West Nile Virus
Medical Author: John P. Cunha, DO
Medical Editor: Melissa Conrad Stöppler, MD
What is West Nile virus?
West Nile encephalitis is an infection of the brain that is caused by a virus known as the West Nile virus. First identified in Uganda in 1937, the virus is commonly found in Africa, West Asia, and the Middle East. "Encephalitis" means inflammation of the brain. One of the causes of encephalitis is viral and bacterial infections, including viral infections transmitted by mosquitoes.
West Nile virus had not been previously reported in the U.S. prior to an outbreak in New York in September 1999. According to the U.S. Centers for Disease Control and Prevention (CDC), there were 4,261 cases of disease caused by the West Nile virus internationally in 2006; however, the CDC says case reports are understated and estimates that there were 42,000 cases, most of which were unreported.
West Nile virus also is called West Nile fever or West Nile encephalitis.
Where did the West Nile virus come from?
To date, the West Nile virus has been commonly found in humans,
birds, and other vertebrate animals in Africa, Eastern Europe, West
Asia, and the Middle East. Prior to 1999, the West Nile virus had not
been recognized in the Western Hemisphere.
The source of the outbreak in New York was probably the Middle East. The American strain of the virus is almost indistinguishable from a virulent strain found in a goose on an Israeli farm in 1998. Thousands of people travel between New York and the Middle East each year. The virus may well have hitchhiked a ride to New York.
How do people get West Nile virus?
People get West Nile virus from the bite of a mosquito
(primarily the Culex pipiens mosquito) that is infected with the West Nile virus.
How do mosquitoes get infected with the West Nile virus?
Mosquitoes become infected by feeding on birds that are infected with the virus. The infected birds may or may not become ill. The birds are vectors. A vector is an intermediate carrier of the virus that is important for the virus' life cycle and transmission cycle.
Crows are the birds that are most vulnerable to infection by the West Nile virus. They are often killed by the virus. Although 17 species of birds have been found to be infected by the virus, the common dust-colored house sparrow is probably a principal bird reservoir for the virus in New York. Sparrows can harbor the virus for five days or more at levels high enough to infect mosquitoes that bite them.
The infected mosquitoes then transmit the virus when they bite and suck blood from people and animals and, in the process, inject the virus into their victim.
The incubation period (the time from infection to the development of symptoms) is five to 15 days.
Next: Can one person contract the virus from another? »
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