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New research reveals an important part of how mosquitoes invade human warmth to find and bite people.
Mosquitoes are one of the deadliest animals on the planet. Hundreds of thousands of people die each year from mosquito-borne diseases such as malaria, dengue, West Nile virus and yellow fever, most of them children. Another 200 million are infected and suffering from the symptoms.
The discovery that is emerging in science today offers the possibility of one day being able to fool or turn off the temperature sensors of the insects so that they do not spread disease.
“Sensory systems like these are great targets for developing new ways to repel mosquitoes or confuse them so they don’t bite us, or to find new ways to trap and kill these disease-spreading creatures,” says Paul Garrity, Professor of Biology at Brandeis University.
Mosquitoes around the teapot
At the beginning of the 20th century, Frank Milburn Howlett, a British scientist in India, noticed that mosquitos were always floating around his teapot at tea time. As an experiment, he filled a loose gauze bag with the insects and placed it near a test tube filled with hot water.
When the heat from the tube reached the animals, “the effect was most interesting,” he wrote in a 1910 research report. The mosquitoes were drawn to the side of the bag that was closest to the rising hot air. Howlett also observed that mosquitos did not appear to attack cold-blooded animals, suggesting that it was body heat that attracted them to humans.
Other research since then has shown that mosquitoes rely on the carbon dioxide we exhale, the smells we give off, and visual cues to find us for distances of many feet. But when they’re just inches away, body temperature plays an important role in controlling them.
Only the females of the species behave like this. As was later learned, they use the protein in our blood to feed their eggs. Males only feed on fruit and plant nectar.
Heating cells and cooling cells
Last year, Garrity and several colleagues published an article in Neuron magazine that turned conventional thinking about the temperature sensor receptors at the top of fly antennas.
Traditionally, these receptors were thought to act like thermometers, measuring the ambient temperature to let the fly know whether the environment is hot or cold. Instead, Garrity and colleagues found that the receptors only sensed if the temperature was changing and let the fly know if it was getting hotter or colder.
For this reason, Garrity renamed these temperature sensors cold rooms and heating cells. They are so sensitive that they can detect a temperature change of a few hundredths of a degree per second.
Mosquitoes, which are close evolutionary relatives of flies, also have cold rooms and heating cells.
While it makes sense to look at insects’ heating cells to understand what draws them to human warmth, Garrity’s group considered an alternative – and counterintuitive – hypothesis. Perhaps it wasn’t that the insects were flying towards the heat; Maybe they flew away from the cold. This would mean the cold rooms are the ones to focus on.
With and without an IR21a receptor
The specific cold cells Garrity and his colleagues studied for their scientific work rely on a molecular receptor called IR21a. IR stands for ionotropic receptor, a group of proteins that help neurons transmit signals. IR21a facilitates the transmission of a signal that the temperature around the insect is dropping.
In their experiment, the researchers switched off the mosquito gene that is responsible for producing the IR21a receptor. Then they put about 60 of the mutant insects in a shoebox-sized container with a plate on the back wall that was heated to a body temperature of nearly 98.6 degrees, and gave the mosquitoes a puff of carbon dioxide to mimic human breath.
While non-mutated mosquitoes quickly gathered on the body temperature plate and tried to eat, the mutated mosquitoes largely ignored the plate. Without the IR21a receptor, they would no longer be able to focus on the hottest spot in their vicinity.
In a second experiment, the mosquitoes were placed in a small lattice cage. The researchers placed two vials of human blood over the cage, one of which was heated to 73 degrees (room temperature) and the other to 88 degrees (the surface temperature of a human hand). Compared to non-mutant mosquitoes that go through the same set-up, the mutants showed a decreased preference for the 88-degree blood.
“You’re getting cooler …”
According to Garrity, the IR21a receptor is activated whenever mosquitoes approach a cooler temperature. Since humans are usually warmer than their surroundings, this means that IR21a is silent when a mosquito approaches a human. However, should the animal deviate from its course and move away from its warm-blooded prey, IR21a is activated and only switched off when the insect course has been corrected.
Ultimately, tracking the change in temperature is extremely useful in helping these animals determine exactly where to bite us, since blood vessels are the warmest point on our skin.
Garrity says the IR21a seems to be acting like an “annoying alarm”. It always starts when the female mosquito goes in the direction of cooler climes. When looking for people, they seem driven to do whatever they can to turn the sound down. “
The gene for IR21 comes from a sea creature that lived over 400 million years ago and eventually gave birth to both modern crustaceans (such as lobsters and crabs) and insects. After the ancestors of modern insects finally ventured ashore, the gene was passed on to the common ancestors of flies and mosquitoes. When the evolutionary trajectories of these insects diverged about 200 million years ago, each species developed different uses for the IR21a receptor. Flies use it to avoid heat, mosquitoes to find heat and to feed on human blood.
The newspaper is co-authored by Brandeis and the Harvard TH Chan School of Public Health.
Source: Brandeis University