<x-charset iso-8859-1>The following is from SCIENCE---apparently a lot of mosquito related articles in
tomorrow's issue.
> Michael Gochfeld
==============================================================
>
>
> What Mosquitoes Want: Secrets of Host Attraction
> Martin Enserink
>
> Why do mosquitoes feast on some people and leave others alone? Researchers
> are trying to find out, hoping it will help them design the perfect mosquito
> trap
> Life isn't fair. Whereas some people never seem to get bitten by
> mosquitoes--and often don't even seem to notice the critters-- others spend
> their evenings frantically swatting them, usually to no avail. If you're in
> the latter category, you've probably wondered: Why me? Is it thin skin? My
> gorgeous body odor? Sumptuous blood vessels begging to be punctured? Or is
> it all between the ears, as some people say, and you simply fuss and fret
> more about mosquito bites?
>
> Rest assured, it's not your imagination: Several studies have shown that to
> mosquitoes, all people really aren't created equal. Besides factors such as
> heat and carbon dioxide, mosquitoes use odors to find their victims, and
> humans appear to exude different amounts of the volatile compounds the
> insects love.
>
> By studying mosquito behavior, entomologists are trying to tease out these
> favorite smells. It's a complex story, they say. Millions of years of
> evolution have resulted in sophisticated odor-based navigation systems that
> differ greatly from one mosquito species to the next, depending on where it
> lives and which host it prefers. Even so, chemical and behavioral
> studies--often using human volunteers as bait--have helped identify some of
> the smells that tempt several mosquito species. And recently, molecular
> researchers have begun identifying the receptors that pick up these odors
> and translate them into neural signals.
>
> Researchers hope to use odor cues to lure mosquitoes into the perfect trap
> or otherwise outwit them--say, by designing repellents that foul their sense
> of smell. Garden parties and golf getaways might be the first beneficiaries;
> indeed, one U.S. company is already marketing the $500 to $1200 Mosquito
> Magnet, which purportedly attracts mosquitoes by emitting a compound called
> 1-octen-3-ol, as well as heat, CO2, and water vapor.
>
> But the ultimate goal is a far cry from such pricey gadgets, says Willem
> Takken of Wageningen Agricultural University in the Netherlands, a pioneer
> in the field. He'd like a simple, $1 or $2 trap that people in developing
> countries could affix to their doorposts to keep out the mosquitoes that
> spread deadly diseases. Key targets are Anopheles gambiae, the species that
> transmits malaria, and Aedes aegypti, which spreads dengue and yellow
> fever.
> --------------------------------------------------------------------------------
> Footwork. Anopheles gambiae (left) bites mainly on the feet and ankles; An.
> atroparvus prefers the face.
>
> Blood, sweat, and cheese
> For almost a century, researchers have been trying to divert mosquitoes
> from their pursuit of human blood. The field blossomed in the 1950s, when
> dozens of entomologists in several countries set out to discover what
> attracts females--the only mosquitoes that bite--to their hosts. Anthony
> Brown of the University of Western Ontario in London, Canada, for instance,
> built human-shaped steel tanks, which he called robots, dressed them up, and
> then counted the number of mosquitoes that landed on them in a forest. He
> found, among other things, that the robots became more attractive if their
> skin was 37ºC (the temperature of the human body) than at lower
> temperatures, if they exhaled CO2, or if they wore a wet jerkin--or, better
> still, one soaked in human sweat.
>
> By the mid-1960s, most research on host attraction had stopped, in part
> because DDT made mosquito extermination so easy. Lately, however, emerging
> resistance and second thoughts about insecticide use have sparked a renewed
> interest in alternative control methods.
>
> Scouting for potentially attractive compounds, researchers are taking a
> closer look at the more than 300 chemicals present on human skin. Martin
> Geier of the University of Regensburg, Germany, for instance, takes skin
> rubbings and then chemically removes a certain group of compounds--say, the
> ketones or the fatty acids. If one group attracts mosquitoes, it can be
> further separated into its individual components, he says.
>
> To test how compelling single compounds or mixtures are, researchers use a
> specialized instrument called an olfactometer, whose central part is a
> Y-shaped wind tunnel. Two different odors are blown into the short legs of
> the Y; when mosquitoes are set loose at the other end, they fly upwind and,
> like quiz show contestants choosing between two doors, decide whether to go
> left or right. Researchers can also fixate mosquitoes, apply miniature
> electrodes to their nerves, and test whether exposing them to a whiff of
> some compound elicits an electrical signal.
>
> Recent studies have confirmed what Brown and others discovered half a
> century ago: that for most mosquito species, CO2, heat, and moisture are key
> attractants. But these lead a mosquito to any warm-blooded animal--bird,
> cow, or human. That might be fine for species that aren't too picky, such as
> Culex pipiens, a West Nile vector in the United States. But those that dine
> almost exclusively on humans, such as An. gambiae and Ae. aegypti, need much
> more specific attractants.
>
> Hunting for cues, Bart Knols, a researcher in Takken's group, noticed in
> 1995 that An. gambiae had a predilection for biting its victims on the feet
> and ankles--even when their entire bodies were exposed. (This clearly set it
> apart from related species, such as An. atroparvus, a mosquito from Holland
> that goes mainly for the face.) A native of the Dutch province of Limburg,
> Knols also realized that foot odor bears a remarkable resemblance to the
> pungent cheese from that region. And sure enough, An. gambiae turned out to
> be heavily attracted to the smell of Limburger cheese.
>
> The finding, after making snickering headlines around the globe, led
> researchers to tempt different mosquitoes. "It became sort of a madhouse,"
> Knols recalls. "People started taking Limburger cheese all over the world."
