Seventeen-year-old Helena Leal doesn't like mosquitoes, but they like her.
“I always get bitten by mosquitoes more often than my family and friends,” says the Davis High School senior. “I get fairly bad reactions to mosquito bites, including swelling and itching, so I always have to be meticulous in using repellents.”
Thanks to her lifelong interest in those pesky female mosquitoes that target her for a free (blood) meal, Helena Leal now joins the ranks of published scientists. She's the lead author of UC Davis-based research published Dec. 21 in the journal Nature Scientific Reports.
“It was great to have the opportunity to do research addressing a question I've had ever since I was a child,” she said of the work, “Attraction of Culex Mosquitoes to Aldehydes from Human Emanations,” completed in the UC Davis Department of Molecular and Cellular Biology lab of her father, chemical ecologist Walter Leal. In addition to the father-daughter team, co-authors are UC Davis student researchers Justin K. Hwang and Kaiming Tan of the Leal lab.
Aldehydes are organic compounds—many fragrances are aldehydes—while skin emanations are odorants.
“Helena has always been curious about mosquito bites,” said her father, a distinguished professor who unravels the molecular mechanisms that make the olfactory system of insects so sensitive and selective. “Her research suggests that some people attract more mosquitoes—and receive more bites—because of their ratio of aldehydes, not because of natural repellents. We all produce these aldehydes but their ratios differ from person to person. The previous hypothesis is that people attract more mosquitoes because they lack a natural repellent. We found that the natural repellent is not produced in enough amount to repel.”
“Our paper shows that the aldehyde ratio is very important,” Helena said. The researchers studied the southern house mosquito, Culex quinquefasciatus. Female mosquitoes feed on humans and other vertebrates to acquire nutrients to develop their eggs, she noted. Infected mosquitoes can transmit such viruses as West Nile, Zika and the St. Louis encephalitis virus.
“The public has a genuine interest in understanding how mosquitoes find their hosts,” Helena wrote. “A Google search at the time of this writing, for example, produced almost 15 million results for the questions ‘why do mosquitoes find me?' Likewise, the question ‘why do mosquitoes prefer one person and not another?' generated more than 13 million results.”
An article in the July 13, 2013 edition of the Smithsonian magazine related that an estimated 20 percent of people “are especially delicious for mosquitoes and get bit more often on a consistent basis.”
Various factors contribute to differential attraction, including pregnancy, malaria infection, alcohol/beer consumption, skin microbiota, genetic makeup, and even blood type.
Fellow researcher Kaiming Tan said he was "delighted to see Helena able to facilitate her learning in a laboratory setting with a team of university students at UC Davis, a world-class research institution." He praised her interests in asking scientific questions, leading to solutions, as "useful skills in her future education endeavors."
"Dr. Leal's continuing support and mentoring on this student-run research project motivate us to pursue our goals," Tan added. "It is my pleasure to be on this team and pass down my knowledge to the next generation of college students."
Almost a century ago scientists identified carbon dioxide CO2 as a primary mosquito attractant, and it's commonly used to trap bloodseeking female mosquitoes. “However, no evidence is available that suggests that CO2 mediates differential attraction, i.e., carbon dioxide emission levels do not explain the common observation that mosquitoes systematically prefer one person to another,” the research team wrote. They agreed that skin emanations “play a key role in the mosquito finding a host” and that “these emanations are complex in nature and contain hundreds of compounds, but as far as mosquitoes are concerned, a handful of compounds activate the mosquito's olfactory system.”
“Anecdotes related to preferential mosquito bites are very common, but to date there is no complete explanation as to why one out of two people systematically receives more mosquito bites than the other when both are equally accessible,” the researchers wrote in their abstract. They tested the hypothesis that two constituents of skin emanations, 6-methyl-5-heptan-2-one (6MHO) and geranylacetone (GA), are natural repellents and may account for differential attraction in different ratios. “We studied skin emanations from two human subjects, confirmed in behavioral assays that female southern house mosquitoes are significantly more attracted to subject A (attractant) than to subject N (nonattractant), and tested their 6MHO/ GA ratios 2/23 in a dual-choice olfactometer.”
