Chemical cocktail in the skin triggers disease-spreading mosquitoes

Aedes aegyptii mosquito biting a person. Credit: CDC

Mosquitoes that spread Zika, Dengue and Yellow Fever are guided to their victims by a scent from human skin. The exact composition of that fragrance has not yet been determined.

A team led by UC Riverside found that the combination of carbon dioxide plus two chemicals, 2-ketoglutaric and lactic acid, produces an odor that causes a mosquito to locate and land on its victim. This chemical cocktail also encourages probing, the use of piercing mouthparts to find blood.

This chemical mixture seems to specifically attract female Aedes aegypti mosquitoes, Zika vectors, and chikungunya, dengue and yellow fever viruses. This mosquito is native to Africa, but has spread to tropical and subtropical regions worldwide, including the US

This new research finding, and how the team discovered it, is detailed in the journal Scientific Reports. “While others have identified compounds that attract mosquitoes, many of them don’t have a strong, rapid effect. This one does,” said Ring Cardé, a UCR entomologist.

Mosquitoes use a variety of signals to locate their victims, including carbon dioxide, visibility, temperature and humidity. However, recent research by Cardé shows that skin odors are even more important for locating a bite site.

“We have shown that mosquitoes land on visually indistinct targets imbued with these two odors, and these targets are not associated with heat or moisture,” Cardé said. “That leaves skin odor as the main guiding factor.”

Given the importance of scent in helping mosquitoes feed successfully on humans, Cardé wanted to discover the exact chemicals that make our scent so potent to the insects. Part of the equation, lactic acid was identified as a chemical element in the fragrance cocktail as far back as 1968.

Since then, several studies have shown that carbon dioxide combined with ammonia and other human-produced chemicals also attract these mosquitoes. However, Cardé, who has studied mosquitoes for 26 years, found that these other chemicals were not strong attractants.

Chemical cocktail in the skin triggers disease-spreading mosquitoes

A sock and glass beads used to trap human sweat odor for the mosquito research. Credit: Jan Bello/UCR

“I suspected there was something undiscovered about the chemistry of scents that lure the yellow fever mosquito,” Cardé said. “I wanted to capture the exact blend.”

Methods that chemists typically use to identify these chemicals wouldn’t have worked for 2-ketoglutaric acid, Cardé said. Gas chromatography, which separates chemicals based on their molecular weight and polarity, would have missed this acid.

“I think these chemicals may not have been found before because of the complexity of the human odor profile and the minute amounts of these compounds present in sweat,” said chemist Jan Bello, formerly of UCR and now at the insect control company Provivi.

In search of mosquito attractants, Cardé turned to Bello, who extracted connections from the sweat in his own feet. He filled his socks with glass beads and walked around with the beads in his socks for four hours per scent collection.

“Wearing the beads almost felt like a massage, like squeezing stress balls full of sand, but with your feet,” Bello said. “The most frustrating thing about doing it for a long time is they get stuck between your toes, so it would get uncomfortable after a while.”

The inconvenience was worth the investment. Bello isolated chemicals from the sweat deposited on the sock beads and observed the mosquitoes’ response to those chemicals. This created the most active combination.

Future studies are planned to determine whether the same compound is effective for other mosquitoes, and why there is such variation in how individuals tend to be bitten. “Some are more attractive than others to these mosquitoes, but no one has yet determined why this is,” Cardé said.

While this discovery may not lead to insights for the development of new repellents, the research team is hopeful that their discovery could be used to attract, trap and potentially kill disease-spreading mosquitoes.

“In the end, we’re very happy to have found these connections, because we weren’t always sure we would. We suspected they existed, but premonitions don’t always work,” Cardé said.

Why are some people mosquito magnets and others not? Entomologist points out metabolism, body odor and mindset

More information:
Scientific Reports (2022). DOI: 10.1038/s41598-022-19254-w

Provided by University of California – Riverside

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