For a bloodthirsty, global health threat, the African malaria mosquito, Anopheles gambiae, has a surprisingly distinctive taste. It prefers to feed on humans over other animals, is more attracted to some humans than others, and even then seems to have a particular fondness for feet.
“Despite being quite small, the African malaria mosquito has a very powerful sense of smell,” says Conor McMeniman, said a vector biologist at Johns Hopkins University. “And it can also be quite finicky.”
Scientists have spent decades deciphering the chemistry of mosquito attraction, identifying exactly what odors they are attracted to and why some people are mosquito magnets. To investigate such questions, researchers often released the insects into small lab wind tunnels filled with used socks or sweat-coated glass beads.
But dr. McMeniman wanted to better mimic the way the mosquito selects its targets in the real world. “We really wanted to create a spacious kind of home away from home for the Anopheles gambiae mosquito,” he said, “where we battle multiple sources of human odor against each other to see what they like best.”
And so he and his colleagues built a 4,300-square-foot “escape cage” at the Macha Research Trust, a health research institute in southern Zambia. The structure became the centerpiece of an elaborate experimental system involving sleeping volunteers, custom-made canvas tents, precision-heated hot plates, and mosquito-tracking cameras.
While the research is in its early stages, the scientists hope that learning more about “the sensory biology of how mosquitoes track and hunt humans” could lead to better mosquito decoys and repellents and ultimately new strategies for tackling malaria, said Dr. McMeniman.
Here’s how they conducted their first studies, which were published in Current Biology in May.
Build the flight cage
The Macha Research Trust is remote. “Not an excavator, cement truck or crane around,” said Chris Book, the institute’s administrative director. Local workers mixed the concrete, poured the foundation and placed the steel beams by hand, while a tailor’s company sewed together the mesh that encloses the structure.
Release the (small) beasts
Anopheles gambiae is “the night owl of the mosquito world,” said Dr. McMeniman, so the experiments started in the evening. At 8 p.m., a researcher entered the fly cage with a container of 200 hungry mosquitoes, released the bugs and then quickly left. (The mosquitoes were lab-bred and did not carry the parasites that cause malaria.)
Produce the people
At 10 p.m., six volunteers climbed into individual canvas tents around the perimeter of the escape cage. The volunteers, wearing scrubs, were asked to refrain from using scented personal products and from eating onions, garlic or other smelly foods. Then they slept. As they did, a low-speed fan blew their scents through a flexible aluminum tube—a repurposed piece of air-conditioning ducting—sewn into the base of each tent.
Follow the mosquitoes
The odors traveled through the channels and into the escape cage, about 50 feet away. Each tube spread its fragrant plumes over a small aluminum plate, which had been warmed to the temperature of human skin. The researchers used infrared cameras to record the mosquitoes landing on each hot plate, “which is a good sign they’re ready to bite,” said Dr. McMeniman. They then counted the number of landings on each participant’s board.
Collect the body odors
While the volunteers slept, the scientists collected air samples from each tent. They were later analyzed in the lab to determine each person’s “smell signature.”
Suck up the mosquitoes
In the morning, the scientists collected the mosquitoes with vacuum cleaners on their backs. “We use ourselves as bait,” said Dr. McMeniman. “So once they land on us, we can suck them up.”
Reset and reuse the arena
For six nights, the scientists tested the same six volunteers against each other. They found that the mosquitoes were most attracted to a participant who released high levels of carboxylic acids, which are produced by skin microbes and sebum, an oily residue secreted by glands in the skin. The volunteer who was least attractive to the mosquitoes not only emitted low levels of carboxylic acids, but also high levels of eucalyptol, a plant-derived compound common in a variety of foods and known to repel mosquitoes.
Expand the experiments
The researchers hope to expand their pool of volunteers in Zambia to conduct further research into the chemicals and microbes that mosquitoes are attracted to. They are also interested in building similar experimental arenas on other continents to investigate the preferences of local mosquito species. “Because you can sit in your backyard and have a conversation with someone and swat at the mosquitoes all the time, while your friend sitting next to you barely gets bit,” said Mr. Book. “What is the difference?”