### Climate Change Change Expands Mosquito Habitats, Spreading Dengue Fever
By Kaitlin Sullivan, Anne Thompson, and Patrick Martin
Meg Norris was traveling in Argentina in April when she first experienced symptoms of dengue fever. Despite the warm weather in Salta, just south of the Bolivian border, the 33-year-old from Boulder, Colorado, zipped up a fleece sweatshirt to stop shivering.
"I thought it was sun poisoning," she said.
That night, she woke up drenched in sweat, alternating between feeling extremely hot and freezing cold. By morning, her eyes were sore, and her lymph nodes were swollen. For the next week, there was nothing to do but sleep, stay hydrated, and endure the body aches that give the illness its nickname, "breakbone fever."
### Record-Breaking Dengue Outbreak in Latin America
Latin America is experiencing its worst dengue fever outbreak on record. In the first 4½ months of 2024, case numbers are already 238% higher than at the same time last year, which itself ended with a record 4.1 million cases, according to the Pan American Health Organization. Current cases are more than 400% higher than the five-year average.
An unusually wet and warm summer season, driven by the El Niño weather pattern, has created ideal conditions for the mosquitoes that spread dengue, allowing them to hatch in large numbers and carry higher amounts of the virus.
Experts warn that this could be a preview of future dengue fever trends. Climate change is producing unusually warm conditions, expanding the range of mosquito-borne diseases.
"That’s concerning for places where dengue hasn’t occurred before in recent history: North America and Europe," said Dr. Albert Ko, a professor of epidemiology of microbial diseases at the Yale School of Public Health.
Dengue is a viral fever caused by four different viruses and spread through mosquito bites. It's common in many tropical regions worldwide but has begun appearing in more temperate climates. The mosquitoes that carry dengue fever, Aedes aegypti, are now regularly found in the southern U.S., and have recently been detected as far north as the Bay Area and Washington, D.C. A 2019 study predicted that an additional 2 billion people will be at risk for dengue fever by 2080.
"We are definitely worried," Ko said.
### Why Are Dengue Cases Rising Worldwide?
Dengue outbreaks have historically occurred in the Americas every three to four years, said Dr. Gabriela Paz-Bailey, dengue branch chief in the division of vector-borne diseases at the Centers for Disease Control and Prevention. "But now we are seeing them every year," she said.
Part of the reason for this increase is tied to climate changechange.
A warming climate expands the mosquitoes' habitat and allows them to breed year-round, rather than only in the warmer months. The hotter temperatures also cause the viruses to replicate faster, meaning mosquitoes end up carrying many more viral copies, increasing the likelihood of infection from a bite.
"We are also seeing dengue cause outbreaks at times when they usually don’t occur," Ko said.
South America's dengue cases this year were not only unusually high but also occurred unusually early in the season. Similarly, Puerto Rico, which typically sees dengue outbreaks in summer and fall, declared a public health emergency in late March after being overwhelmed by dengue fever cases, with more than 400 people hospitalized.
In recent years, the epidemic has spread to parts of southern Brazil and northern Argentina, areas previously less affected by dengue, Ko said.
"That gives us a snapshot of what we may see here in North America in the coming decades," Ko said.
### How Might Dengue Establish Itself in the U.S.?
The presence of Aedes aegypti mosquitoes in regions outside their usual range does not mean they are already carrying dengue viruses, but their appearance is a warning of what might come, Ko added.
Locally transmitted dengue fever infections—meaning the infected person did not contract the virus abroad—are still rare in the continental U.S., but have recently been reported for the first time in some states. Last October, California health officials reported the state's first case of locally transmitted dengue in Pasadena. Local transmission has also been recorded in Arizona, Florida, and the southern coast of Texas. Last summer's record-breaking heatwaves in Europe saw local dengue transmission in France, Italy, and Spain.
"I think this means dengue will become more common," said Paz-Bailey, noting that the main concern remains the significant rise in cases where the virus is already endemic.
This summer, she does not expect significant dengue outbreaks on the U.S. mainland but anticipates that some travelers returning from regions with higher-than-usual cases may bring the virus back home.
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