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Global Burden of Dengue
The mosquito-borne viral disease known as “breakbone fever,” is three times more prevalent than originally thought, according to a research paper published today in Nature and co-authored by dengue expert Thomas Scott of UC Davis.
In their research paper, titled “The Global Distribution and Burden of Dengue,” Scott and the 17 other team members estimated that 350 million people are infected each year--more than triple the World Health Organization’s current estimate of 50 to 100 million.
Professor Simon Hay of the University of Oxford led the research as part of the International Research Consortium on Dengue Risk Assessment, Management and Surveillance.
“Dengue takes an enormous toll on human health worldwide, with as many as 4 billion people at risk," said Scott, a UC Davis professor of entomology and worldwide expert on the epidemiology and prevention of dengue. He chairs the mosquito-borne disease modelling group in the Research and Policy for Infectious Disease Dynamics (RAPIDD) program of the Science and Technology Directory, Department of Homeland Security, Fogarty International Center, National Institutes of Health.
“The results of our study and infrastructure that created the dengue maps fill a critical gap in the battle against dengue,” said Scott, who maintains field research programs in Iquitos, Peru, and Khamphaeng Phet, Thailand. “Prior to this, without rigorously derived dengue estimates that can be continuously updated, it was not possible to know with confidence where and when to direct interventions for greatest potential impact or to objectively assess the effectiveness of regional and global control efforts. That kind of knowledge was among the most important missing information for developing enhanced dengue prevention programs.”
The highly infectious tropical and subtropical disease is spread by the bite of an infected female Aedes aegypti, a day-biting, limited flight-range mosquito that prefers human blood to develop its eggs. Dengue is caused by four distinct, but closely related, viruses. The most severe form of disease is life-threatening dengue hemorrhagic fever or DHF.
The researchers assembled known records of dengue occurrence worldwide and used a formal modelling framework to map the global distribution of dengue risk. They then paired the resulting risk map with detailed longitudinal information from dengue cohort studies and population surfaces to infer the public health burden of dengue in 2010.
“There are currently no licensed vaccines or specific therapeutics, and substantial vector control efforts have not stopped its rapid emergence and global spread,” the researchers wrote.
Dengue has now begun to appear along the southern border of the United States, including Texas. Florida has also reported cases of dengue.
Of the 96 million clinically apparent dengue infections, Asia bears 70 percent of the burden, the research paper revealed. India alone accounts for around one-third of all infections.
Professor Thomas Scott, a worldwide expert on dengue, is pictured in Kenya.
Global dengue risk. Areas in red indicate high risk for dengue occurrence while green areas indicate low risk. (Map courtesy of Jane Messina)
Fighting Dengue
And compelling.
In an article published this week in the Proceedings of the National Academy of Sciences (PNAS), the UC Davis medical entomologists and their colleagues found that human movement—people going from house-to-house to visit their friends and relatives—is a key component to driving the virus transmission. (Read PNAS paper)
The research site is Iquitos, nestled in the heart of the Amazon rain forest of northeastern Peru. It's considered one of the world’s primary “open laboratories” to study the transmission of the virus.
The Aedes aegypti mosquito is a day-biting mosquito and we humans are its favorite host/target.
The ground-breaking research shows why it's crucial to focus on people movement, not just on the traditional mosquito control-and-prevention methods, such as applying insecticides and eliminating water-filled containers that can provide a larval habitat.
As lead author/medical entomologist Steve Stoddard said: "This finding has important implications for dengue prevention, challenging the appropriateness of current approaches to vector control."
“Interestingly, it didn’t matter how far away the visited houses were," Stoddard said. "The mosquito that transmits dengue virus prefers to stay in small areas, say in less than a 100-meter radius, but the distance between houses was often much greater than this. So it only makes sense that humans are frequently spreading the virus around as they commute between their homes and the homes of their friends and family. Altogether the data demonstrate what we expected, that human movements are really key to the transmission of this mosquito-borne virus.”
Said Scott, professor of entomology at UC Davis and director of the Mosquito Research Laboratory: “Dengue takes an enormous toll on human health worldwide, with as many as 4 billion people at risk—half of the world’s population--and 400 million new infections each year. The results from our study are focusing attention to the role human social networks in virus invasion and epidemic spread.
"At our Peru study area, we found that infection risk is based on the places a person visits and transmission dynamics are driven by overlapping movements of people who recently visited the same places, like the homes of their family and friends.”
Bottom line: The scientists found that people movement not only defined individual infection risk and local patterns of incidence, but resulted in the rapid spread of the virus and marked heterogeneity in transmission rates.
Next phase of the research? It's aimed at "understanding how variation in human behavior influences transmission and applying that knowledge in enhanced disease prevention strategies,” said Scott, the principal investigator of a National Institutes of Health (NIH)-funded grant.
With some 4 billion people worldwide at risk, and with 400,000 million new infections each year, dengue is indeed taking its toll. Every year some 500,000 people with severe dengue are hospitalized, and 2.5 percent die.
Aedes aeypti, also known as the dengue mosquito. (CDC Photo)
Street scene in Iquitos, Peru. (Photo courtesy of the Thomas Scott lab, UC Davis)
An Important Message
It's Halloween tomorrow (Wednesday) but what's really frightening is Aedes aegypti, a mosquito that transmits the deadly dengue. According to the World Health Organization (WHO), dengue is the world's most rapidly spreading mosquito-transmitted disease.
Some 2.5 billion people, or about 40 percent of the global population, are at risk from dengue, WHO says. The disease infects between 50 to 100 million people a year. The most severe form afflicts some 500,000 a year, killing an estimated 2.5 percent or 22,000.
