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A The virus can infect humans, birds, mosquitoes, horses and some
other mammals.
West Nile fever is a case of mild disease in people, characterized
by flu-like symptoms. West Nile fever typically lasts only a few
days and does not appear to cause any long-term health effects.
More severe disease due to a person being infected with this virus
can be West Nile encephalitis, West Nile meningitis or West Nile
meningoencephalitis. Encephalitis refers to an inflammation of the
brain, meningitis is an inflammation of the membrane around the
brain and the spinal cord, and meningoencephalitis refers to inflammation
of the brain and the membrane surrounding it.
West Nile virus has been commonly found in humans and birds and
other vertebrates in Africa, Eastern Europe, West Asia, and the
Middle East, but until 1999 had not previously been documented in
the Western Hemisphere. It is not known from where the U.S. virus
originated, but it is most closely related genetically to strains
found in the Middle East.
It is not known how long it has been in the U.S., but CDC scientists
believe the virus has probably been in the eastern U.S. since the
early summer of 1999, possibly longer.
One of the species of mosquitoes found to carry West Nile virus
is the Culex species which survive through the winter, or "overwinter,"
in the adult stage. That the virus survived along with the mosquitoes
was documented by the widespread transmission the summer of 2000.
The continued expansion of West Nile virus in the United States
indicates that it is permanently established in the Western Hemisphere.
In the temperate zone of the world (i.e., between latitudes 23.5°
and 66.5° north and south), West Nile encephalitis cases occur primarily
in the late summer or early fall. In the southern climates where
temperatures are milder, West Nile virus can be transmitted year
round.
©
Centers for Disease Control
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