Although a mosquito bite is usually just a minor nuisance, in many regions, it can evoke fear. One type of mosquito, Aedes aegypti, is responsible for transmitting viruses that lead to over 100 million instances of dengue, yellow fever, Zika, and other illnesses each year. Another type, Anopheles gambiae, carries the parasite responsible for malaria. According to the World Health Organization, malaria alone results in more than 400,000 fatalities annually. This ability to spread diseases has rightfully earned mosquitoes the title of the most dangerous animal.
Male mosquitoes pose no threat, but female mosquitoes require blood for the development of their eggs. It’s understandable why extensive research has been conducted for over a hundred years to understand how mosquitoes locate their hosts. Scientists have found that these insects do not rely on just one single cue; rather, they use a combination of various sensory inputs from different ranges.
A research team from UC Santa Barbara has identified an additional sense in mosquitoes’ known abilities: the ability to detect infrared (IR). When combined with CO2 and human scent, IR radiation from a source at a temperature close to human skin increased the mosquitoes’ overall host-seeking behavior twofold. The mosquitoes consistently moved toward the infrared source while searching for hosts. The researchers further explored the location and functioning of this infrared sensor at both a structural and biochemical level. Their findings are detailed in the journal Nature.
“The mosquito we study, Aedes aegypti, is remarkably adept at locating human hosts,” stated co-lead author Nicolas DeBeaubien, a former graduate student and postdoc at UCSB in Professor Craig Montell’s lab. “Our research provides new insights into how they accomplish this.”
Guided by thermal infrared
It is well-known that mosquitoes like Aedes aegypti utilize multiple indicators to find hosts from afar. “These include CO2 from our breath, odors, vision, heat emitted from our skin, and humidity from our bodies,” explained co-lead author Avinash Chandel, a current postdoc in Montell’s lab. “However, each of these indicators has its limitations.” The insects struggle with vision, and factors like strong winds or quick movements from a human can disrupt their chemical tracking. Hence, the researchers contemplated if mosquitoes could pick up a more reliable directional signal, such as infrared radiation.
Within a proximity of about 10 cm, these insects can detect heat emitted from our skin, and they can sense the skin temperature right after landing. These two abilities correspond to two types of heat transfer: convection, the movement of heat through a medium like air, and conduction, which is heat exchanged through direct contact. However, heat can also travel longer distances as electromagnetic waves, typically within the infrared (IR) spectrum. Certain animals, like pit vipers, can sense thermal IR from warm prey, prompting the team to investigate whether mosquitoes, particularly Aedes aegypti, have this capability too.
The researchers placed female mosquitoes in a cage to analyze their host-seeking behavior in two areas. Each area was exposed to human odors and CO2 at levels similar to what humans exhale. However, only one area was complemented by an IR source at skin temperature. A barrier prevented heat transfer through conduction and convection between the source and chamber. They then counted how many mosquitoes exhibited vein probing, suggesting they were searching for a feeding site.
When the 34º Celsius IR source (around skin temperature) was added, the mosquitoes’ host-seeking behavior doubled, indicating that infrared radiation is a newly recognized sense that mosquitoes use to detect us. Moreover, the researchers found that this capability is effective up to about 70 cm (2.5 feet).
“What surprised me the most about this research was how significant the effect of IR turned out to be,” said DeBeaubien. “Once we adjusted all the parameters correctly, the results were a clear affirmation.”
Past studies had failed to demonstrate any influence of thermal infrared on mosquito behavior, but senior author Craig Montell believes this may be due to research methods. A diligent scientist might aim to study thermal IR in isolation, introducing only an infrared signal without accompanying cues. “Yet, relying solely on one cue does not prompt host-seeking behavior. It’s only when combined with other signals, like increased CO2 and human odors, that IR has an impact,” noted Montell, who holds the title of Duggan and Distinguished Professor of Molecular, Cellular, and Developmental Biology. His team confirmed that the same conclusion applied even in tests with only IR: infrared alone does not influence behavior.
A trick for sensing infrared
Mosquitoes cannot perceive thermal infrared radiation in the same manner as they detect visible light. The energy of IR is too low to trigger the rhodopsin proteins, which are responsible for detecting visible light in animal eyes. Electromagnetic waves with wavelengths longer than about 700 nanometers won’t activate rhodopsin; IR emitted by body heat is approximately 9,300 nm long. In fact, there is no known protein activated by radiation at such extended wavelengths, according to Montell. However, alternative mechanisms for detecting IR may exist.
