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West Nile Virus

The 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.


The first step in the transmission cycle of West Nile Virus (WNV) occurs when a mosquito bites an infected bird or animal and acquires 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 infection, as are robins, blue jays, and other birds. Scientists have identified more than 138 bird species that can be infected and more than 43 mosquito species that can transmit WNV.

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

Most cases of human disease occur in elderly people and in people with impaired immune systems. WNV also can be transmitted through blood transfusions and organ transplants from WNV-infected donors.

Health experts also believe it is possible for WNV to be transmitted from a mother to her unborn child and through breast milk.


People who contract West Nile Virus (WNV) usually experience only mild symptoms—fever, headache, body aches, skin rash, and swollen lymph glands. If WNV enters the brain, however, it can cause life- threatening encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord).


In December 2005, the Food and Drug Administration (FDA) approved the Procleix West Nile Virus (WNV) Assay for the direct detection of viral nucleic acid in donors of whole blood and blood components, tissues, and organs. It can be used to screen organ donors whose heart is still beating as well as cadaveric (non-heart-beating) donors.

In March 2007, FDA approved the ProcleixTIGRIS system, the first fully automated WNV nucleic acid test for screening blood, tissues, and organs. The ProcleixWNV screening tests are manufactured by Gen-Probe Inc., and marketed by Chiron Corp.

In August 2007, FDA approved a biologics license application for another automated WNV nucleic acid test (cobas TaqScreen WNV Test) manufactured by Roche Molecular Systems Inc., for the same indication. It is noteworthy that neither the Procleix WNV screening tests nor the cobas TaqScreen WNV Test are intended for use on cord blood specimens or as an aid in diagnosing WNV infection.


There is no specific treatment for WNV infection. In cases with milder symptoms, people experience symptoms such as fever and aches that pass on their own, although even healthy people have become sick for several weeks. In more severe cases, people usually need to go to the hospital where they can receive supportive treatment including intravenous fluids, help with breathing and nursing care.



National Institute of Allergy and Infectious Diseases, USA.

Centers for Disease Control and Prevention, USA.