“Although repelling at high doses, 6MHO/GA mixtures were not active at the levels emitted by human skin,” they wrote. “We found, however, differential attraction elicited by the aldehydes in the ratios produced by subjects A and N. When tested in a dose commensurate with the level released from human skin and in the ratio produced by subject A, the aldehyde mixture significantly attracted mosquitoes. By contrast, an aldehyde mixture at the same ratio released by subject N did not attract mosquitoes. We, therefore, hypothesized that aldehydes may play a role in the commonly observed differential attraction.”
Helena worked on the project during the summer and into the fall. The Davis High School scholar recently received a scholarship from the Mexican-American Yolo County Concilio at its recognition dinner and scholarship fundraiser. She told the crowd that she plans to major in cognitive science with a focus on artificial intelligence.
Her other interests? "I play lacrosse and tennis on my school teams, but I also love to dance. I currently spend a lot of time working on the school yearbook as an editor. I am a big fan of live music and I am learning how to play the guitar."
Her brother, Gabriel, now 20, also did research in the Leal lab and saw his work on the odorant-binding protein of the yellow mosquito published. In addition, Gabriel worked on almond research (unpublished). Now a student at Dartmouth, Gabriel is majoring in economics and minoring in engineering and philosophy. Older brother Augusto, 26, holds a degree in political sciences from Princeton University with a minor in economics.
The Leal lab is “particularly interested in elucidating the roles of odorant-binding proteins, odorant receptors, and odorant-degrading enzymes,” said Leal, adding that “Insects are extremely successful animals whose lives intertwine with ours. They may be vegetarian and harmful to our food supply as well as vectors of pathogens that inflict tremendous suffering and human loses. Thus, a better understanding of the molecular basis of insect olfaction may facilitate the design of eco-friendly chemicals to controls insects of medical and agricultural importance.”
The Leal lab is credited with discovering the exact mode of action of DEET, “the gold standard” of insect repellents for the past six decades. “DEET doesn't mask the smell of the host or jam the insect's senses,” Leal said. “Mosquitoes don't like it because it smells bad to them. That's why they avoid it.” The also discovered the DEET receptor, which is also sensitive to plant defense compounds.
“It's interesting what will repel and what will attract mosquitoes,” said Helena Leal. “It appears that mosquitoes will always find me--and 20 percent of the rest of the population—as especially delicious. They're out for blood—our blood. Let's keep them at bay with repellents.”
(Editor's Note: Walter Leal is a fellow of the Entomological Society of America and former chair of the UC Davis Department of Entomology and Nematology.)
Chemical ecologist and mosquito researcher Walter Leal, distinguished professor in the UC Davis Department of Molecular and Cellular Biology, and his lab collaborated with scientists in Recife to ask “Does Zika Virus Infection Affect Mosquito Response to Repellents?”
The work, funded by the National Institutes of Health, was published Feb. 16 in Scientific Reports of the journal Nature. The researchers used mosquitoes originating from colonies reared by UC Davis medical entomologist Anthony Cornel and from colonies in Recife.
“We used assays mimicking the human arm to test the mosquitoes infected with the Zika virus,” Leal said, “and we asked whether the Zika infection affects mosquito response to repellents.” They tested DEET and Picaridin, considered the top two mosquito repellents. “We discovered that DEET works better than Picaridin against the southern house mosquito, Culex quinquefasciatus, and the yellow fever mosquito, Aedes aegypti, whether infected or not.”
The researchers also found that old mosquitoes that already had a blood meal “were less sensitive to repellents,” said Leal, adding “It was not clear whether this was due to the virus, but mostly likely because of age. ”
“Lower doses--normally used in commercial products--work well for young mosquitoes,” Leal said, “but the old ones are the dangerous ones because they may have had a blood meal infected with virus and there was enough time for the virus to replicate in the mosquito body.”
“The bottom line: to prevent bites of infected mosquitoes, higher doses of repellent are needed. The data suggest that 30 percent DEET should be used. Lower doses may repel nuisance young mosquitoes, but not the dangerous, infected, old females.”
Leal, a native of Brazil, collaborates with Rosangela Barbosa and Constancia Ayres of the Oswaldo Cruz Foundation (FIOCRUZ-PE), Recife, Brazil. The work with infected mosquitoes was conducted at FIOCRUZ-PE.
