Scientific Method / Science & Exploration

Mosquitoes can smell your ankles

Studying the mosquito's sensory pathways helps scientists find new repellents.

She knows where you're exposed!

The deadliest animals in the world, female mosquitoes, target their prey—us—because they are attracted to the carbon dioxide that we exhale. But just finding breathing human beings isn't really enough to guarantee the one big sip of blood the mosquito needs to get the nutrients necessary for laying eggs. Most mosquitoes can't bite through heavy clothing, so they need to head for exposed skin like ankles and ears. Finding that perfect spot to suck our blood depends on more than just carbon dioxide. In fact, experiments have shown that if you turn off the mosquito's ability to sense carbon dioxide, they can still detect and target human odors.

A team of scientists from the University of California Riverside studied mosquito sensory perception and wanted to figure out how the mosquitoes sense our skin: which of our many body odor compounds do they recognize and what organs sense those compounds? They found that the same olfactory neurons on the mosquito's maxillary palp—the little sensory appendages around the mouth—that sense carbon dioxide also play a major role in identifying human body odors.

To test this, they needed some human body odors. They collected a bit of distinctive fragrance by having a couple of subjects rub their feet on some glass beads. They then watched as the test mosquito, Aedes aegypti, homed in on the beads even when placed in a small wind tunnel.

Once they had a working odor target, they gave the mosquitoes a dose of butyryl chloride, which was known to knock out the specific neurons that sense carbon dioxide, at least temporarily. When they released these exposed mosquitoes on the same foot odor beads, the insects struggled to find the beads. Less than 20 percent found the target (by using some of their other sensory neurons that detect human odors), but they were slower than the controls, which hit the target about 70 percent of the time.

Armed with this indication that they had identified a key odor detection system, the scientists screened thousands of potential odor compounds through a chemical structure analysis to find the chemicals that were likely to trigger the mosquito's sensory system. From that list, they chose a small subset that was cheap, approved for human safety, and smelled nice enough.

They found some strong attractant chemicals, but more important, they found some strong inhibitors. Because no one really wants to attract more mosquitoes, right? Brief exposure to one chemical, which smells like green fruit, can shut down the mosquitoes' skin-seeking system for more than five minutes. They had brave human volunteers donate their arms to test out the fruity repellent.

The researchers even found a use for the attractants! The team tested a method for making effective mosquito traps using a powerful attractant chemical that smells nicer than your feet (minty) and lures in the insects at a pretty high rate.

Carefully engineering mosquito repellents and traps through this high-tech route isn't just about preventing a few itchy, annoying bites. Mosquitoes transmit some of the world's deadliest diseases, including malaria and dengue fever. Trapping them, or masking the human scent that attracts them, would be a huge benefit to public health if it worked on a large scale. Because the UC Riverside team chose to test chemicals already approved for safe human use, their candidates should be able to be scaled up and tested for real-world effectiveness pretty quickly. Even if these two specific chemicals don't pan out, this new insight into how mosquitoes find us will no doubt help us learn how to hide from them.

Cell, December 2013. DOI: 10.1016/j.cell.2013.11.013

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