> But the stinky dairy product turned out to be an acquired taste, he says;
> just those few mosquitoes that feed primarily on humans were strongly
> attracted.
>
> Knols says the common denominator between feet and cheese is obvious: a
> bacterium used in cheese production, called Brevibacterium linens, which is
> a close relative of Brevibacterium epidermis, a bug known to reside in the
> warm, humid clefts between human toes. Both turn glycerides into a specific
> set of breakdown products, such as fatty acids. Takken's group is now trying
> to find out exactly which products provide the draw.
>
> Over the years, researchers have found that individual species have their
> own idiosyncratic tastes for various attractants. Ae. aegypti find lactic
> acid--which humans produce on their skin but other mammals don't--sublime;
> to An. gambiae, it's only so-so. With ammonia, it's the other way around.
> And even in Aedes, Geier explains, lactic acid alone isn't all that
> attractive; rather, it boosts the appeal of several other compounds.
>
> Complicating matters, explains Ring Cardé of the University of California,
> Riverside, an effective trap depends not just on the right attractants but
> also on the physical properties of the odor plume. Cardé has spent most of
> his career studying how male moths home in on females by navigating
> pheromone plumes--which, from the insect's viewpoint, consist of a series of
> small odor filaments swirling through the air. More recent work in
> mosquitoes, carried out by Cardé's colleague Teunis Dekker, suggests that
> they, too, use the fine structure of an odor plume to navigate, and Cardé
> believes that the shape and structure of a plume will determine any trap's
> efficacy.
>
> Takken and others hope that molecular researchers, who joined the field
> just 2 years ago, will help make sense of it all. They are making some
> headway: In a study published on page 176, a team led by Laurence Zwiebel at
> Vanderbilt University in Nashville, Tennessee, has scoured the newly
> sequenced An. gambiae genome for so-called G protein-coupled receptors,
> which include odor receptors.
>
> The team, working with researchers at the University of Notre Dame in
> Indiana, the University of Illinois, Urbana-Champaign, and Celera Genomics
> in Rockville, Maryland, found 79 odor-receptor candidates, only five of
> which had been known before. Of these, 64 were expressed solely in the
> mosquitoes' olfactory tissues--evidence that they're probably involved in
> odor recognition. And at least one of the candidate receptors is produced
> only in mature females--an important clue that it might be involved in host
> seeking. So far, the group hasn't been able to link any of the odors known
> to attract An. gambiae to any of the receptors. Still, the study is a "nice
> breakthrough," says Takken, that might speed the discovery of other, more
> powerful attractants.
> --------------------------------------------------------------------------------
> Death trap. The Mosquito Magnet's popularity is growing in the United
> States (bottom). Something much simpler and cheaper is needed to divert
> mosquitoes in developing countries.
>
> Building the perfect trap
> Whether chemical lures can be fashioned into an irresistible mosquito trap,
> much less one that would be cheap and effective in developing countries,
> isn't clear. But there is a precedent. In many East African countries,
> simple traps have helped virtually eradicate tsetse flies, the carriers of
> sleeping sickness and a livestock disease called nagana. (One trap consists
> of a simple black-and-blue cloth, baited with acetone and octenol--or,
> alternatively, buffalo urine--and sprayed with insecticide.)
>
> Mosquitoes, however, could pose a more daunting challenge. One tsetse fly
> produces only a handful of offspring over her lifetime, making the
> population vulnerable to even a slight increase in mortality. By contrast,
> mosquito mothers can produce hundreds of young. It might also be "very
> difficult," says Geier, to produce a trap that can compete with the real
> thing: living, breathing humans who emit not just smell but also heat and
> moisture. (A trap could do that too, of course, but it would quickly get too
> complicated and costly.) But even if they only reduced the number of
> mosquitoes, "traps could have a fantastic impact," says Takken. "We all
> agree that no single measure will ever solve the malaria problem
> completely."
>
> Short of that ambitious goal, traps might also be effective in monitoring
> the risk of epidemics and focusing control efforts. Some countries already
> use a relatively unsophisticated trap developed by the U.S. Centers for
> Disease Control and Prevention (CDC) to keep track of pathogens. But this
> trap, which relies on just CO2, light, or a combination, catches a motley
> array of insects--often not those most relevant to human health. To catch
> An. gambiae, says Takken, a human needs to be nearby, and because the
> attractiveness of people varies, so does the nightly catch. Spiking such a
> trap with a specific odor blend could lead to a much better and more
> reproducible haul, he says.
>
> Other mosquito-thwarting strategies on the drawing board are clever new
> repellents. If, for instance, researchers could find compounds that
> overstimulate crucial odor receptors, they might be able to disorient the
> insects, dooming them to a life of aimless buzzing, Zwiebel says. It might
> even be possible, he says, to tinker with the receptors that help mosquitoes
> find nectar or places to lay their eggs.
>
> In the meantime, attraction studies with human volunteers suggest another,
> more down-to-earth approach to keeping mosquitoes at bay. Among his human
> subjects, chemist Ulrich Bernier of the U.S. Department of Agriculture in
> Gainesville, Florida, has found some people who are almost never bitten. His
> team has isolated compounds from their skin--he declines to discuss which
> ones--that he believes might be a clue to the protection. Someday, he
> speculates, they could serve as a natural, less toxic alternative to DEET.
>
> Splashing yourself or your house with somebody else's body odor might not
> sound all that enticing. But at the levels needed to keep bugs away, Bernier
> assures, humans won't smell a thing.
>
>
>
> ------------------------------------------------------------------------
> Name: mosquitoswhat they want.pdf
> mosquitoswhat they want.pdf Type: Portable Document Format (application/pdf)
> Encoding: base64
</x-charset>
Received on Fri Oct 04 11:24:22 2002
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