Leal, Barbosa and Ayres co-authored the paper with Fangfang Zeng and Kaiming Tan, both of the Leal lab; and Rosângela M. R. Barbosa, Gabriel B. Faierstein, Marcelo H. S. Paiva, Duschinka R. D. Guedes, and Mônica M. Crespo, all of Brazil.
The World Health Organization (WHO) recommends that people traveling to or living in areas with Zika virus (ZIKV) outbreaks or epidemics adopt preventive measures, including the use of insect repellents, to reduce or eliminate mosquito bites. Prior to the Feb. 16 published research, it was not known whether the most widely repellents are effective against ZIKV-infected mosquitoes, “in part because of the ethical concerns related to exposing a human subject's arm to infected mosquitoes in the standard arm-in-cage assay,” the researchers pointed out.
They used a previously developed, human subject-free behavioral assay, which mimics a human subject to evaluate the top two recommended insect repellents.
Scientists isolated the Zika virus (ZIKV) nearly seven decades ago from a sentinel rhesus monkey while they were trying to unravel the cycle of sylvan yellow fever virus in Uganda..
According to the Centers for Disease Control and Prevention:
- Zika is spread mostly by the bite of an infected Aedes species mosquito (Ae. aegypti and Ae. albopictus). These mosquitoes bite during the day and night. The virus can also be sexually transmitted.
- Zika can be passed from a pregnant woman to her fetus. Infection during pregnancy can cause certain birth defects.
- There is no vaccine or medicine for Zika.
- Local mosquito-borne Zika virus transmission has been reported in the continental United States.
In a groundbreaking discovery, a scientific team of Brazilians and Brazilian-born chemical ecologist Walter Leal of the University of California, Davis, has announced that the Zika virus has been detected in wild-caught Culex quinquefasciatus mosquitoes in Recife, the epicenter of the Zika epidemic.
Scientists from the Fiocruz Institute, Pernambuco, confirmed the discovery July 21. The detection could have widespread repercussions, as the Culex mosquitoes are more common and widespread than the yellow fever mosquito, Aedes aegypti, known as the primary carrier of the Zika virus.
Leal, who collaborates with Fiocruz Institute researcher Constancia Ayres in a National Institutes of Health-sponsored project on the investigation of Zika in the C. quinquefasciatus, said that the Brazilian lab earlier discovered that Culex had the capability of transmitting the virus. Although the scientists were able to infect the lab mosquitoes with the virus, they had not found the virus in wild-caught mosquitoes—until now.
“This could have major repercussions here in the United States and in other parts of the world,” said Leal, a distinguished professor in the UC Davis Department of Molecular and Cellular Biology who is co-chairing the International Congress of Entomology meeting Sept. 25-30 in Orlando, Fla. The conference is expected to draw some 7000 entomologists throughout the world.
Leal said more work needs to be done to see if Culex mosquitoes are playing a role in the current epidemic. In an interview July 21 with health reporter Jennifer Yang of the Toronto Star, Canada's largest daily, he commented: “It looks like there were more vectors than we thought, and this is one of them. We don't have to panic, but we have to know. And now that we know, we have to take care of the Culex.”
A. aegypti is already established in California; it has spread to at least seven counties since its discovery in Clovis, Fresno County, in June 2013, according to medical entomologist Anthony Cornel of the UC Davis Department of Entomology and Nematology and the UC Kearney Agricultural Research and Extension Center, Parlier.
The Zika virus, which can result in birth defects in pregnancy, can be transmitted through exposure to infected blood or sexual contact. The Centers for Disease Control estimates that between 400,000 and 1.3 million cases have been discovered across South, Central, and North America, where the disease was previously unknown.
Leal and a group of 18 students just hosted a Zika Public Awareness Symposium on May 26 at Giedt Hall, UC Davis campus. The podcast can be accessed at https://video.ucdavis.edu/media/Zika+Virus+Public+Awareness+Symposium/0_n3aupf5c
That's what UC San Francisco medical student Joshua Lang wrote in his piece, With Summer Coming, Can the Zika Virus Be Contained?, published April 14 in The New Yorker.
