Female Aedes aegypti mosquito, a carrier of the Dengue fever, feeding on the human skin, 2005. Image courtesy Centers for Disease Control (CDC) / James Gathany. (Photo by Smith Collection/Gado/Getty Images)
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For the past century, most mosquito-borne diseases like dengue, malaria and yellow fever have been considered problems of the tropics, not the continental United States. That is starting to change. Florida began dealing with locally acquired dengue over a decade ago. Malaria cases acquired in Texas and Florida made headlines in 2023. And now a new paper in the CDC journal Emerging Infectious Diseases documents what may be the most telling example in a seven-week chain of sustained dengue transmission in Los Angeles County.
Fourteen people contracted dengue in LA County in the fall of 2024. Eight of them clustered within a mile of each other in the San Gabriel Valley. The other six were scattered across five neighborhoods more than 20 miles apart, each possibly a separate introduction from a returning traveler.
Much of the southern United States sits at the environmental margin for vector-borne disease transmission. The mosquitoes are present. The climate is warm enough for part of the year. Infected travelers arrive regularly. What has kept dengue from establishing is that conditions have been just barely insufficient to sustain local chains. The LA outbreak shows what happens when “just barely insufficient” tips the other way, even briefly.
Timeline and geographic distribution of 14 locally acquired dengue cases, Los Angeles County, August–November 2024. Source: Vaughan et al., Emerging Infectious Diseases 32(5), 2026.
CDC
What Seven Weeks Means
Dengue requires a relay. A mosquito bites an infected person, the virus replicates inside the mosquito for eight to twelve days, and the mosquito bites someone new. That person incubates the virus for five to seven days before becoming infectious to the next mosquito. One full cycle takes two to three weeks. A seven-week chain means the relay completed at least twice, probably three times, without breaking.
The fourteen confirmed cases are almost certainly a fraction of the total. About three-quarters of dengue infections produce no symptoms or symptoms too mild to trigger a medical visit. Many dozens more people were likely infected in LA County and never knew it.
The dengue virus transmission cycle. A mosquito bites an infected person, the virus replicates inside the mosquito over eight to twelve days, and the mosquito passes it to someone new.
CDC
A Connected World
Why did this happen in 2024? LA County recorded 222 travel-associated dengue cases that year, up from 35 in 2022 and 75 in 2023. More than 13 million cases were reported across the Americas in 2024, surpassing previous records. Every traveler who returns to LA with active dengue viremia is a potential spark. With 222 sparks landing in a county where the dominant mosquito vector, Ae. aegypti, has been around since 2013, it would have been surprising if none of them caught.
This is how what is global becomes local. LA County did not develop a dengue problem because its mosquito control failed. It developed one because other places in the world were surging with dengue and enough infected people came home to a place where the vector was waiting. The ongoing hantavirus outbreak that started on the cruise ship MV Hondius is another reminder that pathogens move at the speed of global travel.
Reported dengue cases in the Americas, 1980–2024. The 2024 total of nearly 13 million cases was three times the previous record set in 2023. Data: OpenDengue V1.2 (Clarke et al. 2024); 2024 total from PAHO.
John M. Drake
Climate change pushes both sides of the equation, expanding the range and season length of Aedes mosquitoes in temperate regions while intensifying transmission in the tropics that feeds the flow of infected travelers.
The Threshold Problem
Diseases propagate themselves. The difference between no local dengue and a little local dengue is not a matter of degree. It is a difference in kind.
Below the transmission threshold, every introduction dies out on its own. Public health can afford to be slow, because the math is on its side. Above the threshold, each infection generates more infections, and the burden compounds. Surveillance, diagnosis and vector control all have to work quickly and in coordination, and all of it costs more than prevention did.
Florida has learned this over the past decade. Key West saw its first modern dengue outbreak in 2009. Miami-Dade and other South Florida counties began reporting sporadic locally acquired cases shortly after. By 2022–2023, Florida was recording over 60 locally acquired cases in a single season, managed through sustained investment in surveillance, rapid diagnostic capacity and aggressive vector control.
California is earlier on that curve. The LA County investigation showed that the median time from symptom onset to a dengue test was 10 days. One case was initially diagnosed as West Nile virus. Four of the fourteen patients were found by public health teams going door to door, not by the medical system. In a place where dengue has never been endemic, the diagnosis requires someone to think of it first.
What Ended the Outbreak?
The 2024 outbreak ended when the mosquitoes went dormant in late October. The timeline of the California outbreak shows cases accumulating through August, September and October, then stopping. The public health response was aggressive, 318 household surveys, nine field operations, multilingual outreach. Whether it would have broken the chain on its own is unknowable. The season ended first.
Climate change is lengthening that season. The global dengue burden is growing. The introduction pressure will keep rising. Every one of these forces pushes a marginal system closer to the threshold. It is almost always cheaper to keep a disease out than to manage it once it arrives.