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West Nile virus 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 West Nile virus 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. West Nile virus (WNV) is a member of the Flaviviridae family. Other diseases caused by flaviviruses include yellow fever, Japanese encephalitis, dengue, Saint Louis encephalitis, and tick-borne encephalitis.
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. As of Nov. 14, 2012, more than 5,100 human cases of West Nile disease, including 229 deaths, have been reported in the United States. Almost eighty percent of cases have been reported from 12 states (Arizona, California, Colorado, Illinois, Louisiana, Michigan, Mississippi, Nebraska, Ohio, Oklahoma, South Dakota, and Texas) with over one-third of cases reported from Texas, 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 West Nile virus (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 the Centers for Disease Control and Prevention. 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 West Nile virus (WNV) will not show any symptoms at all, according to the Centers for Disease Control and Prevention (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 the CDC.
People typically develop symptoms between 3 and 14 days after they are bitten by an infected mosquito.
Diagnosis and Treatment
According to the Centers for Disease Control and Prevention, preliminary diagnosis of West Nile virus (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 secondary 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 physician if they develop symptoms that could be WNV.
Currently, there are no vaccines to prevent West Nile virus infection in people. The best way to prevent infection is to limit exposure to mosquitoes. For example:
Basic and Clinical Research
NIAID conducts and funds basic and clinical research on:
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 also supports 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 are sequencing several newly isolated WNV strains from Texas. Using genetic analysis, they can compare different WNV strains to see if the virus has changed over time. In addition, the center also provides 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.
Diagnostics and Treatment Research
NIAID conducts and supports the research and development of new diagnostic technologies and treatments for West Nile virus (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:
NIAID supports research on a variety of vaccine approaches that could potentially lead to a safe and effective preventive vaccine for West Nile virus (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:
Last Updated November 16, 2012
Last Reviewed November 16, 2012