It’s one of the deadliest animals in the world and has a taste for human blood: the mosquito.
Mosquitoes spread diseases like malaria, dengue fever, and yellow fever, which kill at least half a million people each year. Now researchers are learning how people taste of mosquitoes, right down to the individual neurons that sense the distinctive, delicious taste of blood.
Female mosquitoes have a sense of taste that is specially tuned to detect a combination of at least four different substances in the blood. The team at Rockefeller University’s Howard Hughes Medical Institute researcher Leslie Vosshall and colleagues report in the journal Neuron on October 12, 2020. The team genetically engineered mosquitoes so that the researchers could see which neurons fire when a mosquito tastes blood.
“This is definitely a technical tour de force,” says Johns Hopkins University School of Medicine neuroscientist Chris Potter, who studies mosquito repellants. Identifying the specific taste neurons associated with blood could be something “that we could use against the mosquito,” he says.
Vosshall and her team already knew a lot about the insect’s other finely tuned senses. In previous work, for example, they found that mosquitoes can detect the repellent DEET with their legs and identified an olfactory receptor that mosquitoes use to differentiate between humans and non-humans.
Little is known, however, about mosquitos’ sense of taste, although it is key to the spread of disease. “If mosquitoes couldn’t detect the taste of blood, they couldn’t theoretically transmit disease,” says Veronica Jové, an HHMI Gilliam Fellow at Rockefeller who led the work in Vosshall’s lab.
Only female mosquitoes feed on blood that they need for their eggs to develop. That puts women in a unique position. They need to distinguish between the sweet nectar they eat for most of their meals and the blood they are on before they lay eggs.
Jové suspected that unlike men, female Aedes aegypti mosquitoes would be able to differentiate between the two substances by taste. In behavioral experiments, she found that female mosquitoes have two feeding modes that use different mouth parts and recognize different flavors.
A nectar feeding mode detects sugar and a blood feeding mode uses a syringe-like “stiletto” to pierce the skin and taste blood. Jové put the mosquitoes into blood-feeding mode by offering them a mixture of four compounds: glucose (a sugar), sodium chloride (salt), sodium bicarbonate (found in both blood and baking soda), and adenosine triphosphate, or ATP. A compound that supplies cells with energy.
Vosshall was curious, so she asked Jové to conjure up an ATP solution in the lab and then took a sip.
It has no taste at all. ATP is that special mysterious stuff that people don’t like. But it has to be incredibly exciting and rewarding for the mosquito. “
Leslie Vosshall, Study Investigator, Howard Hughes Medical Institute, Rockefeller University
Just as a person has taste buds that differentiate between salty, sweet, bitter, sour, and umami flavors, the mosquito stiletto has neurons that specialize in responding to certain flavors.
In order to see these taste neurons in action, the researchers genetically modified mosquitoes with a fluorescent marker that glows when a nerve cell is activated. Then they observed which cells in the stiletto glow in response to different meals. Only a subset was activated by blood, including real blood and the researchers’ artificial mixture.
How does human blood taste to a mosquito? Perhaps the closest we can say is that it’s a little bit salty and a little bit sweet. It’s a bit like trying to describe how a honey bee sees a flower in ultraviolet hues that are invisible to the human eye, or how a bat overhears sonar waves that we cannot hear, says Vosshall. Likewise, a female mosquito can taste things that we cannot. “There is nothing like it in human experience,” she says.
The results indicate how specifically the female mosquito is adapted to find blood. Jové and Vosshall hope that a better understanding of the mosquito’s senses will ultimately lead to new ways to prevent them from biting us and spreading diseases.
One possibility may sound like science fiction, says Vosshall, but there are precedents. “I’ve only given my dogs their monthly flea and tick medications, which are oral,” she says. Perhaps, at some point, something similar could be done for mosquitoes – a drug that people could take before going into a mosquito infested area that would affect the mosquito’s taste for blood.
This idea, which boils down to making people less tasty, begs one final question. Are some people really “tastier” to mosquitoes than others? “We’re all tasty enough for a mosquito,” says Jové. As soon as they discover blood, she says, “We have no feeling that you are very picky.”
Howard Hughes Medical Institute
Jové, V. et al. (2020) Sensory discrimination of blood and flower nectar by Aedes aegypti mosquitoes.