Climate Change Supercharged Peru’s Record-Breaking Dengue Outbreak, Study Finds

In March 2023, a rare cyclone slammed into Peru’s normally dry northwestern coast, dumping record levels of rain across dozens of communities. What followed was something far worse than flooding: Peru’s largest dengue fever outbreak ever recorded, with case rates soaring to ten times their historical average. A new study has now quantified a direct link from that catastrophe back to climate change, and the numbers are hard to ignore.

Dengue fever, spread by the bite of the Aedes aegypti mosquito, has been surging worldwide. Reported cases climbed tenfold compared to 2000, with more than five million cases and 5,000 related deaths reported globally in 2023 alone. In Peru, that year’s outbreak was the worst in the country’s recorded history, killing 381 people. Researchers had long suspected that Cyclone Yaku, the unusual storm that drenched the coast that March, had something to do with it. A team led by scientists from Stanford University and the University of Maryland set out to measure exactly how much, and to trace those conditions back to human-caused climate change. Their results appear in the journal One Earth.

How Cyclone Yaku Triggered a Record Dengue Outbreak

Cyclone Yaku formed off Peru’s coast on March 7th and dissipated by March 20th, but its effects lingered for months. Northwestern Peru, typically one of the driest stretches of coastline in South America, was drenched with extreme rainfall, creating the warm, stagnant pools of water that mosquitoes need to breed. Infrastructure was disrupted, health services were overwhelmed, and communities with little experience managing dengue were left exposed.

To pin down the cyclone’s specific role, researchers identified 49 districts that took the brunt of the extreme rainfall and compared them to a matched pool of 123 other Peruvian districts that reported dengue cases in 2023 but were not hit by Cyclone Yaku. By modeling what the caseload in those 49 districts would have looked like without the flooding, the team was able to separate the storm’s effect from other factors driving transmission.

Researchers estimate that over roughly three months following the cyclone, 22,014 out of 36,709 total reported cases, about 60 percent, were attributable to the extreme rainfall, though the authors note some uncertainty in that figure. Before Cyclone Yaku, the average monthly dengue rate in those same districts was just 0.03 cases per thousand people. At the outbreak’s peak, that figure jumped to 2.68 per thousand, surpassing every prior large outbreak on record in the region by a factor of more than four.

Dengue Struck Hardest in Warm, Urban, and Flood-Prone Districts

Not all affected communities suffered equally. Areas that were warmer, more urban, and more prone to flooding tended to see dramatically higher case counts. Notably, the storm’s rainfall only appeared to drive significant dengue transmission in districts where temperatures during the cyclone stayed above roughly 75 degrees Fahrenheit (24°C). Cooler districts, even those that were badly flooded, did not see the same spike.

That makes biological sense. Mosquitoes thrive in heat; warmer temperatures speed up their breeding cycle and accelerate how quickly the dengue virus matures inside them. When heavy rain creates breeding habitat and the thermometer is already running high, the two factors tend to reinforce each other in the worst possible way.

Cities were also notably harder hit than rural areas. Urban environments, with their pavement, clogged drains, and dense populations, tend to trap floodwater and concentrate human exposure in ways that rural farmland does not. Districts with more people living on land classified as urban infrastructure saw significantly higher outbreak rates than those dominated by agriculture or pasture.

Climate Change Has Nearly Tripled the Odds of Dangerous Dengue Conditions

Most alarming in the research is what the team found about the weather itself. Combing through 203 climate model simulations spanning more than 150 years, researchers found that extreme March rainfall in northwestern Peru is now 31 percent more likely than it was in the preindustrial era. That figure, though, understates the real problem. The simultaneous combination of extreme rainfall and temperatures warm enough to fuel dengue transmission is now nearly three times more likely than before industrial-era greenhouse gas emissions began reshaping the climate.

As the authors put it, “extreme weather, made more likely by climate change, is already having an impact on human health.” Peru is especially exposed: previous research found that warming alone accounts for nearly 40 percent of recent dengue cases in the country, one of the highest shares in the world. Cyclone Yaku can be seen as a compounding disaster, an extreme weather event made more probable by a climate already primed for dengue.

Researchers note that the story does not have to end in epidemic every time a storm makes landfall. Simple protective measures, like screening windows and keeping water storage containers covered, can meaningfully limit transmission after extreme rainfall. Longer-term investments in flood-resilient infrastructure and urban design could also reduce how badly communities are exposed when the next Cyclone Yaku arrives, and given what the climate models show, another one likely will.

In northwestern Peru in 2023, the consequences of decades of warming had a name, a death toll, and a case count. The math connecting them is no longer theoretical.

---