How Can Mosquitoes Find Humans So Easily?

A sophisticated sense of smell makes the Earth’s deadliest animal hard-wired to hunt us

Female mosquito
A female Aedes aegypti mosquito Amy E. Lockwood / Centers for Disease Control and Prevention

Anyone who’s ever felt the subtle sting of a mosquito bite and delivered a sharp slap to the affected area has probably wondered: How are mosquitoes so good at hunting down humans? The moment we step outside, it seems, they arrive en masse, ready to suck nutrient-rich blood from their next victim.

New research offers a possible explanation: Mosquitoes have a highly sophisticated olfactory system that gives them smelling superpowers, suggests a paper published last week in the journal Cell.

For decades, scientists believed that animals' sense of smell worked in a rather basic way: Information-carrying sensory neurons each pick up one specific odor and send that data to the brain. Different kinds of these neurons can combine to identify more complex smells, but each neuron has only one receptor, corresponding to one particular scent.

With this in mind, researchers used gene-editing technology in an attempt to prevent the pesky insects from sniffing out humans. They disabled certain human-odor receptors on sensory neurons in the antennae of female Aedes aegypti mosquitoes. (Only female mosquitoes bite, as they rely on blood to nourish their eggs.)

The scientists expected these tweaks to hamper the neurons’ ability to detect human body odor and pass that information along to the brain. But when they exposed the bugs to human aromas and analyzed their neuronal activity, the researchers realized the mosquitoes were still picking up the scent.

To find out why, they took a closer look at the neurons through RNA sequencing. They found that a single olfactory neuron could contain multiple receptors, not just one as they’d previously assumed. This means that if one human-smell receptor isn’t working for some reason, the bugs have a backup.

Researchers aren’t sure why mosquitoes have this built-in redundancy in their smelling system, but one possible reason could be the vast diversity of odors they might encounter in their search for blood.

“Different people can smell very different from one another,” says Meg Younger, a neurobiologist at Boston University and one of the study’s authors, to Science News’ Erin Garcia de Jesús. “Maybe this is a setup to find a human regardless of what variety of human body odor that human is emitting.”
Zika virus
Zika virus under a microscope Cynthia Goldsmith / Centers for Disease Control and Prevention

Scientists are so keen to understand how mosquitoes find their prey, because the bugs are responsible for passing along dengue, Zika, West Nile, yellow fever, malaria, chikungunya and many other dangerous pathogens to humans. Mosquitos are the deadliest animal on the planet, responsible for more than 700,000 deaths each year and for causing millions of preventable illnesses.

“They’re really the ultimate predator,” says Omar Akbari, a biologist at the University of California San Diego who was not involved in the study, to The Atlantic’s Katherine J. Wu. “You can’t find a single person on Earth that hasn’t been bitten at least once.”

This new research reveals that editing the insects’ smell-related genes likely won’t be an effective way to stop them. Instead, it suggests that scientists and public health experts should direct their attention and resources to other methods, such as creating better traps or repellents. The verdict is still out on the effectiveness of genetic modifications unrelated to smell, such as engineering mosquito offspring to be non-biting males.

As Christopher Potter, a biologist at Johns Hopkins School of Medicine who was not involved in the research, tells New Scientist’s Corryn Wetzel, these and other related studies are “changing the dogma of what we thought we knew about the olfactory system.”