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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 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. This mosquito is often referred to as the house mosquito or the West Nile virus mosquito.

How do mosquitoes get infected with the West Nile virus?

The Culex species that transmits West Nile virus is called the house mosquito because it prefers to lay eggs in small containers of stagnant water, which are common around homes. Humans are not their preferred meal, however, and they become infected by feeding on birds. The infected birds may or may not become ill. The birds are preferred and amplifying hosts of the virus (meaning that the virus reproduces in high numbers) and are 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 nearby people and animals and, in the process, inject the virus into their victim.

Medically Reviewed by a Doctor on 10/16/2017


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