Enter Sarjeet Gill, professor of cell biology and entomology at UC Riverside. He'll speak on on "Bacterial Toxins in Disease Mosquito Vector Control" at a seminar from 12:10 to 1 p.m., Wednesday, Oct. 31 in Room 1022 of the Life Sciences Building, UC Davis.
His longtime colleague and good friend, Bruce Hammock, distinguished professor of entomology at UC Davis, will host him as part of the UC Davis Department of Entomology's fall seminar series.
"Aedes aegypti is an important vector of human diseases, such as dengue fever and yellow fever," Professor Gill says. "Its control has been attempted by eliminating breeding sites, using predators and with chemical insecticides. However, such control is still difficult because of operational limitations and the development of insect resistance. Therefore, Bacillus thuringiensis has been used for decades instead of physical and chemical control methods. B. thuringiensis israelensis is highly active against Aedes aegypti."
"The high insecticidal activity and the low toxicity to other organisms," Gill says, "have resulted in the rapid use of B. thuringiensis as an alternative for the control of mosquito populations. B. thuringiensis israelensis produces a variety of toxins that act synergistically to cause toxicity to larval populations."
Gill says his seminar "will discuss our current understanding of the mode of action of these toxins and provide evidence on how resistance to these toxins has not occurred in Aedes mosquitoes in the field even though B. thuringiensis israelensis has been used for more than three decades."
Gill’s laboratory focuses on two principal research activities. "The first area attempts to elucidate the mode of action of insecticidal toxins from the Gram positive bacteria Bacillus thuringiensis and Clostridium bifermantans," he says. "This research aims to identify novel toxins, and to gain a molecular understanding of how these toxins interact with cellular targets and thereby causing toxicity. The second area focuses on understanding mosquito midgut and Malpighian tubules function, in particular ion and nutrient transport, and changes that occur following a blood meal."
Gill, who received his doctorate from UC Berkeley, joined the UC Riverside Department of Entomology faculty in 1983. He helped establish the Department of Cell Biology and Neuroscience and also served as chair. Currently he is the co-editor of the journal Insect Biochemistry and Molecular Biology.
A noted scientist and a fellow of the American Association for the Advancement of Science, Gill received his doctorate in insecticide toxicology in 1973 from UC Berkeley. See his website.
If you miss his seminar, not to worry. It's scheduled to be recorded and then posted at a later date on UCTV. (See the index of previous Department of Entomology seminars posted on UCTV.)
Aedes aegypti transmits the deadly dengue. (Photo by James Gathany, Centers for Disease Control and Prevention)
Professor Sarjeet Gill at a malaria conference at UC Davis. (Photo by Kathy Keatley Garvey)
Zeroing in on Dengue
So are researchers from the Thomas Scott lab at UC Davis.
Scott, a medical entomologist who directs the state-funded UC Mosquito Research Laboratory, and his field director Amy Morrison, based in Iquitos, Peru, know their foe well.
Their goal: to save lives through research, surveillance and implementation of disease prevention strategies.
Morrison talked about the research efforts today on National Public Radio (NPR).
Morrison told Charles: ""What's fascinating to me about aegypti is it's probably the mosquito that's most closely associated with human beings, and the most adapted to human beings."
The tiger-striped mosquito, is a daybiting mosquito that prefers human blood. Some 2.5 to 3 billion people, primarily in tropical and sub-tropical countries around the world, are at risk for dengue, which Scott describes as "the world's worst insect-transmitted disease." See feature on him on the UC Davis Department of Entomology website, with links to significant research work.
Aedes aegypti is out for blood. And so are the UC Davis-based researchers tracking it.
Aedes aegypti transmits dengue. (Photo courtesy of James Gathany, Centers for Disease Control and Prevention).
Trying to Control Dengue
It's not just high temperatures that play a significant role in the transmission of dengue, a deadly mosquito-borne disease that strikes millions of people in tropical and subtropical countries.
The size and pattern of fluctuations in daily temperature have a large effect on pathogens transmitted by mosquitoes, according to groundbreaking research published today in the Proceedings of the National Academy of Sciences.
The research, led by French, Thailand and U.S. scientists and conceived by medical entomologist Thomas Scott (right) of the UC Davis Department of Entomology, targets the transmission of spread of dengue.
Lead author Louis Lambrechts (left) of the Institut Pasteur, France, who did postdoctoral research in the Scott lab, points out that the influence of average temperatures on dengue virus transmission has long been known–the higher the temperature, the more efficient the virus transmission—but this is the first study linking temperature fluctuations to the transmission of the disease.
Dengue, transmitted by the daybiting Aedes aegypti mosquito, globally infects 50 to 100 million people yearly. At risk are some 2.5 to 3 billion people, primarily in tropical and sub-tropical countries. The most severe form of the disease, dengue haemorrhagic fever (DHF), strikes half a million a year and kills an estimated 5 percent, according to the Centers for Disease Control and Prevention.
Scott said the study helps to explain a long-standing enigma: “What are the underlying causes of seasonal fluctuations in dengue incidence?” Experiments showed that mosquitoes die faster and are less susceptible to virus infection under large temperature swings, which is typical of the low dengue season, than under moderate temperature variation, which is typical of the high dengue season.
Scott, a noted dengue expert whose goal is to save lives through research, surveillance and implementation of disease prevention strategies, has a longstanding interest in the factors that drive seasonal and annual fluctuations in diseases caused by mosquito-transmitted pathogens. “Traditional explanations for the seasonal increase in dengue are not consistent with my experience in Thailand, Peru, and Puerto Rico,” Scott said.
For more about the research, see the UC Davis Department of Entomology web page or access the PNAS paper.
The dengue mosquito, Aedes aegypti. (Photo courtesy of James Gathany, Centers for Disease Control and Prevention).
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