Mosquitoes are targeting us in ways we didn't know about

Male mosquitoes are harmless, feeding on the nectar of flowers. Females would also need this sugar-rich food to survive, but they need blood, a source of protein, to lay eggs. So they have developed several mechanisms to help them locate their victims.

Research into how mosquitoes find hosts, especially humans, has been going on for almost a century. Scientists have already determined that they are attracted to human odor and the carbon dioxide we exhale, among other things.

As explained by Dr. Avinash Chandel from the University of California, mosquitoes detect odors and carbon dioxide and look for prey, but these methods are unreliable. “Insects have poor eyesight, and wind or rapid movement can disrupt chemical signals,” he pointed out. He added that mosquitoes detect human convective heat (the temperature of the air above the heat source) from a distance of about 10 centimeters. In contrast, they detect infrared electromagnetic radiation (IR) from seven times greater distances.

Egyptian mosquitoes were put under the microscope (Aedes aegypti). This species carries viruses that cause more than 100,000 deaths each year, including dengue, yellow fever, Zika and West Nile fever. “These mosquitoes are exceptionally talented at finding human hosts,” said Dr. Nicolas DeBeaubien, who also participated in the study. “This study sheds new light on how they do it,” he added.

Female Egyptian mosquitoes were placed in two cages. Human odor molecules and carbon dioxide were pumped into each cage at concentrations similar to those in exhaled air. A source of infrared radiation at human skin temperature was also placed in one zone.

They found that in the infrared cage, female mosquitoes were twice as active in searching for prey. “What struck me most about this work was how strong a signal infrared was. Once all the parameters were dialed in, the results became undeniably clear,” Dr. DeBeaubien said.

Lead author of the study and head of the lab where the research was carried out, Professor Craig Montell, explained that IR radiation only matters to mosquitoes if the prey also emits other signals. “No single cue stimulates host-seeking activity. It is only when elevated levels of carbon dioxide and human odour are present in the vicinity of the prey that the insects become more interested,” Professor Montell said.

As Dr. DeBeaubien described, mosquitoes do not detect infrared radiation in the same way they detect visible light. That's because the IR energy is too low to activate rhodopsin, a photosensitive pigment responsible for the perception of visible light, found in the retinas of cephalopods, arthropods (including insects), and vertebrates.

Biologists at the University of California also discovered that mosquitoes have heat-sensitive neurons on the tips of their antennae. Removing these structures caused the parasites to stop “seeing” infrared rays. The researchers found a temperature-sensitive protein, TRPA1, on the antennae and identified the gene that codes for it. Animals lacking this gene did not respond to infrared light.

Mosquitoes have also been found to have other proteins that are sensitive to small temperature fluctuations. These proteins are not only responsible for detecting visible light, but are also detectors of taste and temperature.

“Half of the world's population is at risk of mosquito-borne diseases. Climate change and widespread travel have meant that the Aedes aegypti mosquito is no longer confined to tropical and subtropical countries,” Dr. Chandel noted. “This study could lead to better methods of controlling mosquito populations, for example by using infrared light in mosquito traps,” Dr. Chandel said.

The publication was published in August in the journal Nature.