Diseases historically absent from the United States have been showing up in Florida, Texas, California and other U.S. states in recent years. To understand why, look to Peru. That's where researchers from Stanford and other institutions analyzed the connection between a cyclone and a massive outbreak of dengue fever, a mosquito-borne viral disease that can cause fever, rash, and life-threatening symptoms like hemorrhage and shock. Their findings, published March 17 in One Earth, reveal that warmer, wetter weather linked to climate change is making disease epidemics more likely.
"Health impacts of climate change aren't something we're waiting for," said study lead author Mallory Harris, a postdoctoral scholar at the University of Maryland who conducted the research as a PhD student in biology at Stanford. "They're happening now."
Standing water + heat = sick people
Dengue fever, transmitted by Aedes aegypti and Aedes albopictus mosquitoes, sickens an estimated tens of millions of people worldwide each year, according to the World Health Organization, and has surged more than 10-fold globally since 2000. A 2023 cyclone and coastal El Niño in a normally dry region of Peru was followed by a dengue fever outbreak 10 times larger than normal.
Using a statistical technique developed in economics, the researchers asked what share of this historic outbreak was due to the unusual 2023 weather, by modeling what would have happened without the storm. In collaboration with scientists at the Peruvian Ministry of Health and the Latin American Center of Excellence in Climate Change and Health, the team estimated that 60% of dengue cases in the hardest hit districts were directly caused by extreme rainfall and warm temperatures during the cyclone. That translates to roughly 22,000 additional people falling ill who otherwise would not have.
The link goes like this: heavy rains flood low-lying areas, knock out water and sanitation infrastructure, and create pools of water ideal for breeding Aedes aegypti and Aedes albopictus mosquitoes. Warm weather turbocharges mosquito breeding and disease transmission processes. By comparison, cooler areas hit by the cyclone saw no significant effect of extreme precipitation on dengue incidence.
"While we often observe large dengue outbreaks following extreme weather events, this is the first time scientists have been able to pinpoint the role of climate change and precisely measure the impact of a particular storm on dengue—one of the most rapidly-growing infectious diseases," said study senior author Erin Mordecai , an associate professor of biology in the Stanford School of Humanities and Sciences and co-lead of the Disease Ecology in a Changing World program based at the Center for Human and Planetary Health .
Stanford climate modelers Jared Trok and Noah Diffenbaugh then analyzed simulations comparing precipitation in March across 1965-2014 to a pre-industrial baseline. The result: extreme precipitation conditions like those seen in 2023 are now 31 percent more likely in northwestern Peru than they were before industrialization. When combined with warming temperatures, the probability of climate conditions like those that fueled the 2023 dengue epidemic has nearly tripled.
Fighting a tiny enemy
The findings are both a warning and the seed of a possible solution. Targeted mosquito control and vaccination in high-risk urban districts could all blunt the impact of a mosquito-borne disease surge, according to the researchers. Investments in urban flood resilience, such as better drainage, sturdier housing, and more reliable water infrastructure could also help stave off the threat.
"This research provides Peru's Ministry of Health an initial estimate to quantify the specific health impacts of extreme climatic events," said study coauthor Andrés Lescano of the Latin American Center of Excellence for Climate Change and Health. "That can be used as a reference to advocate for greater public health investments in preparation and response."
Similar analyses could be applied to hurricanes, monsoons, and other extreme events around the world. They could help governments prepare before mosquito-borne outbreaks take hold, and better understand the impact climate change is already having on human health.
"As extreme weather events become more frequent with climate change, we need to think strategically and act decisively to prevent mosquito borne epidemics," Harris said.
Mordecai is also a senior fellow at the Stanford Woods Institute for the Environment ; a faculty fellow in the Center for Innovation in Global Health , the Center for Human and Planetary Health , and the King Center on Global Development ; and a member of Bio-X . Trok is a Ph.D. student in Earth system science at Stanford. Diffenbaugh is the William Wrigley Professor and Kimmelman Family Senior Fellow in the Stanford Doerr School of Sustainability , and the Olivier Nomellini Family University Fellow in Undergraduate Education
Coauthors of "Extreme precipitation, exacerbated by anthropogenic climate change, drove Peru's record-breaking 2023 dengue outbreak," also include Kevin Martel and César Munayco of the Peruvian Ministry of Health and the Latin American Center of Excellence for Climate Change and Health; Mercy Borbor Cordova of the Escuela Superior Politécnica del Litoral in Ecuador.
The study was funded by the Achievement Rewards for College Scientists Scholarship; the National Institutes of Health; Montgomery County, Maryland; the National Science Foundation; the Stanford Center for Innovation in Global Health; the King Center on Global Development; the Stanford Woods Institute for the Environment; the Fogarty International Center; the National Institute on Aging, and the National Institute of Environmental Health Sciences.
