Sophisticated measures used by mosquitoes for human tracking

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Healthcare (Commonwealth Union) – Mosquitoes have always been dreaded across the world for spreading many diseases. One species, Aedes aegypti, transmits viruses responsible for more than 100 million cases of diseases like dengue, yellow fever, and Zika virus each year. Another species, Anopheles gambiae, spreads the parasite that causes malaria. According to the World Health Organization, malaria alone leads to over 400,000 deaths annually. Mosquitoes have rightfully earned the title of the deadliest animal due to their ability to spread disease.

While male mosquitoes are harmless, female mosquitoes require blood to develop their eggs. Unsurprisingly, scientists have been studying how mosquitoes locate their hosts for over a century. Research has shown that mosquitoes don’t rely on just one signal; instead, they use various sensory cues from different distances to find their targets.

A group of researchers from UC Santa Barbara has now uncovered a new sensory ability in mosquitoes: infrared detection. When combined with CO2 and human scent, infrared radiation—similar to the heat emitted by human skin—doubles the mosquitoes’ host-seeking activity. During host-seeking, the mosquitoes were drawn to the infrared source. The team also pinpointed the location of the mosquitoes’ infrared sensor and explained its function on a structural and biochemical level. These findings were published in Nature.

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

Avinash Chandel, a co-lead author and postdoctoral researcher at UCSB in Montell’s group pointed out that it is well known that mosquitoes, such as Aedes aegypti, rely on multiple signals to find hosts from afar. He further indicated that these include carbon dioxide from our breath, odors, visual cues, heat from our skin, and the moisture we release. But each of these signals has its drawbacks.

Due to the fact that mosquitoes have poor eyesight, and factors like strong winds or rapid movement can disrupt their ability to follow chemical cues had posed a question. This led the researchers to consider whether mosquitoes might use a more dependable directional signal, such as infrared radiation.

When mosquitoes get within about 10 cm, they can detect the warmth from our skin, and once they land, they can sense our skin temperature directly. These two sensations align with two of the three ways heat is transferred: convection, where heat moves through a medium like air, and conduction, where heat is transferred through physical contact. But heat can also travel over greater distances when converted into electromagnetic waves, particularly in the infrared (IR) part of the spectrum. IR radiation can heat objects upon contact. Certain animals, like pit vipers, can detect thermal IR from warm-blooded prey, so the researchers wondered if mosquitoes like Aedes aegypti might have a similar ability.

The team conducted an experiment by placing female mosquitoes in a cage and measuring their host-seeking behavior in two separate zones. Both areas contained human odors and CO2 levels matching those in exhaled breath. However, only one zone was exposed to IR radiation from a source at skin temperature. A barrier was placed to block any heat transfer by convection or conduction. The researchers then counted how many mosquitoes started probing, as if searching for a blood vessel.

When IR radiation from a 34ºC source (roughly the temperature of human skin) was added, the mosquitoes’ host-seeking activity doubled. This finding suggests that infrared radiation is another sensory tool mosquitoes use to locate humans. The researchers found that this ability works effectively from up to 70 cm (2.5 feet) away.

The researchers’ findings may lead to new strategies for controlling mosquito populations. By utilizing heat signatures similar to human skin temperature, mosquito traps could be enhanced in their efficiency. Furthermore, the study sheds light on the effectiveness of loose-fitting garments in deterring mosquito bites. These clothes not only hinder the mosquito’s access to the skin, but they also facilitate the dispersal of heat between the skin and fabric, making it difficult for mosquitoes to detect the body’s warmth.

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