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Mosquitoes Sense Infrared From Body Heat to Help Track Humans Down

Mosquitoes Sense Infrared From Body Heat to Help Track Humans Down

While a mosquito bite is often no more than a temporary bother, in many parts of the world it can be scary. One mosquito species, Aedes aegypti, spreads the viruses that cause over 100,000,000 cases of dengue, yellow fever, Zika and other diseases every year. Another, Anopheles gambiae, spreads the parasite that causes malaria. The World Health Organization estimates that malaria alone causes more than 400,000 deaths every year. Indeed, their capacity to transmit disease has earned mosquitoes the title of deadliest animal.

Male mosquitoes are harmless, but females need blood for egg development. It’s no surprise that there’s over 100 years of rigorous research on how they find their hosts. Over that time, scientists have discovered there is no one single cue that these insects rely on. Instead, they integrate information from many different senses across various distances.

A team led by researchers at UC Santa Barbara has added another sense to the mosquito’s documented repertoire: infrared detection. Infrared radiation from a source roughly the temperature of human skin doubled the insects’ overall host-seeking behavior when combined with CO2 and human odor. The mosquitoes overwhelmingly navigated toward this infrared source while host seeking. The researchers also discovered where this infrared detector is located and how it works on a morphological and biochemical level. The results are detailed in the journal Nature.

“The mosquito we study, Aedes aegypti, is exceptionally skilled at finding human hosts,” said co-lead author Nicolas DeBeaubien, a former graduate student and postdoctoral researcher at UCSB in Professor Craig Montell’s laboratory. “This work sheds new light on how they achieve this.”

Guided by thermal infrared

It is well established that mosquitoes like Aedes aegypti use multiple cues to home in on hosts from a distance. “These include CO2 from our exhaled breath, odors, vision, [convection] heat from our skin, and humidity from our bodies,” explained co-lead author Avinash Chandel, a current postdoc at UCSB in Montell’s group. “However, each of these cues have limitations.” The insects have poor vision, and a strong wind or rapid movement of the human host can throw off their tracking of the chemical senses. So the authors wondered if mosquitoes could detect a more reliable directional cue, like infrared radiation.

Within about 10 cm, these insects can detect the heat rising from our skin. And they can directly sense the temperature of our skin once they land. These two senses correspond to two of the three kinds of heat transfer: convection, heat carried away by a medium like air, and conduction, heat via direct touch. But energy from heat can also travel longer distances when converted into electromagnetic waves, generally in the infrared (IR) range of the spectrum. The IR can then heat whatever it hits. Animals like pit vipers can sense thermal IR from warm prey, and the team wondered whether mosquitoes, like Aedes aegypti, could as well.

The researchers put female mosquitoes in a cage and measured their host-seeking activity in two zones. Each zone was exposed to human odors and CO2 at the same concentration that we exhale. However, only one zone was also exposed to IR from a source at skin temperature. A barrier separated the source from the chamber prevented heat exchange through conduction and convection. They then counted how many mosquitoes began probing as if they were searching for a vein.

Adding thermal IR from a 34º Celcius source (about skin temperature) doubled the insects’ host-seeking activity. This makes infrared radiation a newly documented sense that mosquitoes use to locate us. And the team discovered it remains effective up to about 70 cm (2.5 feet).

“What struck me most about this work was just how strong of a cue IR ended up being,” DeBeaubien said. “Once we got all the parameters just right, the results were undeniably clear.”

Previous studies didn’t observe any effect of thermal infrared on mosquito behavior, but senior author Craig Montell suspects this comes down to methodology. An assiduous scientist might try to isolate the effect of thermal IR on insects by only presenting an infrared signal without any other cues. “But any single cue alone doesn’t stimulate host-seeking activity. It’s only in the context of other cues, such as elevated CO2 and human odor that IR makes a difference,” said Montell, the Duggan and Distinguished Professor of Molecular, Cellular, and Developmental Biology. In fact, his team found the same thing in tests with only IR: infrared alone has no impact.

This story was originally published by the University of California, Santa Barbara on August 22, 2024.