Meanwhile, at UC Davis, plans are underway for a public "Zika Public Awareness Symposium," set Thursday, May 26 from 6:30 to 8:30 p.m. in Room 1001 of Giedt Hall.
"It is very important that students and the public-at-large learn how to prevent a possible Zika epidemic as this is the first virus known to be transmitted both sexually and by mosquitoes," said coordinator Walter Leal, a chemical ecologist and professor in the UC Davis Department of Molecular and Cellular Biology. Leal, who collaborates with fellow mosquito researchers in his native Brazil, is the co-chair of the International Congress of Entomology (ICE) meeting, to take place Sept. 25-30, 2016 in Orlando, Fla.
"This symposium is important to host because Zika is a rapidly spreading across the globe so people need to be informed," said Amarita Singh, one of the 18 students organizing the symposium. "Recent studies have shown that this particular virus not only affects pregnant women, but is a threat to all. Since there is no vaccine or medication, it is better to educate now to help prevention."
"I became more aware of the disease when Professor Leal began to discuss it in our biochemistry class, which is what originally sparked my interest," Singh said. "After learning about how dangerous the disease was and the discovery of the first case in Yolo County, I decided to take the opportunity in organizing the symposium."
Singh added: "It is incredible how much research has been done on Zika virus in the short amount of time. I am hopeful that in the near future that a vaccine will be developed, but until then we should do everything in our power to prevent this horrible virus from spreading. My concerns are that people may not be well informed which allows the virus to spread even more rapidly."
Said James Warwick, also one of the student organizers: "I think one of the scariest things about Zika in the United States is the lack of public knowledge about it. The scientific community needs to research the virus and develop a vaccine, but can't without funding. And without public pressure, securing funding is going to be slow. Also, the sheer suddenness in which Zika has burst onto the global scene has left both scientific research and public knowledge behind. That is why we are hosting the symposium, to bring the public up to speed, update them on the current research, and to give them the knowledge they need to protect themselves against transmission. As a byproduct, public awareness will increase public pressure to get the world on the right track to stop the spread of Zika."
"The virus itself is extremely alarming," Warwick said. "It can cause extreme neurological and immunological defects, as we see in babies born to infected mothers. But the direct hosts' symptoms usually consist only of temporary pain, rashes, fever, and the like; and only one in four or so people infected with Zika become symptomatic. So there is a very real possibility that a person could be infected, not be aware of it, and potentially pass on the undetected infection to sexual partners or their children. This is made all the more plausible by the fact that many people have never heard of Zika."
A flier (below) distributed at the campuswide UC Davis Picnic Day noted that the "Zika virus is a risk to all of us, not just to pregnant women." At the May 26th symposium, folks will "learn the symptoms and learn the facts and science about Zika and how you can protect yourself from this disease."
The scientific-based symposium will include "expert panels and speakers throughout the United States and the world, including those working on the front lines of the Zika epidemic." Speakers will be announced soon.
Attendance to the symposium is free, but due to limited space, those planning to attend are asked to RSVP to firstname.lastname@example.org
However, not everyone wants to use DEET, a synthetic insect repellent. There's that smell, for one thing. "Properties that people do not like in addition to the smell is that DEET is a solvent for plastic," says chemical ecoloigst Walter Leal of the University of California, Davis. "So, one gets eyeglass frames and watchbands dissolved by DEET."
There's also "the misconception that everything synthetic is bad."
So what is it with DEET that repels mosquitoes? What odorant receptor is involved? Mosquitoes, as we know, detect smells with their antennae.
The Leal lab today (Oct. 27) published research in the Proceedings of the National Academy of Sciences (PNAS) that pinpoints the exact odorant receptor that repels them. They also identified a plant defensive compound that might mimic DEET, a discovery that could pave the way for better and more affordable insect repellents.
For more than six decades, DEET has been known as the gold standard of insect repellents. More than 200 million people worldwide use the chemical insect repellent, developed by scientists at the U.S. Department of Agriculture and patented by the U.S. Army in 1946.
So when Leal and his team--project scientist Pingxi Xu, postdoctoral scholar Young-Moo Choo, and agricultural and environmental chemistry graduate student Alyssa De La Rosa-- published their groundbreaking research, “Mosquito Odorant Receptor for DEET and Methyl Jasmonate,” they drew global attention.
