Nearly Half Of Californians Live In Areas Where Dengue Could Spread In Summer, Study Finds

Dengue Has Already Found a Foothold in California, and Climate Change Could Make It Far Worse

A mosquito-borne illness once considered a distant, tropical problem has been quietly taking root in California, and a new study finds that tens of millions of residents already live in areas where summer conditions are ripe for local outbreaks.

Dengue fever is often described as a severe flu, and in serious cases it can become dangerous and require urgent medical care. For decades it has been closely associated with tropical regions in Asia, Latin America, and Africa. But its geography is shifting. California recorded its first two locally transmitted dengue cases in Los Angeles County in 2023, followed by a cluster of cases across southern California in 2024.

Now, researchers have built a detailed risk model mapping exactly where and when transmission conditions are most favorable across the state, and the picture is sobering. Roughly 18.2 million California residents, nearly half the state’s population, currently live in areas where conditions during peak summer months are at least as favorable for dengue transmission as they were when those first local cases appeared.

Published in The Lancet Regional Health – Americas, the study does not predict that dengue will sweep the state. But it makes clear that the window for localized outbreaks is open, has likely been open longer than health officials realized, and that climate change is working to push it wider.

Three Conditions That Fuel Dengue Transmission in California

To understand where dengue can take hold, researchers identified three ingredients that must all be present at the same time: the right mosquitoes, the right temperatures, and infected travelers returning from places where dengue is common.

Aedes aegypti is the species of greatest concern, one that prefers to feed on humans and thrives in urban environments. First detected in California in the early 2010s, it has since spread rapidly across northern, central, and southern parts of the state. It is also the most efficient at transmitting dengue virus.

Temperature matters because dengue transmission is highly sensitive to heat. Peak transmission occurs around 29 degrees Celsius, roughly 84 degrees Fahrenheit. Below a certain threshold, the virus cannot complete its life cycle inside the mosquito fast enough to be passed on, which makes summer in the Central Valley and the greater Los Angeles and San Diego regions particularly concerning.

California’s size and global connectivity make infected travelers a persistent liability. About 140 travel-associated dengue cases are reported in the state each year, and researchers note this is likely an undercount. Every one of those cases is a potential spark.

How Researchers Built the Dengue Risk Model for California

Researchers developed a two-part risk measurement. One part combined estimates of where Ae. aegypti is likely present with how favorable local temperatures are for the virus to spread. The other layered on an estimate of how many infected travelers were moving through each neighborhood in a given month.

To map mosquito presence, the team used trap surveillance data collected across California between 2016 and 2023, combined with satellite-based climate data, blending two computer modeling techniques to improve reliability.

To estimate travel-associated cases at the neighborhood level, researchers drew from four surveillance datasets spanning more than a decade, including reports from the California Department of Public Health and the Centers for Disease Control and Prevention. Neighborhoods with larger populations tracing their backgrounds to dengue-prone regions tended to see more travel-associated cases. When tested against actual reported cases at the county level, the model’s predictions held up well.

To anchor the risk estimates in reality, the team calculated scores for the specific locations and months tied to California’s eight confirmed local transmission events in 2023 and 2024, spanning Pasadena, Long Beach, Baldwin Park, Panorama City, Escondido, El Monte, Hollywood Hills, and San Bernardino. Any location scoring at or above that level in a given month was considered at elevated risk.

California’s Dengue Risk Map Shows a Seasonal, Expanding Threat

Not all of California faces the same level of concern. In August, the peak month, areas most clearly above the baseline are concentrated in the Los Angeles and San Diego metro areas and parts of the Central Valley. Outside of June through October, risk across the entire state drops to near zero as temperatures fall too low for transmission, a pattern that makes the seasonal nature of the current threat clear.

Geography creates natural brakes in some places. San Francisco stays too cool for efficient transmission even in summer. Parts of the western Central Valley have adequate summer temperatures but few infected travelers passing through. Inland desert areas have both low mosquito presence and rare viral introductions working against sustained spread.

Climate change could erase some of those advantages. Under moderate warming, an additional 4.1 million Californians beyond the current 18.2 million could be living in areas that cross the transmission threshold by mid-century, with that number climbing to 10 million by end-of-century. The Bay Area and coastal southern California are flagged as places likely to see the largest increases as rising temperatures remove the thermal limits that currently keep transmission in check.

Global dengue cases hit a record high of over 14.1 million in 2024, meaning infected travelers arriving in California are only likely to become more frequent. Whether the state can detect and contain those introductions before local transmission takes hold is the challenge health officials now face.

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Disclaimer: This article is based on a published scientific modeling study and is intended for informational purposes only. It does not constitute medical advice. The risk estimates described reflect modeled conditions and are not official public health designations. Readers with health concerns should consult a qualified medical professional.

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