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A sweet target

THE WASHINGTON POST – To unravel the age-old question of why mosquitoes eat some people alive but spare others, scientists built a large, open-air arena in Zambia and piped in the smells of a half-dozen humans slumbering in nearby tents. They discovered that malaria-carrying mosquitoes, like the ones that carry yellow fever, are drawn to specific chemicals found on people’s skin.

They also identified a lucky individual whose distinctive body odour appeared to be relatively unappetising, opening up a new avenue in the search for ways to deter bites.

At close range, mosquitoes use visual cues and body warmth to seek their prey. But when they’re out of visual range – which could be a few dozen feet away – they are thought to track carbon dioxide and other chemicals found in body odour and breath. The precise mixture that mosquitoes find most compelling remains an area of active research.Experiments on mosquitoes are typically conducted in relatively small boxes or wind tunnels free from the aromatic cacophony of the real outdoors. But such experiments tend to mimic mosquito decision-making at close range. So scientists built a huge new arena to get to the root of mosquitoes’ human-seeking instincts in the wild.

“I like to think of it as the world’s largest perfumerie for mosquitoes, where they can chose whose scent they like,” said an assistant professor of molecular microbiology and immunology at the Johns Hopkins Malaria Research Institute Conor McMeniman who led the study, published in the journal Current Biology.

HOW DO MOSQUITOES CHOOSE THEIR TARGETS?

Mosquitoes are humans’ most lethal predators, carrying diseases such as malaria, yellow fever and dengue fever that kill more than half a million people each year.

Anopheles gambiae, one of the species that carries malaria, is a particularly menacing hunter. There are various estimates of how far the mosquitoes fly, but they tend to range less than half a mile a day, according to one study from West Africa. They typically feed around midnight, flying into the open eaves of people’s homes. In Zambia, 2,000 people die from malaria each year.

PHOTO: ENVATO
PHOTO: ENVATO

To find out how these mosquitoes stalk their sleeping victims, scientists built an outdoor testing arena that’s about the size of two tennis courts, or 2,000 times the size of a typical laboratory setup.

They let the mosquitoes acclimate to their open-air laboratory. Then, the researchers did everything they could to conjure the right mood.

At stations scattered around the arena, air conditioning ducts delivered the bouquet of different humans sleeping in nearby tents. At each station, the piped-in aromas suffused hot plates warmed to human body temperature, alongside puffs of carbon dioxide.

With an infrared camera, scientists watched which hot plates became mosquito discos.

They found that heat and carbon dioxide weren’t enough to attract the insects without the added element of human body odour.

“This study adds a lot,” said a neurobiologist and chief scientific officer Leslie Vosshall of the Howard Hughes Medical Institute whose laboratory recently discovered that skin chemistry determined whether people were mosquito magnets for a different species, Aedes aegypti, that carries yellow fever.

“It’s a different mosquito – it’s a much more important mosquito,” Vosshall said of Anopheles gambiae. “This mosquito kills orders of magnitude more people… it’s a real apex predator of humans.”

The study found that the mosquitoes were particularly attuned to the oily secretions that hydrate skin and protect it from microbes. Chemical compounds called carboxylic acids are a strong draw – both in the new study and in Vosshall’s work with Aedes aegpyti.

But one individual in the new study was relatively unappealing, the researchers found.

Their signature scent included an unusually low amount of carboxylic acids and high eucalyptols, a substance found in many plants, raising the possibility that diet may play a role, McMeniman said.

WHAT’S NEXT FOR MOSQUITO RESEARCHERS?

Now that the researchers have shown that their testing arena works, they are planning a much larger experiment in which they will pit 120 sleeping people against each other in rounds of multiple choice competition, to find out who is irresistible to mosquitoes and who isn’t.

They hope to decipher what combination of chemicals makes a person more or less attractive than others. They will also study the extent to which factors like diet or the microorganisms on people’s skin – the skin microbiome – influences their attractiveness.

McMeniman also dreams of building a similar facility in the United States to test other species of mosquito that spread disease and ruin backyard barbecues.

The insights gained from such experiments could lead to new ways to repel mosquitoes, perhaps by finding ways to alter or mask skin chemistry, making people less alluring.

But the search for an explanation for why mosquitoes prefer some humans to others will probably defy a simple answer. Previous experiments found that pregnant women are more likely to attract mosquitoes. Drinking alcohol attracts mosquitoes. Using certain kinds of soap, even ones that leave behind a smell dominated by a chemical known to repel mosquitoes, paradoxically increase people’s attractiveness to mosquitoes.

“What really matters to the mosquito is not the most abundant type of chemical, it’s really those chemical interactions and relative abundances,” said an assistant professor of biochemistry at Virginia Tech Clément Vinauger. He recently tested four commonly used soaps and found that three increased humans’ attractiveness to Aedes aegypti mosquitoes, while one – Native Coconut and Vanilla body wash – seemed to decrease it, probably because mosquitoes don’t like coconut oil.

“The short answer,” Vinauger said, “is it’s a complex problem”. – Carolyn Y Johnson

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