Mosquitoes can be dangerous disease vectors, and they infect and kill hundreds of thousands of people and sicken millions more with illnesses like dengue, malaria, Yellow Fever, and West Nile virus. Researchers have now learned more about how mosquitoes use human warmth to locate their targets and get their blood meals. Understanding this process may help scientists stop the mosquitoes' temperature-sensing mechanisms from working, which could prevent the spread of the harmful diseases they carry. The findings have been reported in Science.
“Sensory systems like these are excellent targets for developing new ways to repel or confuse mosquitoes to keep them from biting us or to create new ways to help trap and kill these disease-spreading creatures,” said Brandeis University Professor of Biology Paul Garrity.
Mosquitoes have been studied for decades, and researchers know that females attack warm-blooded animals while leaving cold-blooded animals alone; they need the blood to nourish their eggs. When hunting for blood over long distances, the insects also use visual cues and sense humans' odor and the carbon dioxide we exhale. They are guided to food over small distances by sensing temperature.
Garrity and colleagues reported surprising findings in Neuron last year. Receptors that sit on the tips of fly antennas and sense temperature were thought to function like thermometers, getting a read on the local environment. The researchers' work showed, however, that these receptors are actually detecting temperature changes and whether the insect was in a place that was becoming hotter or colder. As such, the sensors, which can sense changes down to a few hundredths of a degree, were dubbed the Cooling Cells and Heating Cells by the researchers.
Mosquitoes have Cooling Cells and Heating Cells as well. Garrity's team hypothesized that Cooling Cells enable mosquitoes to fly away from cold (and not toward heat). In this work, the researchers focused on ionotropic receptor 21a (IR21a).
They deleted the gene that encodes for the receptor in mosquitoes. In one test, around 60 of these mutated mosquitoes were put in a container that was placed against a 98.6-degree plate, and gave the insects a puff a carbon dioxide. Normal mosquitoes gathered on the area facing the heated plate while mutant mosquitoes mostly ignored that area. A second experiment showed that when presented with vials of 73 or 88-degree blood, mutants were less likely to prefer warmer blood.
Garrity noted that IR21a is activated as mosquitoes sense cooler temperatures, so IR21a is not active when mosquitoes get close to people. If the mosquito starts to move away from a person, however, the receptor is then activated, inactivating only when the mosquito gets back on track to attack the human.
IR21a is like "an annoying alarm. It goes off whenever the female mosquito heads towards cooler climes. When they are seeking humans, they seem to be driven to do whatever it takes to turn down the sound,” Garrity explained.
The gene coding for IR21 dates back 400 million years to a marine animal that is an ancestor of crustaceans and insects. When flies and mosquitoes diverged around 200 million years ago, the gene began to serve different purposes, allowing mosquitoes to find human blood by detecting warmth and flies to get away from heat.