Scientists have released three million genetically-modified (GM) mosquitoes in the Cayman Islands to fight dengue fever.

Batches of the male mosquitoes were released in a 40-acre area, from May to October, to mate with wild female counterparts of the same species, so they wouldn’t be able to produce any offspring. Only female mosquitoes bite humans and spread the disease.

By August, mosquito numbers in that region dropped by 80 percent compared with a neighbouring area where no sterile male mosquitoes were released, the Daily Mail reported.

“This test in the Cayman Islands could be a big step forward,” said Andrew Read, professor of biology and entomology at Pennsylvania State University.

“Anything that could selectively remove insects transmitting really nasty diseases would be very helpful,” he said, according to the journal PLoS Biology.

Dengue is a potentially fatal mosquito-borne disease that can cause fever, muscle and joint pain, and hemorrhagic bleeding.

More than 2.5 billion people are at risk and the World Health Organisation estimates there are at least 50 million cases every year. There is no treatment or vaccine.

Unlike malaria, which is also spread by mosquitoes, dengue outbreaks are unpredictable and mosquito nets are of limited use because dengue-spreading mosquitoes also bite during the day.

Researchers at Oxitec Limited, an Oxford-based company in Britain, created sterile male mosquitoes by manipulating the insects’ DNA.

For years, scientists have been working to create mutant mosquitoes to fight diseases like malaria and dengue, which they say could stop outbreaks before the start. But, others suspect it could be an environmental nightmare.

“If we remove an insect like the mosquito from the ecosystem, we don’t know what the impact will be,” said Pete Riley, campaign director of GM Freeze, a British non-profit group that opposes genetic modification.

“Nature often does just fine controlling its problems until we come along and blunder into it,” he concluded.

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