How effectively human-biting mosquitoes are so effective at tracking us down is currently unknown, but it is important because they don’t just make us itch. They also carry dangerous diseases such as zika, dengue, West Nile virus, and malaria, which can be fatal.
In fact, stopping these pesky insects in their tracks could save up to half a million lives each year from those diseases.
“In each of those instances where a mosquito evolved to bite people – which has only happened two or three times – they become evil vectors,” said Carolyn “Lindy” McBride, assistant professor of ecology and evolutionary biology at the Princeton Neuroscience Institute in New Jersey.
So she wants to understand how to find and target people.
Mosquitoes can smell us
“Mosquitoes mostly choose what to bite based on the smell,” said McBride, whose lab focuses on the Aedes aegypti mosquito species, which are specifically designed to bite humans.
Only female mosquitoes suck blood because they need it to make their eggs. Knowing how a potentially disease-causing female mosquito is spying on a person while ignoring other warm-blooded animals is a key question.
Once this is better known, much more effective repellants – or baits to lure mosquitoes away from humans – could be made to save lives, said Christopher Potter, associate professor of neuroscience at the Center for Sensory Biology at Johns Hopkins University.
If scientists can control their sense of smell, “we can really control what these mosquitoes are doing,” said Potter, who studies another human-specific mosquito, Anopheles, that carries malaria.
Our smells are complicated
This question is not easy to answer, as every animal odor is made up of hundreds of chemical compounds mixed together in certain proportions.
“The actual chemicals that are in human odor are basically the same as the chemicals that are in animal odor – it is the proportions and relative abundance of these compounds in human mixtures that are unique,” said McBride, whose research focuses on these topics.
Every time a hungry female mosquito flies by, she performs complex chemical calculations in her tiny brain to figure out what is a human, what a dog, and what a flower is.
A library of smells
“To investigate this, we decided to record neural activity in the brain of women and expose them to natural human and animal odor extracts,” wrote Zhilei Zhao, a PhD student at McBride’s lab, on a Twitter thread detailing the lab’s work has been described. It took four years to develop “the necessary genetic reagents, odor delivery systems, and analytical approaches,” wrote Zhao.
McBride’s lab team created a library on the chemical composition of animal odors. “This dataset doesn’t really exist – so we decided to collect it ourselves,” said Jessica Zung, a PhD student in McBride’s lab.
Zung has so far collected scent samples from around 40 different animals, including guinea pigs, rats, quail, and more.
A common connection stood out
Something jumped out when comparing some of these samples with the 16 human samples. Decanal, a simple, common compound, is particularly common in human skin, said Zung.
Omnipresent in nature and in humans, Decanal comes from a different, more complex compound. Zung rummaged in the archives to find research from the 1970s (much of it originally conducted to find an acne cure) detailing how decanal is left when a component of our skin’s natural oils, sapienic acid, is left over , disintegrates. This acid (as indicated by its name) is only found in humans. It is what is likely to result in high decanal levels that help the mosquitoes smell their way to us, but more studies need to be done.
Understanding what the mosquitoes are sniffing out is only part of the story; Knowing how to do it is also important. To see exactly how mosquitoes use this sense, scientists bred genetically engineered Aedes aegypti mosquitoes “so that we could cut their little heads open and put them under a fancy microscope and actually see how neurons fire when they smell human and animal odors are exposed.” “McBride said.
The research team already knew that mosquitoes have around 60 different types of neurons that sense smells. When they looked into the insects’ brains, they thought they could see a lot of activity. But it was surprisingly quiet, which meant that the signal was maybe quite simple, except for a few types of neurons.
“One type of neuron was very responsive to humans and animals. Another type of neuron responded to both – but it was much more responsive to humans than to animals,” said McBride of the work. So it can be as simple as this mosquito brain to compare just two types of neurons.
This type of research has only been possible since the technology to study mosquitoes’ brains in detail became available, which was only recently. “It has traditionally been very difficult to study this at the level we are now,” said Potter.
An example of rapid development
Incredibly, mosquitoes that target humans have evolved to be able to do so over the past 5,000 years. So it’s a “really amazing example of rapid evolution,” said McBride.
The Aedes aegypti, also known as the “yellow fever mosquito”, also carries dengue, zika, and chikungunya. The animal is native to Africa and, according to McBride, likely found its way on slave ships in the 17th century to its current range in the southern United States and Central and South America.
Together, these diseases kill and become ill thousands of people annually, which is why mosquitoes have been named “the deadliest animal in the world” by the US Centers for Disease Control and Prevention. McBride and Potter both hope that their work can be used by others working on repellants and attractants to prevent disease.
Keeping them out is easy
As for inside knowledge of how to avoid getting bitten in your own yard, McBride said she uses a fan.
“Let it blow air over where you are sitting outside or over the grill or under the table where it will bite your feet.” It’s not that you blow the scent around to throw the mosquitoes off their tracks, she said.
It’s just because, McBride said, these deadly creatures “aren’t great fliers.”
Starre Vartan, a former geologist, is a science writer and dog runner who lives on an island in Puget Sound near Seattle and still picks up rocks wherever she goes.