LONDON, Feb 8 (Thomson Reuters Foundation) - Drought has
increased the severity of outbreaks of the deadly West Nile
virus in the United States, and may double the size of epidemics
over the next 30 years, scientists said on Wednesday.
Outbreaks of the mosquito-borne virus have occurred every
year since the virus spread to North America in 1999, and in
some years caused only a few hundred severe cases nationally.
But in each of three years - 2002, 2003 and 2012 - about
3,000 people suffered brain-damaging meningitis or encephalitis,
and almost 300 died.
In some states the number of cases varied 50-fold from year
"We thought epidemics would coincide with the most ideal
temperatures for (virus) transmission," Marm Kilpatrick, an
associate professor of ecology and evolutionary biology at the
University of California, Santa Cruz, said in a statement.
"Instead, we found that the severity of drought was far more
The scientists said more severe drought in the United States
in the next 30 years may double the size of future epidemics,
particularly affecting areas which have not experienced previous
Populations which have been exposed to large outbreaks
develop immunity to the virus, limiting the size of subsequent
Worsening drought will be caused by rising temperatures and
affect even areas with increased rainfall, they said.
Most people infected with West Nile virus experience no
symptoms. About 20 percent develop fever and other symptoms, and
less than 1 percent develop a serious illness, according to the
U.S. Centers for Disease Control and Prevention.
It is not yet clear how drought increases transmission of
the virus, the scientists said.
Mosquitoes pick up the virus when they feed on infected
birds, and then pass it on to humans. This may be more likely to
happen in drought periods, the scientists said.
They found that in Colorado, drought increased the
proportion of mosquitoes infected with the virus, but not the
abundance of mosquitoes.
The findings were published on Wednesday in the journal
Proceedings of the Royal Society B.
(Reporting by Alex Whiting @Alexwhi, Editing by Ros Russell.;
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