Only a few of the over 3,000 mosquito species in the world have evolved to be specialized in sucking blood from humans.
It is not known how mosquitoes that bite humans can track us so well, but this matters because they do more than just make us itch. These mosquitoes can carry deadly diseases like Zika, West Nile virus, dengue and malaria.
Stopping these insects could prevent up to a half-million deaths each year from the diseases they spread.
'In every case where a mosquito evolved to bite people -- which only happened twice or three times -- it becomes a nasty disease vector,' said Carolyn 'Lindy' McBride. She is an assistant professor at Princeton Neuroscience Institute, New Jersey, who teaches ecology and evolutionary biology.
She wants to know how they target and find humans.
Mosquitoes are able to smell us
McBride's lab studies the Aedes Aegypti mosquito, which has evolved to bite people specifically.
Only female mosquitoes sucking blood to produce eggs. It is important to understand how a female mosquito that may carry a disease can detect a human while ignoring warm-blooded mammals.
Christopher Potter, associate Professor of Neuroscience at Johns Hopkins University’s Center for Sensory Biology, says that once this is better understood, it will be possible to make much more effective repellents - or baits to lure mosquitoes from humans - and save lives.
Potter, who studies Anopheles mosquitoes that carry malaria, said that if scientists could control their senses of smell they would be able to control the behavior of mosquitoes.
The answer is not easy to give, as any animal odor is composed of hundreds of chemical components mixed in certain ratios.
McBride's research on these issues focuses on the fact that 'the actual chemicals that are present in human odors are the same as those found in animal mixtures. It's just the ratios and relative abundance of compounds that is unique.
Every time a female mosquito is hungry, its tiny brain does complex chemical calculations to determine what's an animal, what's human and what's flower.
The library of scents
In a tweet thread that described the lab's research, Zhilei Zhu, a graduate students in McBride lab, wrote: 'To investigate we decided to record the neural activity in females' brains while they were exposed to natural human and animals odor extracts.' Zhao reported that it took four years to create 'the necessary genetics reagents and odor delivery systems as well as analytical approaches'.
McBride’s lab created a library that contains the chemical compositions of animal odors. Jessica Zung is a graduate research student at McBride’s lab.
Zung has collected smell samples from more than 40 animals, including rats, quail, and guinea-pigs.
One compound stood out.
When comparing some of these to the 16 samples from humans, something stood out. Zung says that decanal, an ordinary compound, is especially abundant in the skin of humans.
Decanal is ubiquitous in nature, but in humans it comes from a more complex compound. Zung delved into the archives and found research done in the 1970s, primarily to cure acne. The study detailed how sapienic acids, a component of the natural oils of the skin, are broken down. As its name suggests, this acid is found only in humans. This is what leads to high levels of decanal, which helps mosquitoes find us. But more research needs to be done.
Knowing what mosquitoes smell is important, but also how they do so is equally as vital. Scientists wanted to see how mosquitoes used this sense. They bred Aedes Aegypti genetically modified mosquitoes. 'We could cut open their tiny heads, put them under an expensive microscope, and watch neurons firing as they are exposed to animal and human odors' McBride explained.
Researchers knew mosquitoes had about 60 different types neurons that detect odors. So when they examined the brains of insects, they expected to see lots of activity. It was quieter than expected, which meant that the signal may have been quite simple and consisted of only a few types of neurons.
One type of neuron was very sensitive to both animals and humans. McBride explained that another type of neuron also responded, but much more strongly with humans. It may be as simple a comparison as the brain of a mosquito comparing only two types neurons.
The technology for studying mosquito brains in depth has only recently become available. Potter stated that it has been very difficult to study mosquito brains at the level of detail we are doing now.
Rapid evolution: An example
McBride noted that mosquitoes have evolved in the last 5,000 to target humans. This is a "really amazing example" of rapid evolution.
Aedes Aegypti - also known as the yellow fever mosquito - can carry dengue, Zika, and chikungunya. According to McBride, the critter is believed to have originated in Africa. It likely arrived in its current range of the southern United States, Central and South America and Central Asia on slave ships in the 1600s.
The US Centers for Disease Control and Prevention have called mosquitoes 'the deadliest animal in the world' because of these diseases. McBride, Potter and others hope that their work can be used to develop repellents and lures for the prevention of disease.
It is easy to keep them at bay
McBride has some insider tips on how to avoid being bitten by mosquitoes in your backyard. She uses a fan.
"Blow air where you are sitting outside, over the BBQ or under the table. They're biting on your feet." She said that it's not as if you are blowing the smell around to get the mosquitoes away.
McBride explained that this is because the deadly creatures 'aren't great flyers'.