West Nile virus (WNV) is an emerging infectious disease that made its first appearance in North America in 1999. The microbe that causes the infection belongs to a group of disease-causing viruses known as flaviviruses that are usually spread by ticks or mosquitoes.
Other well-known diseases caused by flaviviruses include:
WNV is most commonly found in Africa, West Asia, Europe, and the Middle East.
The first step in the transmission cycle of West Nile virus 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 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 North America.
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 get West Nile virus usually have only mild symptoms:
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).
Although licensed West Nile virus vaccines exist for horses, there are no specific vaccines or treatments for human WNV disease.