Researchers studying dengue epidemics in Southeast Asia from the last decade and a half have stumbled upon a surprising result—each of the eight countries in their study region suffered from a larger-than-usual dengue fever epidemic in 1997–1998. Temperatures rose 1.5°C higher than normal in this region during that time. The warming coincided with the strongest El Niño of the century.
“All the provinces in this entire, very large region had epidemics exactly at the same time,” said epidemiologist Wilbert van Panhuis of the University of Pittsburgh, Pa., lead author of the study.
He and his colleagues published their findings Monday in the Proceedings of the National Academy of Sciences of the United States of America.
Dengue fever is an infectious, mosquito-borne disease found most often in the tropics and subtropics. It causes severe headaches, joint pain, and, if left untreated, severe bleeding. There is no remedy, although van Panhuis said a vaccine is being developed.
The mosquito that spreads the virus, Aedes aegypti, grows in standing water, which often collects around cities and suburbs in rain buckets, garbage cans, piles of refuse, or even old tires. Because of water’s role in the insect’s life cycle, epidemiologists see annual dengue outbreaks associated with rainy seasons, said van Panhuis. It’s the large epidemics of the disease, which occur periodically, that puzzle scientists.
In the study, the researchers looked specifically at 273 provinces across Thailand, Cambodia, Laos, Vietnam, Malaysia, Singapore, the Philippines, and Taiwan—an area covering 10 million square kilometers. Working with international collaborators, the researchers cobbled together epidemiological data on dengue fever. The major epidemics of 1997–1998, during the strong El Niño, leapt out of the 18 years’ worth of data. In that year, the outbreak affected 450,000 people. In a normal year, this number reaches about 200,000.
“High temperatures across a large area can really support major dengue epidemics to be occurring simultaneously,” van Panhuis said.
El Niño’s Effects
El Niño is a periodic climate phenomenon in which warm ocean water builds up in the eastern equatorial portion of the Pacific basin. In the United States, an El Niño can bring more precipitation to California and the Gulf Coast, dryness to the Ohio Valley and Pacific Northwest, and other effects, but the impact on weather isn’t always certain.
In Southeast Asian countries such as Indonesia, Thailand, and Malaysia, El Niño generally spurs hotter temperatures. Mosquitoes thrive in hot, wet climates. Because the region is already very humid, elevated heat alone can hasten the spread of the disease there, van Panhuis said.
When temperatures rise, mosquitoes grow faster and feed more frequently, said Cameron Webb, a medical entomologist at the University of Sydney in Sydney, Australia, who was not involved in the research.
“This means that, once a mosquito takes an infected blood meal from one person, the period until they can then pass the virus on to another person is reduced,” Webb explained.
The Current El Niño
With a new El Niño, which climate models indicate could prove as intense as the 1997–1998 event, under way, “there is a possibility that the temperature will rise again in Southeast Asia and that a large dengue epidemic will follow,” van Panhuis said.
Already, Taiwan and the Philippines have experienced an increase in the number of cases of dengue, he noted.
Still, other factors can influence a dengue outbreak, van Panhuis said. Those include whether residents of an affected area have previously contracted the virus and are now immune or if the virus has mutated slightly, which happened during the 1997 outbreak. If the epidemic happens now, that could make residents immune next year, he explained.
Even with increased temperature and precipitation, “we can’t predict that a dengue outbreak will happen,” said Ciro Ugarte, director of the Department of Emergency Preparedness and Disaster Relief at the Pan-American Health Organization, which is the regional office for the Americas of the World Health Organization. During that same El Niño in 1997–1998, some places in Central and South America saw increased dengue cases, but others didn’t, recalled Ugarte, who did not participate in the new study.
If the El Niño–dengue fever connection proves to be robust in some places, health officials could take extra cautionary measures whenever El Niño conditions began to build, such as readying hospitals for an influx of patients or improving mosquito control, van Panhuis said.
Hoping for “a regional, systematic process of disease monitoring and prediction,” he noted that “we’re much better at predicting climate and weather than we are at predicting diseases. If there’s a link [with El Niño], then climate data can really help us predict disease better.”
—JoAnna Wendel, Staff Writer
Citation: Wendel, J. (2015), Dengue fever epidemics linked with El Niño, study says, Eos, 96, doi:10.1029/2015EO037169. Published on 9 October 2015.