One is punctured, the other is always skipped. It remains puzzling how mosquitoes choose their victim. What is particularly special is that the stinging animals can smell you from almost 100 meters away.
American researchers have tried to find out how this is possible by building a huge 1000 cubic meter test area in Zambia. “The test arena is 2,000 times larger than a normal lab setting to test the attraction of human scents to mosquitoes. In this natural environment, mosquitoes are exposed to normal wind, temperature and humidity, and moonlight, for example. They also have a lot more room to navigate and can track people’s scent over a much longer distance. The conditions in our test area reflect much better what it really is like in nature. We see it as a ‘home away from home’ for the mosquitoes,” says researcher Conor McMeniman of Johns Hopkins University On Scientias.nl.
Heat and a little CO2
The test arena comprises a ring of equally spaced landing sites that are warmed to human body temperature, around 35 degrees. Every night, the researchers released 200 hungry malaria mosquitoes and monitored their activity with infrared cameras. They mainly looked at how often the mosquitoes settled on the landing sites, which is a sign that they are ready to strike.
But more is needed than a warm human body: CO2 also had to be present and preferably also an attractive body odor. For this, the researchers found six people willing to sleep in a tent nearby. Their nighttime body odor was pumped out of the tent and taken to the landing sites.
“These mosquitoes mainly prey on humans in the hours before and after midnight,” McMeniman explains. “They follow scent trails and air currents and enter a house to bite between 10 p.m. and 2 a.m. We wanted to study mosquito preferences when they were most active and also use the body odor of sleeping humans.”
Why do mosquitoes especially go out in the middle of the night? “It is probably an evolutionary strategy of this malaria mosquito to avoid the harsh climate conditions during the day,” the researcher explains. “Moreover, the chance that you will be killed as a mosquito is much smaller, because people are asleep.” During the day the mosquitoes don’t do much. “They rest among the leaves near human habitations, where the climatic conditions are more suitable for them. For example, there are more shady and humid places where they can sit out the hot day until their nocturnal hunt can begin again.”
Feel the heat
But how do they find that tasty human snack? “Because of that, mosquitoes have wonderfully fine-tuned senses that can pick up on human presence, such as good vision in low light, temperature sensors and an excellent sense of smell. The animals combine these sensory skills to bring them to the right people,” explains the researcher. “At a distance of about 60 meters from a possible target, a mosquito is the first to smell the odor of the CO2, which we all exhale. As it approaches, it picks up traces of other chemicals from our body odor and breath. A mosquito can also see us from 5 to 15 meters away and from 1 meter away it feels the warmth of our skin. Meanwhile, the mosquito’s nervous system processes all these signals to ensure that it can choose the best bite.”
Because not everyone is equally tasty. Night after night, the researchers discovered that some people were more attractive than others. For example, one of the volunteers, who had a very different body odor from the others, consistently attracted very few mosquitoes. That was due to the chemical composition of his body odor.
The best fragrance blend
In the end, the researchers were able to identify 40 chemicals that were released by all the volunteers, albeit in different amounts. It’s probably a specific scent mix that the mosquitoes follow. Although body odor differed from night to night for each participant, the researchers did find some stable patterns. People who were more attractive to mosquitoes consistently released more carboxylic acids, which are likely produced by skin microbes. The person who was the least attractive emitted much less of the acids, but about three times as much eucalyptol, a compound found in many plants.
They were thrilled to see how effective the mosquitoes were at finding their perfect meal in that huge arena. “Seeing how the mosquitoes can still pick up the best scents in that wide open space in a field in Zambia only confirms how powerful these animals are at finding their prey,” said researcher Stephanie Rankin-Turner.
Finally an answer to that one question
“But what surprised us most was how the different odors consistently attracted or disliked the mosquitoes,” says McMeniman. “In particular, there was one volunteer who released almost no carboxylic acid, but a lot of eucalyptol, a substance known for its mosquito-repellent properties. This was found in the body odor of all volunteers, which is likely due to their diet as it is found in many spices and food additives, but it was much more prevalent in the most mosquito-unattractive participant.”
There is much more to do in the future. “We want to use our method to determine on a larger scale which people are attractive to mosquitoes and which are not. In this way we hope to better understand which factors, such as diet, the composition of the microbiome or the blood type, have an influence. As a result, we may finally be able to answer the age-old question of why mosquitoes like some people better than others.”