Environews Innovations

A 468 VOLUME 113 | NUMBER 7 | July 2005 • Environmental Health Perspectives

OutsmartingOlfactionThe Next Generation ofMosquito Repellents

Environmental Health Perspectives • VOLUME 113 | NUMBER 7 | July 2005 A 469

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The world’s most dangerous animal weighs abouttwo milligrams and pursues its human prey atspeeds of barely a mile per hour. Surprised?Don’t be. The dubious honor belongs to the

lowly mosquito—a fragile creature whose bite infectsmillions with lethal diseases, such as malaria, dengue,and West Nile encephalitis. For centuries, humanshave slathered on insect repellents to deter the buzzingmenace—the first recorded repellents were document-ed by Herodotus around 400 B.C. But these productshave always been far from optimal. Even DEET(N,N´-diethyl-n-toluamide), the world’s most popularand efficacious repellent, has numerous shortcomings:it can require frequent applications, it must beapplied to all exposed body parts, and it won’t protectagainst some dangerous mosquito species, includingAnopheles albimanus,the chief malaria vectorin Central America.

Today, the need formore effective repellentsis increasingly urgent,experts say. According tothe World Health Or-ganization, global climatechange is expandingmosquitoes’ range, heightening the risk of disease formillions of additional people. The Centers for DiseaseControl and Prevention notes that dengue and West Nilevirus are both moving from developing countries towardsthe United States, where concerns over mosquito expo-sure are rising. Malaria—which by various estimates killsbetween 1 million and 3 million people worldwide eachyear—is also a growing problem in many regions. Thisis in part because Plasmodium, the mosquito-borne para-site that causes malaria, is fast becoming resistant toexisting treatments, such as chloroquine.

Many experts believe that better repellents could helpto control mosquito-borne diseases. These next-genera-tion repellents must be effective against anopheline mos-quitoes that carry malaria. They should also be cheapand nontoxic, and should last long enough to protecthumans as they sleep, when they are most vulnerable.

Where will these repellents come from? The answers,scientists increasingly say, will be found in genomic

Tuned in to scents. Mosquitoes“smell” with their antennae.

Innovations | Outsmarting Olfaction

A 470 VOLUME 113 | NUMBER 7 | July 2005 • Environmental Health Perspectives

Innovations | Outsmarting Olfaction

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research. According to this view, knowl-edge of the genes and proteins that mos-quitoes rely on to sense their environmentcould lead to new repellents that directlyinterfere with the insects’ ability to detecthuman beings.

A Sense of the FutureToday, genomic information about mos-quitoes is accruing rapidly. Scientists withthe International Anopheles GenomeProject, a consortium based at the PasteurInstitute in Paris, have already decodedthe genome of An. gambiae, the dominantmalaria vector in Africa. Another genome,for Aedes aegypti, a vector for both yellowfever and dengue, is now being sequencedwith funding from the National Instituteof Allergy and Infectious Diseases. Usingthese data sets, scientists are identifyingthe genes that control mosquito sensorysystems, including the olfactory system.

Mosquitoes rely on smell to guidethem towards mates, food, and of course,sources of blood meals. The process ishighly specific. For example, only femalemosquitoes are attracted to blood sources,which they require as nourishment fortheir eggs. (Mosquitoes’ main nutrimentcomes from other sources, mainly plantnectar.) Olfaction is also species-specific.An. gambiae, for instance, prefers to bitehumans. “You could be in a room full ofcows, and that mosquito will find you andbite you,” says Laurence Zwiebel, an asso-ciate professor of biology at VanderbiltUniversity. Because this species has

evolved to target humans, it will stayindoors where it can get to them more eas-ily, he adds.

To better understand the molecularbiology of mosquito olfaction, consider thefollowing scenario: You’re asleep in the sum-mertime, in a warm room with an openwindow. Your body and the bacteria thatreside on your skin are giving off a molecu-lar cloud of human-specific odorants. Thesemolecules drift through the air and soonreach a mosquito perched on the wall. In aninstant, odorant-binding proteins (OBPs)located on the creature’s antennae bind themolecules and transport them towardsreceptors located on the surfaces of theolfactory neurons. The odorant–receptorlinkage activates a cue in the mosquito’s ner-vous system, which alerts the insect to yourpresence. Moments later, the mosquitolands on your unprotected shoulder andbegins to feed.

OBPs are now widely viewed as one ofthe most likely targets for next-generationrepellents. Compounds that interfere withthese proteins could block mosquitoes’ability to smell and thus detect humans.“This really is a unique approach,” saysLeslie Vosshall, head of the Laboratory ofNeurogenetics and Behavior at RockefellerUniversity. “Since we now know quite a bitabout the basic workings of insect olfactorysystems, we can focus our search for newrepellents that interrupt proteins known tobe important for smell. We’re optimisticthat this rational approach to repellentdesign will uncover new compounds that

are safer and more effective than those thatare currently available.”

