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WASHINGTON (Reuters Life!) - Genetically altered mosquitoes that cannot fly may help slow the spread of dengue fever and could be a harmless alternative to chemical insecticides, U.S. and British scientists said on Monday.They genetically altered mosquitoes to produce flightless females, and said spreading these defective mosquitoes could suppress native, disease-spreading mosquitoes within six to nine months.There is no vaccine or treatment for dengue fever, which is endemic in the tropics and is particularly prevalent in Asia and the western Pacific. The disease, which causes severe flu-like symptoms and can kill, is spread through the bite of infected female Aedes aegypti mosquitoes."This could be the first in a new wave of products that might supplant insecticides," researcher Anthony James of the University of California, Irvine, said in a telephone interview.There are an estimated 50 million cases of dengue fever each year and about 2.5 billion people - two-fifths of the world's population - are at risk, mostly in Africa and southeast Asia, according to the World Health Organization.James's team, including a group from the British biotechnology firm Oxitec Ltd., altered mosquito genes to disrupt development of the insects' wing muscle.The genetic modification grounded only the virus-carrying females and did not affect the males' ability to fly, they wrote in the Proceedings of the National Academy of Sciences. The idea would be to distribute tens of thousands of eggs that would hatch out these genetically modified males, which would proceed to create a new generation of flightless, and thus doomed, daughters.Because eggs are so small and easy to distribute, there would be far more genetically modified mosquitoes than natives, so they could in effect blot out the dengue-carrying population."We stack the numbers in our favor by releasing a lot of these things," James said."The technology is completely species-specific, as the released males will mate only with females of the same species," added Oxitec's Luke Alphey, who led the study.Alphey said using genetically modified mosquitoes would be an environmentally friendly alternative to chemical insecticides and would be egalitarian."All people in the treated areas are equally protected, regardless of their wealth, power or education," he said.Both Oxitec and Oxford University have applied for a patent.The current work is focused on mosquitoes that carry dengue fever, but the researchers said it could be adapted to other species that spread malaria and West Nile fever.