Mosquitoes hunt humans via infrared

According to researchers, mosquitoes can detect infrared radiation from body heat to locate humans.

Although a mosquito bite is often only a temporary nuisance, it can be frightening in many parts of the world.

One species of mosquito, Aedes aegyptispreads the viruses that cause more than 100,000,000 cases of dengue, yellow fever, Zika and other diseases each year. Another, Anopheles gambiaespreads the parasite that causes malaria.

The World Health Organization estimates that malaria alone causes more than 400,000 deaths each year. Their ability to transmit disease has even earned mosquitoes the title of deadliest animal.

Male mosquitoes are harmless, but females need blood to develop eggs. It’s no surprise that more than 100 years of rigorous research has gone into how they find their hosts. In that time, scientists have discovered that there is no single signal these insects rely on. Instead, they integrate information from many different senses over different distances.

Now researchers have added a new sense to the mosquito’s documented repertoire: infrared detection.

Infrared radiation from a source that was about the temperature of human skin doubled the insects’ overall host-seeking behavior when combined with CO2 and human scent. The mosquitoes overwhelmingly navigated toward this infrared source during 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 described in detail in the journal Nature.

“The mosquito we study, Aedes aegypti“The virus is exceptionally adept at finding human hosts,” said co-lead author Nicolas DeBeaubien, a former doctoral student and postdoctoral researcher at the University of California, Santa Barbara in the lab of professor Craig Montell. “This work sheds new light on how they achieve this.”

How Mosquitoes Find People

It is common knowledge that mosquitoes love mosquitoes Aedes aegypti Use multiple signals to find remote hosts.

“These include CO2 from our exhaled breath, smells, vision, (convective) heat from our skin, and humidity from our body,” explained co-lead author Avinash Chandel, a current postdoctoral fellow at UCSB in Montell’s group. “However, each of these signals has limitations.”

The insects have poor eyesight, and strong winds or rapid movement from a human host can disrupt their chemical senses. So the authors wondered whether mosquitoes could detect a more reliable directional cue, such as infrared radiation.

Within about 10 cm, these insects can detect heat rising from our skin. And they can feel the temperature of our skin directly as soon as they land. These two senses correspond to two of the three types of heat transfer: convection, heat carried away through a medium such as air, and conduction, heat through direct contact.

But energy from heat can also travel longer distances when it is converted into electromagnetic waves, generally in the infrared (IR) range of the spectrum. The IR can then heat whatever it hits. Animals such as vipers can sense thermal IR from warm prey, and the team wondered whether mosquitoes, like Aedes aegypticould also be possible.

The researchers placed 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 as 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, preventing heat exchange by conduction and convection. They then counted how many mosquitoes began probing as if searching for a vein.

By adding thermal IR from a source at 34º Celsius (about skin temperature), the insects’ host-seeking activity was doubled. This makes infrared radiation a newly documented sense that mosquitoes use to locate us. And the team found that it remains effective out to about 70 cm (2.5 feet).

“What struck me most about this work was how powerful IR turned out to be,” DeBeaubien says. “Once we got all the parameters right, the results were unmistakably clear.”

Previous studies have found no effect of thermal infrared on mosquito behavior, but lead author Craig Montell suspects that comes down to methodology. A diligent scientist might try to isolate the effect of thermal IR on insects by presenting only an infrared signal with no other cues. “But a single signal alone does not stimulate host-seeking activity. Only in the context of other cues, such as elevated CO2 and human odor, does IR make a difference,” says Montell, a professor of molecular, cellular and developmental biology. In fact, his team found the same thing in IR-only tests: infrared alone has no impact.

An infrared trick

Mosquitoes can’t detect thermal infrared radiation the same way they can detect visible light. The energy of IR is far too low to activate the rhodopsin proteins that detect visible light in animal eyes. Electromagnetic radiation with a wavelength longer than about 700 nanometers doesn’t activate rhodopsin, and IR generated by body heat is about 9,300 nm. In fact, no known protein is activated by radiation with such long wavelengths, Montell says. But there’s another way to detect IR.

Think of the heat radiated by the sun. The heat is converted into IR, which travels through empty space. When the IR reaches Earth, it hits atoms in the atmosphere, transferring energy and warming the planet.

“You have heat that is converted into electromagnetic waves, which is converted back into heat,” Montell says, noting that the IR coming from the sun has a different wavelength than the IR generated by our body heat, because the wavelength depends on the temperature of the source.

The authors speculated that our body heat, which generates IR, might hit certain neurons in the mosquito, activating them by warming them up. That would allow the mosquitoes to detect the radiation indirectly.

Scientists knew that the tips of a mosquito’s antennae have heat-sensitive neurons. And the team found that removing these tips eliminated the mosquitoes’ ability to detect IR.

Another lab did indeed find the temperature-sensitive protein, TRPA1, in the tip of the antenna. And the researchers saw that animals without a functional trpA1 gene, which codes for the protein, could not detect IR.

The tip of each antenna has pin-in-well structures that are well adapted to detecting radiation. The well protects the pin from conductive and convective heat, allowing the highly focused IR radiation to enter and heat the structure. The mosquito then uses TRPA1, essentially a temperature sensor, to detect infrared radiation.

How does it work?

The activity of the heat-activated TRPA1 channel alone may not fully explain the range over which mosquitoes were able to detect IR. A sensor relying solely on this protein may not be useful at the 70 cm range the team observed. At this distance, the peg-in-pit structure likely does not collect enough IR to heat it enough to activate TRPA1.

Fortunately, Montell’s group thought there might be more sensitive temperature receptors, based on their earlier work in fruit flies in 2011. They had found a few proteins in the rhodopsin family that were quite sensitive to small increases in temperature.

Although rhodopsins were originally thought to be solely light detectors, Montell’s group discovered that certain rhodopsins can be activated by a variety of stimuli. They found that proteins in this group are quite versatile, involved not only in vision, but also in taste and temperature perception. 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 eliminated the mosquito’s sensitivity to IR. But insects with defects in either rhodopsin, Op1 or Op2, were unaffected. Even knocking out both rhodopsins at once did not completely eliminate the animal’s sensitivity to IR, although it significantly weakened the senses.

Their results indicated that more intense thermal IR, such as what a mosquito would experience at a shorter distance (say, about 1 foot), activates TRPA1 directly. Meanwhile, Op1 and Op2 can be activated at lower levels of thermal IR, and then indirectly trigger TRPA1. Because our skin temperature is constant, increasing the sensitivity of TRPA1 effectively extends the range of the mosquito’s IR sensor to about 2.5 feet.

Looking ahead

Half the world’s population is at risk of mosquito-borne diseases, and about a billion people are infected each year, Chandel says. In addition, climate change and global travel have increased the scope of Aedes aegypti outside of tropical and subtropical countries. These mosquitoes are now present in places in the US where they were not present a few years ago, including California.

The team’s discovery could provide a way to improve methods for suppressing mosquito populations. For example, incorporating thermal IR from sources close to skin temperature could make mosquito traps more effective. The findings also help explain why loose-fitting clothing is particularly good at preventing bites. Not only does it prevent the mosquito from reaching our skin, it also allows the IR to diffuse between our skin and the clothing, so the mosquitoes can’t detect it.

“Despite their small size, mosquitoes are responsible for more human deaths than any other animal,” DeBeaubien says.

“Our research increases understanding of how mosquitoes target humans and offers new opportunities for controlling the transmission of mosquito-borne diseases.”

Source: UC Santa Barbara