Global cases of flu-like dengue fever reached a historic high last year, with over 14.6 million cases and more than 12,000 related deaths reported to the World Health Organization.
Global warming is driving up those infection numbers in a big way, a new study suggests.
Transmitted by the cold-blooded Aedes aegypti mosquito, the dengue virus is common across tropical and subtropical regions in Asia and the Americas.
Nearly one-fifth of cases in recent decades can be directly attributed to rising temperatures, researchers at Stanford, Harvard and Arizona State universities and the National Bureau of Economic Research found.
Analyzing local dengue caseloads and climate data within 21 countries where the virus is endemic, the scientists found that rising temperatures were responsible for an average of 18 percent of reported cases between 1990 and 2014. That’s about 260,000 infections caused by climate change each year, based on reported caseloads.
“This is not just hypothetical future change, but a large amount of human suffering that has already happened because of warming-driven dengue transmission,” said Erin Mordecai, a Stanford infectious disease ecologist and senior author on the study.
Reported cases likely represent just a fraction of the total dengue burden, Mordecai noted. Accounting for underreporting, global warming may have caused as many as 4.6 million infections each year across the study area, the authors wrote.
Their research answers questions about climate change that have long troubled infectious disease experts, said Heidi Brown, an epidemiologist studying mosquito-borne diseases at the University of Arizona. Brown was not involved in the study.
In laboratory experiments, warming up to a certain point, around 29 degrees Celsius (84 degrees Fahrenheit), is known to accelerate the reproduction and development of both Aedes aegypti mosquitoes and the virus they carry, Brown said.
Supercharged reproduction leads to larger mosquito populations and a higher likelihood of disease transmission, Brown explained. But demonstrating that effect in the real world is much more difficult.
While many studies have noted a link between temperature and dengue transmission, the new study is among the first to separate warming from all other factors that influence dengue, such as human migration, land use, precipitation and population immunity.
“This study is showing us that within these places where [dengue] is endemic, the trends of temperature effects are rather robust,” Brown said.
In the studied areas, the most noticeable effects of warming took place in cooler regions, where average temperatures entered a “Goldilocks zone” ideal for virus transmission, between 25 and 29 degrees Celsius. In already-hot areas, average temperatures quickly exceeded that optimal range and prompted a small decline in dengue cases.
In comparison with a heat wave, where sharp temperature spikes have immediate effects on the human population, the effects of heat on dengue can be deceptively subtle, said Mordecai. After all, the optimal temperature range for dengue transmission is fairly moderate, she said.
“Those are temperatures we experience all the time already. So it’s a little bit harder to notice intuitively that it’s climate change driving this.”
The study projects significant increases in dengue transmission in cooler regions where the virus is endemic. Slight declines in dengue in already-hot areas will be overshadowed by warming-fueled dengue spikes elsewhere, the study’s authors predict.
Even if greenhouse gas emissions can be reduced, they anticipate a 49 percent increase in the global dengue burden from warming via past pollution. But if emissions worsen, that growth rises to 76 percent.
“There is a real public health benefit to climate mitigation here,” said Mordecai. “We project that it’s going to get much worse than it already is, but there is a substantial benefit from emissions reduction.”
Mordecai noted that the study was limited to regions where dengue is already regularly transmitted. The research did not assess how warming might drive the geographic spread of the virus into new countries—like the United States—but it could inform predictions about where conditions might soon be ideal for dengue transmission.
“That’s the question we all want to ask,” said Brown, the University of Arizona epidemiologist. “I think what a study like this can do is give us an idea of places where maybe we should start watching for dengue in a different way.”
Scientists have already undertaken research linking severe weather events to climate change, Mordecai said. This study does the same for dengue.
“Climate change is not just affecting the weather,” she said. “It has cascading consequences for human health, including fueling disease transmission by mosquitoes.”
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Anika Jane Beamer
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Anika Jane Beamer covers the environment and climate change in Iowa, with a particular focus on water, soil and CAFOs. A lifelong Midwesterner, she writes about changing ecosystems from one of the most transformed landscapes on the continent. She holds a master’s degree in science writing from the Massachusetts Institute of Technology as well as a bachelor’s degree in biology and Spanish from Grinnell College. She is a former Outrider Fellow at Inside Climate News and was named a Taylor-Blakeslee Graduate Fellow by the Council for the Advancement of Science Writing.