Consider the warmth produced by the sun: it transforms into IR that travels through the vacuum of space. Upon reaching Earth, this IR interacts with atmospheric atoms, resulting in energy transfer and warming the planet. “This is heat converted into electromagnetic radiation, which, upon striking surfaces, is transformed back into heat,” Montell explained. He also pointed out that the IR emitted by the sun has a different wavelength than the IR produced by our body heat, since the wavelength is dependent on the temperature of the emitting source.
The researchers hypothesized that the IR generated by our body heat might interact with specific neurons in mosquitoes, thereby activating them through heating. This mechanism would enable mosquitoes to indirectly sense IR.
It has been established that the extremities of a mosquito’s antennae contain neurons capable of sensing heat. The research team found that when these tips were removed, the mosquitoes could no longer detect IR.
Moreover, another laboratory discovered the temperature-sensitive protein TRPA1 located at the tip of the antenna. The UCSB team noted that mosquitoes lacking a functional trpA1 gene—the gene responsible for the protein—could not perceive IR.
The antenna’s tips contain specially adapted peg-in-pit structures that are optimized for detecting radiation. The pits protect the pegs from conductive and convective heat, allowing the highly directional IR radiation to enter and warm the structure. The mosquito then utilizes the TRPA1 protein—a type of temperature sensor—to detect infrared radiation.
Diving into the biochemistry
The activity of the heat-sensing TRPA1 channel might not entirely account for the distance over which mosquitoes were able to detect IR. A sensor that depends solely on this protein might not function effectively at the 70 cm range found in the study, as there may not be enough IR collected by the peg-in-pit structure to sufficiently heat it and activate TRPA1.
Previous research conducted in 2011 on fruit flies suggested the existence of more sensitive temperature receptors. The team found several proteins within the rhodopsin family that reacted notably to slight temperature increases. While rhodopsins have traditionally been recognized as light-sensing proteins, Montell’s research uncovered that some of these proteins respond to various stimuli, indicating their multifunctionality beyond just vision, encompassing taste and temperature detection as well. In their detailed studies, the researchers noted that two out of ten identified rhodopsins in mosquitoes are present in the same antennal neurons as TRPA1.
When TRPA1 was removed, the mosquitoes lost their sensitivity to infrared (IR) light. However, insects with defects in either of the rhodopsins, named Op1 or Op2, showed no change in their sensitivity. Even when both rhodopsins were knocked out, the mosquitoes still maintained some sensitivity to IR, though it was noticeably reduced.
The findings indicated that stronger sources of thermal IR, similar to what a mosquito would encounter from a nearby target (like a person about a foot away), directly stimulate TRPA1. In contrast, Op1 and Op2 are activated by lower levels of thermal IR and subsequently lead to the activation of TRPA1. Given that human skin temperature remains relatively constant, the enhanced sensitivity of TRPA1 effectively extends the range of a mosquito’s IR detection to approximately 2.5 feet.
A tactical edge
Chandel noted that around half of the global population is vulnerable to diseases carried by mosquitoes, with about a billion people infected each year. Furthermore, climate change and increased global travel have allowed the Aedes aegypti mosquito to expand its habitat beyond tropical and subtropical regions. These mosquitoes have now been discovered in areas of the US, such as California, where they were not seen only a few years ago.
The team’s findings may offer new strategies to reduce mosquito populations. For example, using thermal IR that mimics human skin temperature could enhance the effectiveness of mosquito traps. The results also shed light on why loose-fitting garments help in avoiding bites: they not only prevent mosquitoes from reaching the skin but also allow the IR generated by the skin to disperse between the fabric and the body, making it harder for mosquitoes to detect.
“Though they are small, mosquitoes are responsible for more human fatalities than any other animal,” DeBeaubien remarked. “Our research improves the understanding of how mosquitoes find humans and opens up new avenues for managing the spread of mosquito-borne illnesses.”
Additionally, Vincent Salgado, previously associated with BASF, and his student, Andreas Krumhotz, were part of this research collaboration alongside the Montell team.
