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By Maggie Fox

Hot weather one year may bring more West Nile virus the next, by encouraging mosquitoes that carry it, researchers report in a new study.

That means it might be possible to forecast when the infection will hit people in a certain region, the team at the National Center for Atmospheric Research in Colorado said.

“We've shown that it may be possible to build a system to forecast the risk of West Nile virus disease several weeks or months in advance, before the disease begins to peak in summer,” said Micah Hahn, a scientist with both NCAR and the Centers for Disease Control and Prevention. “Having advance warning can help public health agencies plan and take additional steps to protect the public.”

West Nile was only introduced into the United States in 1999, but it quickly spread to all 50 states as well as Canada and Mexico. Since then, it’s infected hundreds of thousands of people, causing severe illness in about 40,000 and killing more than 1,600.

"Having advance warning can help public health agencies plan and take additional steps to protect the public.”

Outbreaks wax and wane – some years, in some places, there are many cases and the next year there may be none. The worst year for sickness was 2003, when nearly 7,000 people became severely ill and 32 died.

It’s reasonable to think weather might affect mosquito populations, but it’s not as predictable as scientists might hope. And predicting when and if the mosquitoes carry West Nile is even trickier, because the virus circulates first among birds, and then in mosquitoes that bite people.

Hahn’s team found that in the Northeast and Southeast, an annual temperature rise of just 2 degrees F over the average led to a fivefold increased likelihood of an above-average outbreak of West Nile. It doubled the risk for much of the rest of the country, including the mid-Atlantic, Midwest, and Great Plains, they reported in the American Journal of Tropical Medicine and Hygiene.

Temperatures likely affect mosquito breeding cycles, they said.

Rainfall was more complicated. In the eastern United States, drier-than-normal fall and spring seasons seemed to correlate with more West Nile virus activity the following summer. But in much of the West, wetter-than-average winters seemed to lead to more disease circulating.

“West Nile virus represents a significant threat to public health in the United States."

West Nile usually doesn’t cause symptoms in the average healthy person, but it can infect the brain and spinal cord, causing encephalitis and death.

There's not any specific treatment or commercially available vaccine. But one group has made enough progress with an experimental vaccine to test it in people. They just started this month.

“West Nile virus represents a significant threat to public health in the United States, especially among the immunocompromised and the elderly,” said Mark Slifka of the Oregon Health & Science University.

His team and colleagues at Duke University are recruiting 50 volunteers for a phase 1 safety study of a new vaccine. It uses hydrogen peroxide to “kill” the virus used in the vaccine, which they hope will make the vaccine more effective than older methods.

“We believe our vaccine approach will not only be safe and effective for West Nile virus, but it could also provide significant protection against other important human pathogens, including yellow fever, dengue hemorrhagic fever, and, potentially even Ebola,” Slifka said in a statement. He's formed a private company to develop the vaccine.