West Nile Encephalitis (cont.)

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Where did the West Nile virus come from?

To date, strains of the West Nile virus have 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 first recorded epidemics were reported in Israel in the 1950s and in Europe in 1962. A subsequent outbreak occurred in New York in 1999. 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 with an infected traveler.

How do people get West Nile virus?

People get West Nile virus from bites 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, or intermediate carriers, of the virus that is important for the virus' life cycle and transmission cycle.

Among birds, crows are most vulnerable to infection by the West Nile virus. They are often killed by the virus. More than 200 species of birds have been found to be infected by the virus, and 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.


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