Targeting Olfactory ProteinsVosshall has found that the function of awide range of odorant receptors in insectsdepends on a single type of protein “core-ceptor” that facilitates the binding of theOBP–odorant complex to the receptor.Working with fruit flies, she has shown thatmutations in this protein, which in fruit fliesis called Or83b, knocks out the insect’s senseof smell altogether. The protein, she says, ishighly conserved across many differentspecies; in mosquitoes, it now goes by thename GPRor7 (until recently, it was knownas AgOR7). In recent studies, Vosshall gener-ated a strain of fruit flies that lacked theOr83b gene and hence the ability to smell.This finding, remarkable in its own right, wasrendered even more so by a subsequent dis-covery: when she replaced the missing Or83bgene with the GPRor7 gene obtained frommosquitoes, the fruit flies’ olfactory systemwas restored to a normal state. This meansGPRor7 substituted for Or83b, even thoughmosquitoes and fruit flies diverged more than250 million years ago.

Based on these findings, Vosshall sug-gests that GPRor7 is a plausible target for amosquito repellent. Knock out this protein,she says, and a mosquito will be unable tosmell anything—humans included. Herfindings are published in the 22 February2005 issue of Current Biology.

Zwiebel and his colleagues are taking adifferent tack. Instead of targeting a single

Reception conservation. Leslie Vosshall and colleagues (left) developed a fruit fly strainlacking the Or83b gene. The mutation means the fruit flies’ odorant receptors (above,stained red) are not properly localized in the sensory hairs, and the flies do not respondwhen exposed to banana odor. When GPRor7, the mosquito equivalent of Or83b, is putinto the mutant fruit flies, the odorant receptors return to the sensory hairs, and the fliesnow respond to banana odor.

Fruit fly with GPRor7 geneFruit fly with mutated Or83b gene

Response to banana odor Response to banana odor

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protein that regulates mosquitoes’ entire senseof smell, he’s homing in on the olfactory pro-teins that bind human odorants only. In the15 January 2004 issue of Nature, Zwiebelpublished results showing that one olfactoryreceptor in An. gambiae—a protein known asOr1—preferentially binds 4-methylphenol,one of roughly 300 compounds found inhuman sweat. Moreover, the Or1 receptorwas found only in female mosquitoes—fur-ther evidence of the intricate specificity in theinsect’s olfactory machinery.

Today, Zwiebel and his colleagues arestriving to identify other human-specificodorants with the goal of creating repellentsthat inactivate groups of these targets simul-taneously. This approach lessens the likeli-hood that mosquitoes will develop resistanceto the repellents, he says. “It’s almost impos-sible for resistance mutations to appear infour to five receptors concurrently, and theywould not make their way into the mosqui-to population,” he explains. “What we reallywant is a toolbox of behaviorally disruptiveolfactory compounds,” he adds. “Thiswould allow us to fine-tune repellent blendsfor specific geographic regions, where mos-quito populations and olfactory mechanismsmight vary.”

The future repellents mark an additionaldeparture because they could be incorporat-ed into time-release systems that put activeingredients into the air. Thus, there wouldbe no need to apply the repellents to thebody; instead, they would protect a livingspace in its entirety. This is important for

those mosquitoes—An. gambiae in particu-lar—that prefer indoor environments andsleeping humans.

Scratching the SurfaceExperts agree that no new repellent willnecessarily be a panacea for mosquito con-trol. Andrew Spielman, a professor of trop-ical public health at the HarvardUniversity School of Public Health andauthor of the book Mosquito: A NaturalHistory of Our Most Persistent and DeadlyFoe, suggests that all repellents pose a fun-damental dilemma: mosquitoes deterredfrom one protected person will simplyflock in greater numbers to another who isunprotected. “What drives the force of dis-ease transmission is the biting rate,” heexplains. “If half the people are being bit-ten by all the mosquitoes, then those peo-ple and any mosquitoes that feed on themwill wind up being terrifically infected.”But Spielman acknowledges that repellentscan be very useful—“I’m just not sure

about who specifically is going to benefitfrom them,” he says.

When posed with this question, Zwiebelresponds that health officials must take stepsto ensure that no one is left unprotected.“That’s a challenge we need to accept,” hesays. “We need to make sure these productsare universally available and economicallyaffordable.”

In the end, new repellents will be just partof a broader strategy to control mosquitoes, astrategy that Spielman says must also incorpo-rate better environmental management, hous-ing improvements, and greater use of insecti-cide-treated bed nets. Meanwhile, it’s hard tosay which type of rationally designed repellentis likely to emerge first. Will it target one pro-tein, as Vosshall suggests it could, or will ittarget several, as Zwiebel says it must? “At theend of the day,” Zwiebel says, “everyone isgoing forward with the best of intentions, andwe just have to see what comes out.”

Charles W. Schmidt

The hair of the mosquito that bit you? The antenna of the female Anopheles mosquito (above rightand inset) bristles with various olfactory sensillae used for prey detection, flight direction, and egglaying. Laurence Zwiebel (above left, with mosquitoes) is targeting human-specific olfactory recep-tors in these mosquitoes.

Sugge s t ed Read ingJones WD, Nguyen TA, Kloss B, Lee KJ, Vosshall LB. 2005. Functional conservation of

an insect odorant receptor gene across 250 million years of evolution. Curr Biol15(4):R119–R121.

Justice RW, Biessmann H, Walter MF, Dimitratos SD, Woods DF. 2003. Genomicsspawns novel approaches to mosquito control. BioEssays 25:1011–1020.

Spielman A, D’Antonio M. 2001. Mosquito: A Natural History of Our Most Persistentand Deadly Foe. New York, NY: Hyperion Books.