How
Mosquitoes Are Drawn to Human Skin and Breath
Female mosquitoes, which can transmit deadly diseases like
malaria, dengue fever, West Nile virus and filariasis, are attracted to us by
smelling the carbon dioxide we exhale, being capable of tracking us down even
from a distance. But once they get close to us, they often steer away toward
exposed areas such as ankles and feet, being drawn there by skin odors.
Why does the mosquito change its track and fly towards skin? How
does it detect our skin? What are the odors from skin that it detects? And can
we block the mosquito skin odor sensors and reduce attractiveness?
"It was a real surprise when we found that the mosquito's CO2receptor
neuron, designated cpA, is an extremely sensitive detector of several skin
odorants as well, and is, in fact, far more sensitive to some of these odor
molecules as compared to CO2," said Anandasankar Ray, an
associate professor in the Department of Entomology and the project's principal
investigator. "For many years we had primarily focused on the complex
antennae of mosquitoes for our search for human-skin odor receptors, and
ignored the simpler maxillary palp organs."
Until now, which mosquito olfactory neurons were required for
attraction to skin odor remained a mystery. The new finding -- that the CO2-sensitive
olfactory neuron is also a sensitive detector of human skin -- is critical not
only for understanding the basis of the mosquito's host attraction and host
preference, but also because it identifies this dual receptor of CO2 and
skin-odorants as a key target that could be useful to disrupt host-seeking behavior
and thus aid in the control of disease transmission.
To test whether cpA activation by human odor is important for
attraction, the researchers devised a novel chemical-based strategy to shut
down the activity of cpA in Aedes aegypti, the dengue-spreading
mosquito. They then tested the mosquito's behavior on human foot odor --
specifically, on a dish of foot odor-laden beads placed in an experimental wind
tunnel -- and found the mosquito's attraction to the odor was greatly reduced.
Next, using a chemical computational method they developed, the
researchers screened nearly half a million compounds and identified thousands
of predicted ligands. They then short-listed 138 compounds based on desirable
characteristics such as smell, safety, cost and whether these occurred
naturally.
Several compounds either inhibited or activated cpA neurons of which
nearly 85 percent were already approved for use as flavor, fragrance or
cosmetic agents. Better still, several were pleasant-smelling, such as minty,
raspberry, chocolate, etc., increasing their value for practical use in
mosquito control.
Confident that they were on the right track, the researchers then
zeroed in on two compounds: ethyl pyruvate, a fruity-scented cpA inhibitor
approved as a flavor agent in food; and cyclopentanone, a minty-smelling cpA
activator approved as a flavor and fragrance agent.
By inhibiting the cpA
neuron, ethyl pyruvate was found in their experiments to substantially reduce
the mosquito's attraction towards a human arm. By activating the cpA neuron,
cyclopentanone served as a powerful lure, like CO2, attracting
mosquitoes to a trap.
"Such compounds can play a significant role in the control of
mosquito-borne diseases and open up very realistic possibilities of developing
ways to use simple, natural, affordable and pleasant odors to prevent
mosquitoes from finding humans," Ray said. "Odors that block this
dual-receptor for CO2 and skin odor can be used as a way to
mask us from mosquitoes.
On the other hand, odors that can act as attractants
can be used to lure mosquitoes away from us into traps. These potentially
affordable 'mask' and 'pull' strategies could be used in a complementary
manner, offering an ideal solution and much needed relief to people in Africa,
Asia and South America -- indeed wherever mosquito-borne diseases are endemic.
Further, these compounds could be developed into products that protect not just
one individual at a time but larger areas, and need not have to be directly
applied on the skin."
Currently, CO2 is the primary lure in mosquito
traps. Generating CO2 requires burning fuel, evaporating dry
ice, releasing compressed gas or fermentation of sugar -- all of which is
expensive, cumbersome, and impractical for use in developing countries.
Compounds identified in this study, like cyclopentanone, offer a safe,
affordable and convenient alternative that can finally work with surveillance
and control traps.
Ray was joined in the study by the three UCR co-first authors
Genevieve M. Tauxe, Dyan MacWilliam and Sean Michael Boyle; and Tom Guda. Boyle
is now a postdoctoral researcher at Stanford University.
The team tested the efficacy of ethyl pyruvate in the lab on Aedes
aegypti using an arm-in-cage set-up (the experimenter's hand was
gloved and not exposed to mosquito bites or the test chemicals). The
researchers tested the efficacy of cyclopentanone as a lure on C.
quinquefasciatus, the mosquito that spreads West Nile virus and filariasis,
using traps in a modified greenhouse at UC Riverside.
Funding for the research was provided by the National Institute of
Allergy and Infectious Diseases (grants RO1A1087785 and R56A1099778), the
National Science Foundation, the University of California Global Health
Institute and the Bill and Melinda Gates Foundation.
The UCR Office of Technology Commercialization has filed patents
for inventions reported in the research paper. Some pending patents have been
licensed to Olfactor Laboratories Inc. to pursue further development and
commercialization.
Story Source:
The above story is based on materials provided byUniversity of California - Riverside.
The original article was written by Iqbal Pittalwala.
Note: Materials may be edited for content and length. For further
information, please contact the source cited above.
Journal Reference:
Genevieve M. Tauxe, Dyan MacWilliam, Sean Michael Boyle,
Tom Guda, Anandasankar Ray. Targeting a Dual Detector of Skin and CO2 to
Modify Mosquito Host Seeking. Cell, 2013; 155 (6): 1365 DOI: 10.1016/j.cell.2013.11.013
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University of California - Riverside (2013, December 5). How
mosquitoes are drawn to human skin and breath. Science
Daily. Retrieved December 6, 2013, from
http://www.sciencedaily.com/releases/2013/12/131205141852.htm