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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)
Temperature Fluctuations Affect Population Growth Rate of Dengue Mosquito
The PLOS ONE journal published “Effects of Fluctuating Daily Temperatures at Critical Thermal Extremes on Aedes aegypti Life-History Traits," written by lead author Lauren Carrington and four other scientists from Thomas Scott’s Mosquito Research Laboratory and the Center for Vectorborne Diseases (CVEC).
Their work analyzed how natural temperature fluctuations affect the population growth rate of the dengue mosquito. Basically, temperature effects on larval development time, larval survival and adult reproduction depend on the combination of mean temperature and the magnitude of fluctuations.
“The effect of temperature on insect biology is well understood under constant temperature conditions, but less so under more natural, fluctuation conditions,” said Carrington, who completed her three-year postdoctoral fellowship last December in the Scott lab and continues research projects with the lab. She is now based at the Nossal Institute for Global Health, University of Melbourne, Australia.
The research is expected to lead to greater accuracy of applications for mosquito surveillance and disease prevention.
“An improved understanding of mosquito responses to natural temperature variation,” Carrington said, “will enhance the effectiveness of vector control strategies, thereby reducing transmission of mosquito-borne diseases, such as dengue fever.” By using constant temperatures, scientists can under- or -over estimate values, she said.
“In the field, mosquitoes, and other insects, are exposed to a constantly changing environment, with fluctuations in temperature throughout the day, every day. In the lab, however, experimental protocols generally try to minimize as much variability as possible, and temperature is often the first element to be standardized.”
Co-authors are Veronica Armijos, Christopher Barker, Louis Lambrechts and Thomas Scott.
Dengue is spread by an infected female Aedes aegypti mosquito, 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 and the most severe form of disease is life-threatening dengue hemorrhagic fever or DHF.
Some 500,000 people with severe dengue are hospitalized each year, according to the World Health Organization (WHO), and about 2.5 percent of those affected die.
“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,” said Scott, a professor of entomology at UC Davis and active in CVEC.
Dengue mosquito, Aedes aegypti. (Photo courtesy of Centers for Disease Control and Prevention)
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)
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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