Study Uncovers How Mosquitoes Use Infrared Detection To Locate Humans
While a mosquito bite is often just an annoyance, it can be alarming many regions. The Aedes aegpti mosquito spreads viruses causing over 100 million cases of dengue yellow fever, Zika, and other diseases annually. Another species, Anopheles gambiae, transmits the malaria parasite. The World Health Organization estimates that malaria alone results in over 400,000 deaths each year. This makes mosquitoes the deadliest animals due to their disease-spreading capabilities.
Male mosquitoes are harmless, but females require blood for egg development. Over a century of research has focused on how they locate their hosts. Scientists have found that mosquitoes use multiple cues rather than relying on a single signal. They integrate information from various senses across different distances.
Infrared Detection: A New Discovery
A team led by UC Santa Barbara researchers has identified another sense used by mosquitoes: infrared detection. Infrared radiation from a source at human skin temperature doubled the insects' host-seeking behavior when combined with CO2 and human odor. Mosquitoes overwhelmingly moved towards this infrared source during host-seeking activities.
The researchers also pinpointed where this infrared detector is located and its functioning on a morphological and biochemical level. These findings 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
Mosquitoes like Aedes aegypti use multiple cues to locate hosts from afar. "These include CO2 from our exhaled breath, odors, vision, 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 has limitations.
The insects have poor vision, and strong winds or rapid movement can disrupt their chemical tracking abilities. The authors wondered if mosquitoes could detect a more reliable directional cue like infrared radiation.
Within about 10 cm, these insects can detect heat rising from our skin and directly sense skin temperature upon landing. These correspond to two types of heat transfer: convection (heat carried by air) and conduction (heat via direct touch). Heat energy can also travel longer distances as electromagnetic waves in the infrared (IR) range of the spectrum.
Experimental Findings
The researchers placed female mosquitoes in a cage with two zones exposed to human odors and CO2 at concentrations similar to exhalation levels. Only one zone was also exposed to IR from a source at skin temperature. A barrier prevented heat exchange through conduction and convection between the zones.
Adding thermal IR from a 34º Celsius source doubled the insects' host-seeking activity compared to zones without IR exposure. This makes infrared radiation a newly documented sense that mosquitoes use to locate humans effectively 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."
Sensing Infrared Radiation
Mosquitoes cannot detect thermal infrared radiation like visible light because IR energy is too low to activate rhodopsin proteins in animal eyes. Electromagnetic radiation longer than about 700 nanometers won't activate rhodopsin; body heat-generated IR is around 9,300 nm.
No known protein is activated by such long wavelengths of radiation directly. However, there is another way for mosquitoes to detect IR indirectly through heat conversion processes similar to those involving solar radiation heating Earth's atmosphere.
The authors of the study hypothesized that body heat-generated IR might hit specific neurons in mosquitoes, activating them by heating them up indirectly enabling detection of radiation.
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