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Scientists have discovered that mutating a smell-related gene in mosquitoes hinders their ability to sniff out humans from other warm-blooded prey.
Researchers said the findings clearly show how important scent is to mosquito "hunting preferences." And they hope the results will pave the way to better weapons against the mosquitoes that transmit diseases including malaria and dengue fever.
It's well known that certain mosquitoes "specialise in humans," said Leslie Vosshall, a professor at Rockefeller University, in New York City, and senior researcher on the study.
Because they devote their time to moving from one person to the next, she said, they are the mosquitoes responsible for spreading diseases such as malaria - which alone killed close to 700 000 people worldwide in 2010, according to the US Centers for Disease Control and Prevention (CDC).
How the study was done
Scientists have assumed that odour plays a prime role in how mosquitoes zero in on people. They are attracted by other factors - like body heat and the carbon dioxide people exhale - but other warm-blooded creatures also give off those cues.
"None of those factors would be as important as smell," Vosshall said.
And Vosshall's team found clear genetic proof of how important scent is. In experiments with one strain of disease-carrying mosquitoes, the researchers were able to "knock out" a gene involved in odour detection. The result? The bugs lost their ability to distinguish humans from guinea pigs.
It's not surprising that mosquitoes' odour receptors would be key in their preference for humans, said Michel Slotman, who studies disease-transmitting mosquitoes and was not involved with the study.
But the findings offer important details about the insects' scent-detecting systems, according to Slotman, an assistant professor of entomology at Texas A&M University in College Station, Texas.
He said the results raise the possibility of using genetic modification to alter mosquito populations in certain areas where mosquito-borne diseases are endemic. "The idea behind this approach is that natural mosquito populations are replaced by ones that have a gene that modifies their host preference so they no longer prefer humans," Slotman said.
That's just speculation, however. And Slotman added that, "Of course, there are possible complications."
Attracted to human odour
One question, he said, is, would mosquitoes with substantially impaired smell-detection survive in the wild? And even if the engineered mosquitoes survived, what would be the impact? If people were still the most abundant and convenient biting target, Slotman noted, would the "human biting rate" even go down significantly?
For their part, Vosshall said she and her colleagues have no plans "to unleash a race of mutant mosquitoes." Instead, she said she hopes that a clearer understanding of mosquito genetics and hunting preferences will aid the development of better insect repellants.
It's no use, however, for people to try to mask their scent. Humans have an odour, Vosshall noted, and they can't change it.
In a second part of the study, her team found that the mutant mosquitoes were attracted to human skin even when it was protected by the common insect repellant DEET.
"If you coat the arm with DEET," Vosshall said, "the mutants are still very enthusiastic about human skin. But once they land, they fly away."
That, she said, indicates that the "smell pathway" must be one important way by which DEET works. But it's not the only way: The repellant also has some type of action once the insects touch down on the skin, Vosshall said.
DEET has been widely used as an insect repellant for about 50 years, yet no one is sure exactly how it works, Vosshall noted.
Slotman said that based on this and past research, DEET appears to have "multiple modes of action."
According to the CDC, malaria alone infected 219 million people globally in 2010, killing 660 000 - mostly children in sub-Saharan Africa.
A campaign to eradicate malaria worldwide was begun in the 1950s, but it failed - in part because mosquitoes developed a resistance to the insecticides used to kill them.
More information
Learn more about mosquitoes and
malaria from the US Centers for Disease Control and Prevention.
