High temperatures are the driving factor behind massive dengue outbreaks in Southeast Asia, researchers have found.
Though the infection waxes and wanes among many variables like human movement, rainfall, mosquito vector abundance and host immunity, heat emerged as the standout factor in large dengue epidemics.
The World Health Organization reports that dengue infects 100 million people each year and infection rates have increased thirtyfold over the past 50 years (ClimateWire, Nov. 13, 2013). Also known as breakbone fever, the illness presents with painful joints, rashes and fevers, though some infected people don’t show any symptoms at all, making it frustratingly difficult to figure out how many people carry the virus.
Fighting dengue is crucial for global economic development, since it extracts a huge resource cost in afflicted regions that are home to half of the world’s population. And with the climate changing, health officials are concerned about naive populations encountering the disease as the mosquitoes that carry the virus move to new areas.
To get a handle on dengue, an international team of researchers looked at 18 years of data, including more than 3.5 million reported dengue cases across Cambodia, Thailand, Laos, the Philippines, Vietnam, Malaysia, Singapore and Taiwan.
The team published their results yesterday in the Proceedings of the National Academy of Sciences.
"The most important scientific finding is the temperature really seems to drive these large dengue epidemics," said Wilbert van Panhuis, an associate professor of epidemiology at the University of Pittsburgh’s school of public health and the lead author of the report. "We didn’t expect to see such a clear-cut pattern between the association of dengue and temperature."
He explained that dengue predictably rises and falls with the rainy season in ordinary years, but large epidemics erupt when there are periods of prolonged high temperatures. The links were particularly strong during El Niño phases of global climate, where tropical Pacific waters warm up, altering weather patterns around the world.
Big cities key in slowing major outbreaks
During the El Niño years of 1997 and 1998, which resulted in record heat waves, dengue transmission closely tracked warming patterns.
By examining health data from the eight countries in the study at the same time, researchers also uncovered the phenomenon of "traveling waves" in outbreaks. During an epidemic, infections peak in large urban centers across the region at the same time, followed by peaks in the suburbs and then rippling out to rural areas.
The timing between infection peaks in different regions tends to stay constant, which means detecting dengue early in big cities could help health officials anticipate and respond to outbreaks.
The likely mechanism behind this seems to be that people congregate in cities and then travel away with an unwanted souvenir. Some researchers are developing ways to figure out how people move in afflicted regions, using cellphone data to see if they can predict the next dengue hot spot (ClimateWire, Sept. 9).
The findings come as forecasters report a "Godzilla" El Niño brewing in the Pacific Ocean this year (E&ENews PM, Aug. 13).
However, van Panhuis noted that other variables, such as which strain of dengue is circulating in an outbreak, could also play an important role. He noted that some El Niño years didn’t result in huge epidemics, so other factors are at play.
He suggested that health officials could attack the disease during lulls with mosquito control campaigns and postpone and reduce infection peaks later in the season. He added that diseases like dengue should be examined at a regional level and that countries should pool their data in order to discover other disease trends that they could use to their advantage.