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 to 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 important nationally."
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 outbreaks.
Populations which have been exposed to large outbreaks develop immunity to the virus, limiting the size of subsequent epidemics.
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.; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, climate change, resilience, women's rights, trafficking and property rights. Visit http://news.trust.org/climate)