Summary: Researchers have discovered that mosquitoes, specifically Aedes aegypti, can detect infrared (IR) light emitted by human skin, enhancing their ability to find hosts. This newly identified sense, when combined with other cues such as CO2 and human odor, doubles down on their host-finding behavior.
The study revealed that mosquitoes use specific proteins to sense infrared light, providing insight into how these insects find humans and opening up new avenues for controlling mosquito-borne diseases.
Key Facts:
Mosquitoes detect infrared radiation emitted by human skin and guide it to the host. Infrared detection is most effective within 2.5 feet, making it easier to target mosquitoes. Understanding this infrared detection could lead to improved mosquito control methods.
Source: University of California, Santa Barbara
While mosquito bites are often just a temporary annoyance, in many parts of the world they can be frightening: Aedes aegypti, a type of mosquito, spreads viruses that cause diseases such as dengue fever, yellow fever, and Zika, which cause more than 100 million cases of the disease each year.
Another species, the Anopheles mosquito, spreads the parasite that causes malaria. The World Health Organization estimates that malaria alone is responsible for more than 400,000 deaths each year. In fact, mosquitoes’ ability to transmit disease has earned them the title of the most dangerous animal.
While male mosquitoes are harmless, females need blood for the development of their eggs. It’s no wonder that the way mosquitoes find hosts has been meticulously studied for over 100 years. In that time, scientists have discovered that there is no single cue that these insects rely on. Instead, mosquitoes integrate information from different senses at different distances.
A team led by researchers at the University of California, Santa Barbara, has added a new sense to mosquitoes’ known ones: infrared detection. When infrared radiation, which is roughly the same temperature as human skin, combines with carbon dioxide and human odor, the mosquito’s overall host-seeking behavior doubles.
Mosquitoes overwhelmingly move towards this infrared source while searching for a host. 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 very good at finding human hosts,” said co-first author Nicholas Debeaubien, a former graduate student and postdoctoral researcher in Professor Craig Montell’s lab.
“This study sheds new light on how they accomplished this.”
Thermal Infrared Induction
Mosquitoes such as Aedes aegypti are well known to use multiple cues to approach their hosts from a distance.
“These include carbon dioxide from exhaled breath, smell, vision, (convective) heat from the skin, and moisture from the body,” explains co-first author Avinash Chandel, a postdoctoral researcher at UCSB in Montell’s group.
“But each of these clues has its limitations.”
Because mosquitoes have poor eyesight and strong winds or rapid movements of their human hosts can disrupt their chemosensory tracking, the authors wondered whether mosquitoes might be able to sense more reliable directional cues, such as infrared light.
Within about 10 cm, these insects can sense heat rising from their skin, and when they land they can feel the temperature of their skin directly. These two senses correspond to two of the three types of heat transfer: convection (heat being carried away by a medium such as air) and conduction (heat through direct contact).
But thermal energy can travel longer distances when it’s converted into electromagnetic waves, typically in the infrared (IR) range of the spectrum, which can heat up anything it hits. Animals such as pit vipers can detect thermal IR from warm prey, and the team wondered whether mosquitoes such as Aedes aegypti could do the same.
The researchers placed female mosquitoes in cages and measured their host-seeking activity in two zones: each zone was exposed to human odors and carbon dioxide at concentrations similar to those in a person’s exhaled breath, but only one zone was also exposed to infrared skin temperature radiation.
A barrier separating the heat source from the chamber prevented heat exchange by conduction and convection, and the researchers counted the number of mosquitoes that began probing as if searching for a vein.
Adding thermal infrared radiation of 34°C (about skin temperature) doubled the mosquitoes’ host-seeking activity, marking the first time that infrared radiation has been used by mosquitoes to locate humans, and the team found that infrared radiation was effective up to about 70cm (2.5 feet).
“What struck me most about this study was how powerful an IR cue can be,” says DeBeaubien. “Once all the parameters were set properly, the results were unmistakable.”
Previous studies have not observed the effect of thermal infrared radiation on mosquito behavior, but lead author Craig Montell believes this is a methodological issue: A dedicated scientist might present only the infrared signal and try to figure out how thermal infrared radiation affects insects without providing other clues.
“However, a single stimulus alone does not stimulate host-seeking activity; IR is only effective in combination with other stimuli, such as elevated carbon dioxide concentrations or human body odor,” says Montell, the Duggan Distinguished Professor of Molecular, Cellular and Developmental Biology.
In fact, his team found the same thing when they tested infrared light alone: infrared light alone had no effect.
Infrared sensing trick
It’s impossible for mosquitoes to detect thermal infrared light in the same way they detect visible light: infrared light’s energy is too low to activate the rhodopsin protein in the animals’ eyes that detects visible light.
Electromagnetic radiation with wavelengths longer than about 700 nanometers doesn’t activate rhodopsin, and the infrared radiation emitted by body heat is about 9,300 nanometers. In fact, no known proteins are activated by radiation with wavelengths that long, Montell says. But there are other ways to detect infrared light.
Consider the heat radiating from the sun. The heat is converted into infrared radiation and travels through space. When the infrared radiation reaches the Earth, it collides with atoms in the atmosphere, transferring energy and warming the Earth.
“Heat is converted into electromagnetic waves, which are converted back into heat,” Montell says.
