Why do mosquitoes choose you?
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(CNN) -
Of the more than 3,000 species of mosquitoes in the world, only a small number have evolved to specialize in sucking human blood.
It is currently unknown how mosquitoes that bite humans track us so effectively, but it is important as they don't just bite us.
They also carry dangerous diseases like Zika, dengue, West Nile virus, and malaria.
These diseases can be deadly.
In fact, stopping these pesky insects in their tracks could save up to half a million lives lost to these diseases each year.
"In each of those cases where a mosquito has evolved to bite humans, which has only happened two or three times, they become vectors of nasty disease," said Carolyn "Lindy" McBride, assistant professor of ecology. and Evolutionary Biology from the Institute of Neurosciences at Princeton University in New Jersey.
So he wants to understand how mosquitoes find and attack humans.
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Mosquitoes can smell us
"Most mosquitoes choose what (or whom) to bite based on smell," said McBride, whose lab focuses on the Aedes aegypti mosquito species, which evolved to bite humans specifically.
Only females suck blood as they need it to produce their eggs.
Knowing how a potentially disease-carrying female mosquito sniffs a person, while ignoring other warm-blooded animals, is a key question.
Once it is better known, much more effective repellants, or baits, could be made to keep mosquitoes away from humans.
This will save lives, said Christopher Potter, associate professor of neuroscience at the Johns Hopkins University Center for Sensory Biology.
If scientists can control their sense of smell, "we can really control what these mosquitoes do," said Potter, who studies the Anopheles mosquito, another that specifically bites humans.
That mosquito transmits malaria.
Our smells are complicated
It is not an easy question to answer, as any animal odor is made up of hundreds of chemical compounds mixed in specific proportions.
'The actual chemicals found in human odor are basically the same as the chemicals found in animal odor.
It's the proportions and relative abundance of these compounds in human mixtures that are unique, ”said McBride, whose research focuses on these issues.
Every time a hungry female mosquito flies past, it performs complex chemical calculations in its tiny brain, figuring out what is a human, what is a dog, and what is a flower.
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A library of smells
"To investigate, we decided to record neural activity in the brains of females while exposing them to extracts of natural human and animal odors," wrote Zhilei Zhao, a graduate student in McBride's lab, in a Twitter thread describing the work. from laboratory.
It took four years to develop "the necessary genetic reagents, odor release systems, and analytical approaches," Zhao wrote.
Noah Rose, a postdoctoral researcher at Princeton, and Gilbert Bianquinche examine a hole in a tree near Kedougou, Senegal, for Aedes aegypti larvae. More than half of the world's population lives in areas where Aedes aegypti mosquitoes are present.
McBride's laboratory team created a library of the chemical composition of animal odors.
"That dataset doesn't really exist, so we decided to collect it ourselves," said Jessica Zung, a graduate student in McBride's lab.
Zung has collected scent samples from about 40 different animals so far, including guinea pigs, rats, quail, and more.
A common compound of a scent stood out
When comparing some of those to the 16 human samples, something jumped.
Decanal, a simple and common compound, is particularly abundant in human skin, Zung said.
Ubiquitous in the natural world, in humans, the dechannel comes from another more complex compound.
Zung searched the archives to find research from the 1970s (much of which was originally done to find a cure for acne) that detailed how when sapienic acid, a component of our skin's natural oils, breaks down, remains dechannel.
This acid (as its name implies) is only found in humans.
It's what likely leads to the high levels of dechannel that help mosquitoes smell their way to us, but more study is needed.
Understanding what mosquitoes smell is only part of the story.
Knowing how they do it is also important.
To see exactly how mosquitoes use this sense, the scientists bred genetically modified Aedes aegypti mosquitoes "so that we could open their little heads and put them under a microscope and really see how neurons fire when exposed to human and animal odors," he said. McBride.
The research team already knew that mosquitoes have about 60 different types of neurons that detect odors, so when they looked into the brains of insects, they thought they could see a lot of activity.
But it was surprisingly quiet, which means that the signal was perhaps quite simple, with only a couple of neuron types.
'One type of neuron responded very strongly to both humans and animals.
Another type of neuron responded to both, but responded much more strongly to humans than animals, ”McBride said of that work.
So it can be as simple as that mosquito's brain to compare just two types of neurons.
This type of research has only been possible since the technology to study mosquito brains in detail became available, which has only happened recently.
"It has traditionally been very difficult to study this at the level that we are doing now," Potter said.
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Mosquitoes, an example of rapid evolution
Incredibly, the mosquitoes that attack humans have evolved to do this in just the last 5,000 years, making it a "really amazing example of rapid evolution," McBride said.
Aedes aegypti, also known as the "yellow fever mosquito," is also a carrier of dengue, Zika, and Chikungunya.
The creature originated in Africa and likely reached its current range in the southern United States and Central and South America on slave ships during the 17th century, according to McBride.
These diseases combined kill and sicken thousands of people a year, which is why the US Centers for Disease Control and Prevention (CDC) has called mosquitoes "the deadliest animal of the world".
McBride and Potter hope their work can be used by others working on repellants and attractants to prevent disease.
Keeping them away is simple
As for insider knowledge on how to avoid getting stung in your own backyard, McBride said you should use a fan.
"Get him to blow air over where he's sitting outside or on the barbecue or under the table where your feet are itching."
It's not that you're blowing the scent away to scare off mosquitoes, he said.
It's simply because these deadly creatures, McBride said, "aren't great flyers."
- Starre Vartan is a former geologist, science journalist, and dog runner who lives on an island in Puget Sound, near Seattle, who still collects rocks wherever she goes.
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