Researchers have built a mosquito testing arena to discover how the biting insects find people over long distances
We know a lot about mosquito preferences up close, but how do mosquitoes find us from up to a hundred metres away?
Using an ice-rink-sized outdoor testing arena in Zambia, researchers found that human body odour is critical for mosquito host-seeking behaviour over long distances.
The team also identified specific airborne body-odour components that might explain why some people are more attractive to mosquitoes than others.
Most studies of mosquito preference have been performed in confined laboratory settings that probably don’t represent a mosquito’s experience in the wild.
To test how the African malaria mosquito Anopheles gambiae locates and chooses human hosts over a large and more realistic spatial scale, researchers from Johns Hopkins Bloomberg School of Public Health’s Malaria Research Institute and Macha Research Trust teamed up to build a 1,000 m3 testing arena in Choma District, Zambia.
Neuroscientist Diego Giraldo (@dags263), a postdoctoral fellow at Johns Hopkins Bloomberg School of Public Health, and one of the study’s first authors, said: “This is the largest system to assess olfactory preference for any mosquito in the world, and it’s a very busy sensory environment for the mosquitoes.”
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Testing arena
The testing arena contained a ring of evenly spaced landing pads that were heated to human skin temperature (35°C). Each night, the researchers released 200 hungry mosquitoes into the testing arena and monitored their activity using infrared motion cameras.
Specifically, they took note of how often mosquitoes landed on each of the landing pads (which is a good sign that they’re ready to bite).
First, the team compared the relative importance of heat, CO2, and human body odour for attracting mosquitoes.
They found that mosquitoes were not attracted to the heated landing pads unless they were also baited with CO2, but human body odour was a more attractive bait than CO2 alone.
Next, the team tested the mosquitoes’ choosiness. To do this, they had six people sleep in single-person tents surrounding the arena over six consecutive nights, and they used repurposed air conditioner ducting to pipe air from each tent – containing the aromas of its sleeping occupant – onto the heated landing pads.
As well as recording the mosquitoes’ preferences, the researchers collected nightly air samples from the tents to characterise and compare the airborne components of body odour.
Senior author and vector biologist Conor McMeniman (@McMenimanLab), assistant professor at Johns Hopkins Bloomberg School of Public Health and Johns Hopkins Malaria Research Institute, said: “These mosquitoes typically hunt humans in the hours before and after midnight.
“They follow scent trails and convective currents emanating from humans, and typically they’ll enter homes and bite between around 10pm and 2am.
“We wanted to assess mosquito olfactory preferences during the peak period of activity when they’re out and about and active and also assess the odour from sleeping humans during that same time window.”
They found that, night after night, some people were more attractive to mosquitoes than others, and one of the volunteers, who had a strikingly different odour composition from the others, consistently attracted very few mosquitoes.
Stable patterns
The team identified 40 chemicals that were emitted by all of the humans, though at different rates.
Analytical chemist Stephanie Rankin-Turner (@stephrankin2), a postdoctoral fellow at Johns Hopkins Bloomberg School of Public Health, and the study’s other first author, said: “It’s probably a ratio-specific blend that they’re following.
“We don’t really know yet exactly what aspect of skin secretions, microbial metabolites, or breath emissions are really driving this, but we’re hoping we’ll be able to figure that out in the coming years.”
Though each person’s odour profile varied from night to night, the researchers found some stable patterns.
People who were more attractive to mosquitoes consistently emitted more carboxylic acids, which are probably produced by skin microbes.
In contrast, the person who was least attractive to mosquitoes emitted less carboxylic acids but approximately triple the amount of eucalyptol, a compound found in many plants; the researchers hypothesise that elevated levels of eucalyptol may be related to the person’s diet.
The researchers were surprised by how effectively the mosquitoes could locate and choose between potential human meals within the huge arena.
Rankin-Turner said: “When you see something moved from a tiny laboratory space where the odours are right there, and the mosquitoes are still finding them in this big open space out in a field in Zambia, it really drives home just how powerful these mosquitoes are as host seekers.”
The work is published in Current Biology.
Image: An overhead photo (taken using a drone) of the testing arena and surrounding tents. Credit: Julien Adam (CC BY-SA).