In their research, they examined the receptors of the southern house mosquito, Culex quinquefasciatus, which transmits such diseases as West Nile virus.
The researchers set out to investigate two hypotheses regarding DEET's mode of action: activation of ionotropic receptor IR40a vs. odorant receptor(s). “Ionotropic receptor is another family of olfactory receptors, which seem to be the ancestral version when insects were aquatic,” Leal said. “So, the ionotropic receptors normally detect acid, bases, and other water soluble compounds.”
“Vector-borne diseases are major health problems for travelers and populations living in endemic regions,” said Leal. “Among the most notorious vectors are mosquitoes that unwittingly transmit the protozoan parasites causing malaria and viruses that cause infections, such as dengue, yellow fever, chikungunya, and encephalitis.”
Leal said that diseases transmitted by mosquitoes destroy more lives annually “than war, terrorism, gun violence, and other human maladies combined. Every year, malaria decimates countless lives – imagine a city of San Francisco perishing to malaria year after year. The suffering and economic consequences in endemic areas are beyond imagination for those living in malaria-free countries. Both natives and visitors to endemic areas want to keep these ‘infected needles' at bay. In the absence of vaccines for malaria, dengue, and encephalitis, one of the most ancient and effective prophylactic measures against mosquito-borne diseases is the use of DEET.”
Dan Strickman of the Bill and Melinda Gates Foundation, not involved in the study, praised the work. “We are at a very exciting time for research on insect repellents,” said Strickman, senior program officer of the Global Health Program's Vector Control. “ For decades, the field concentrated on screening compounds for activity, with little or no understanding of how chemicals interacted with mosquitoes to discourage biting. Use of modern techniques that combine molecular biology, biochemistry, and physiology has generated evidence on how mosquitoes perceive odors.”
Said zoologist Paul Weldon of the Smithsonian's Conservation Biologist Institute, also not involved in the study: “Since DEET is strictly synthetic and not a natural product, it has been challenging to understand the adaptive nature of the response it elicits. It is not as if the compound emanates from, say, spider webs or fishy water, where avoidance by mosquitoes would make sense. Xu et al. have solved the mystery of where the DEET response comes from: it is in response to plant chemical defenses.”
“This, by the way, also explains why the DEET response is widespread, occurring in many arthropods, including those that are not ectoparasitic -- like cockroaches,” Weldon said. The repellence of other arthropods by DEET may have tipped off some of those investigating the DEET response, but I'm not sure that it did. The focus of research on DEET seems to have been with the organisms in which it just so happened to be discovered -- mosquitoes. The Xu et al. study suggests that there is a much broader array of DEET-sensitive organisms than previously suspected. No doubt, this finding will assist further investigations of it.”
Professor John Pickett, Rothamsted Research, UK, also not involved in the study, called the link between the plant compound and synthetic insect repellent, DEET as a “surprising evolutionary link.”
Pickett, the Michael Elliott Distinguished Research Fellow and Scientific Leader of Chemical Ecology at Rothamsted Research and a foreign associate of the National Academy of Sciences, said: “Not only does this work demonstrate that a mosquito response to the gold standard repellent DEET, as well as the more recently developed repellents, is mediated by a specific odorant receptor (OR136 for the southern house mosquito Culex quinquefasciatus) but that the receptor responds specifically also to methyl jasmonate, involved in plant hormone-based defense against insects, which suggests a surprising evolutionary link between these types of insect interactions.”
The UC Davis researchers pointed out that “insect repellents have been used since ancient times as prophylactic agents against diseases transmitted by mosquitoes and other arthropods, including malaria, dengue fever, and encephalitis. They were developed from plant-based smoke or extracts (essential oils) into formulations with a single active ingredient.”
Progress toward development of better and more affordable repellents has been slow, they said, because scientists weren't sure which odorant receptor was involved. Now they are.
Mosquito researcher Anthony "Anton" Cornel, associate professor with the UC Davis Department of Entomology and Nematology, and based at the UC Kearney Agricultural Research and Extension Center, Parlier, provided the mosquitoes that allowed the Leal lab to duplicate his mosquito colony at UC Davis.
Look for more exciting research to come!