Mosquitoes use body odour and carbon dioxide to seek out their sleeping human targets, and an elaborate field experiment in Zambia shows the whiney insects prefer certain body scents over others.
To test the importance of carbon dioxide and body odour in mosquito preference, a team of researchers from the US and Zambia built a 20m square ‘flight cage arena’. They then released 200 hungry mosquitoes into the arena over subsequent nights and tracked their activity using infrared motion cameras. The study is published in Current Biology.
“These mosquitoes typically hunt humans in the hours before and after midnight,” says author Conor McMeniman, an assistant professor at Johns Hopkins Bloomberg School of Public Health and Johns Hopkins Malaria Research Institute.
“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.”
Evenly spaced, warmed ‘landing pads’ were stationed at the perimeter of the flight arena.
In the first experiment, these landing pads were baited – some with CO2 or body odour and some without. Mosquitoes were not attracted to the heated landing pads unless they were also baited with carbon dioxide. Human body odour was a more attractive bait than carbon dioxide alone.
In the second experiment, six people slept in single-person tents stationed around the arena over six consecutive nights. The tents were connected to the arena by screened aluminium ducting and low speed fans piping each person’s body odour from the tent into the flight arena and onto the landing pad.
As well as recording the mosquitoes’ preferences, the researchers collected nightly air samples from the tents to characterize and compare 40 different chemical components of each person’s body odour.
They found that mosquitoes were consistently attracted to certain body odours over others.
Mosquitoes were more attracted to people whose body odours contained more carboxylic acids (likely produced by skin microbes), over those with lower levels carboxylic acids and elevated levels of eucalyptol (potentially related to a person’s diet).
Eucalyptol – a chemical likely derived from plant-based foods and flavourings in a person’s diet – was highly abundant in the body odour of the least preferred human subject in the six-person cohort.
The researchers were surprised by how effectively the mosquitoes could locate and choose between potential human meals within the huge arena.
“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,” says analytical chemist Stephanie Rankin-Turner.
The research involved the African malaria mosquito, a prolific vector of malaria throughout sub-Saharan Africa, seeking to better understand the factors which drive human attractiveness to the disease vector.
Previous research shows mosquitoes use a range of sensory cues to identify their targets, including visual as well as body emitted heat, odour, carbon dioxide and moisture. These cues have different spatial ranges – warmth reaches up to 50cm, visual cues between 5 – 15m with carbon dioxide and body odour potentially stretching as far as 60m.
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