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What is it?

West Nile virus (WNV) was first discovered as a cause of an infectious disease in Uganda, Africa in 1937. It then spread to the Middle East, Eastern Europe, West Asia and, more recently, the United States. The first US cases were reported in New York in 1999. The virus has spread across the country and made headlines as outbreaks dramatically increased the number of human cases.

WNV belongs to a group of disease-causing viruses known as flaviviruses. These lipid (fat)-enveloped viruses can be spread by insects, usually mosquitoes, to animals, including humans. Most human infections are mild, although in some cases serious illness such as encephalitis can result from infection. It is not contagious person-to-person. In Australia the Kunjin virus is the most closely related virus to WNV.

The most common route of transmission is through a mosquito bite. When a mosquito bites an infected bird, such as a crow (crows are highly susceptible to infection), it can then transmit the virus to another animal it bites. Transmission of the virus commonly peaks during early spring when adult mosquitoes emerge and continues until Autumn. It is estimated that 1 in 200 mosquitoes in the US harbours the virus.

There also has been some recent concern about transmission of WNV through donated blood or organs, as seems to have occurred in the case of four organ transplant recipients. The risk of transmission through blood transfusions is still believed to be very low. Since 2003, the US blood supply has been screened for WNV by nucleic acid-amplification tests (NATs) which has further reduced the risk for transfusion transmission.

As of January 13, 2015, there have been 2,122 reported confirmed human cases of WNV in the U.S. and in 2014 85 deaths attributed to it as the cause. For the most current numbers, visit the Centers for Disease Control and Prevention's West Nile Virus web site.

In Australia, the first laboratory-confirmed human case of WNV infection was reported in a returned traveller in 2009, but to date there have been no cases of WNV infection acquired from within Australia. Surveillance of mosquitoes and birds is being carried out, and blood plasma imported from the US is irradiated to kill viruses.

Last Review Date: July 23, 2015