percent of patients long after recovery from acute disease. The study highlights the continuing need to investigate the pathology behind persistent infections, clinical outcomes, and treatment options.

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Last Updated July 06, 2015