He noted that the wavelength of infrared light coming from the sun varies depending on the temperature of the source and is therefore different from the wavelength of infrared light produced by body heat.
The authors hypothesized that infrared-emitting human body heat might strike and heat up certain neurons in mosquitoes, activating them and allowing the mosquito to indirectly detect the radiation.
Scientists knew that mosquitoes have heat-sensing neurons at the tips of their antennae, and the team found that removing these tips eliminated the mosquito’s infrared sensing ability.
Indeed, another lab discovered a temperature-sensitive protein, TRPA1, in the tips of antennae, and the UCSB team observed that animals without a functional trpA1 gene that codes for this protein were unable to detect IR.
At the tip of each antenna is a peg-in-pit structure well suited to sensing radiation. The pits protect the peg from conductive and convective heat, allowing highly directional infrared light to penetrate the structure and heat it up. The mosquito then detects the infrared light using TRPA1, which is essentially a temperature sensor.
Biochemistry Challenge
It’s possible that activity of the heat-activated TRPA1 channel alone doesn’t fully explain the range at which mosquitoes can detect infrared light: A sensor relying solely on this protein may not be useful at the 70-cm range the team observed. At this distance, the infrared light collected by the peg-in-pit structures may not be strong enough to generate enough heat to activate TRPA1.
Fortunately, Montell’s group suspected that there might be more sensitive temperature receptors, based on previous work they did in fruit flies in 2011. They had found several proteins in the rhodopsin family that were highly sensitive to small increases in temperature.
Although rhodopsins were originally thought of only as light detectors, Montell’s group discovered that specific rhodopsins can be activated by a variety of stimuli. They discovered that this family of proteins is highly versatile, involved not only in vision but also in the sensing of taste and temperature.
Upon further investigation, the researchers discovered that two of the 10 rhodopsins found in mosquitoes are expressed in the same antennal neurons as TRPA1.
Knocking out TRPA1 abolished mosquitoes’ sensitivity to IR, but insects defective in either rhodopsin Op1 or Op2 were unaffected. Knocking out both rhodopsins simultaneously greatly weakened sensation but did not completely abolish the animals’ sensitivity to IR.
Research shows that stronger thermal infrared radiation (such as that experienced by mosquitoes at close range (e.g., about 1 foot)) directly activates TRPA1, while Op1 and Op2 are activated by lower levels of thermal infrared radiation, indirectly stimulating TRPA1. Because human skin temperature remains constant, extending the sensitivity of TRPA1 effectively extends the range of a mosquito’s infrared sensors to about 2.5 feet.
Tactical Advantage
Half the world’s population is at risk of mosquito-borne diseases, with roughly 1 billion people infected each year, Chandell said. Additionally, climate change and global travel have expanded the range of Aedes aegypti mosquitoes beyond tropical and subtropical countries. These mosquitoes are now found in parts of the U.S., such as California, that were not found there just a few years ago.
The team’s findings could provide ways to improve methods for controlling mosquito populations — for example, incorporating a thermal infrared source that is around skin temperature could increase the effectiveness of mosquito traps.
The findings also help explain why loose-fitting clothing is particularly effective at preventing bites: Not only do they prevent mosquitoes from getting close to your skin, but they also allow infrared radiation to diffuse between your skin and your clothing, preventing mosquitoes from detecting it.
“Despite their small size, mosquitoes are responsible for more human deaths than any other animal,” said Debeaubian. “Our study improves our understanding of how mosquitoes target humans and offers new possibilities for controlling the transmission of mosquito-borne diseases.”
In addition to the Montell team, former BASF employee Vincent Salgado and his student Andreas Krumhotz also contributed to the research.
About this Neuroscience Research News
Author: Harrison Tasoff
Source: University of California, Santa Barbara
Contact: Harrison Tasoff – University of California, Santa Barbara
Image: This image is from DeBeaubien and Chandel et al.
Original research: Open Access.
“Thermal infrared radiation guides host-seeking behaviour in Aedes aegypti”, DeBeaubien, Chandel et al., Nature
Abstract
Thermal infrared radiation induces host-seeking behavior in Aedes aegypti mosquitoes
Mosquito-borne diseases affect hundreds of millions of people each year, disproportionately affecting developing countries. The Aedes aegypti mosquito is the primary vector for the viruses that cause dengue, yellow fever and Zika.
To attract humans, female Ae. aegypti mosquitoes must integrate multiple cues, including carbon dioxide from breath, which are detected at medium to long distances, organic odors from the skin, visual cues, and other cues that are detected at very close ranges.
Here we identify cues that Aedes aegypti mosquitoes use as part of their sensory arsenal to find humans. We demonstrate that Aedes aegypti senses infrared (IR) radiation emitted by targets and uses this information in combination with other cues for highly effective medium-range navigation. Detection of thermal IR requires the heat-activated channel TRPA1, expressed in neurons at the antenna tips.
Two opsins are co-expressed with TRPA1 in these neurons, facilitating the detection of low IR intensities. The radiant energy induces localized heating at the tips of the antennae, which is thought to activate temperature-sensitive receptors in thermosensory neurons.
The realization that thermal infrared radiation is an excellent mid-range directional cue expands our understanding of how efficient mosquitoes are at finding hosts.
