California’s warming climate is drawing new attention from health researchers as UC Berkeley scientists examine how dengue fever could become a greater concern in parts of the state where the virus has historically been rare.
A new UC Berkeley led study, published in The Lancet Regional Health Americas, points to a changing risk picture in California. The research does not suggest that dengue is widespread across the state. It does, however, indicate that warmer temperatures, the presence of Aedes aegypti mosquitoes, and travel related virus introductions may create conditions where local transmission becomes more possible in some regions.
Dengue fever, sometimes called “breakbone fever,” is a mosquito borne illness that can cause high fever, joint pain, muscle pain, fatigue, vomiting, bleeding, and, in severe cases, death. Many infections may also show no symptoms. There is no specific treatment for dengue, making prevention and early awareness important for communities where the disease could appear.
The findings arrive as California health officials and researchers continue to monitor Aedes aegypti, the mosquito species closely associated with dengue transmission. The mosquito was first documented by California health officials in the state in 2013 and has since been found in more than half of California’s counties, according to the Berkeley report.
A Disease Once Rare in California Gets New Attention
Dengue has long been associated with tropical and subtropical regions, where the virus circulates through mosquito populations and human hosts. California has not been considered a place where dengue is consistently present. Local transmission remains rare, and many detected cases have been tied to people who recently traveled to areas where dengue is common.
That distinction matters. Researchers are not saying dengue is already embedded in California. Instead, the Berkeley study frames the issue as an emerging risk shaped by climate, mosquito habitat, and human travel.
In 2023, health officials reported a locally acquired dengue case in Pasadena. The Berkeley article noted that a person was hospitalized with dengue and that public health officials later found another nearby person who tested positive but had no symptoms. Neither had recently traveled, leading officials to conclude that local transmission had occurred.
California has reported only about 20 locally transmitted dengue cases to date, according to the Berkeley report. Still, the pattern has placed new focus on whether warming conditions could make it easier for the virus to circulate in select urban and inland areas.
Why Warming Temperatures Matter
Temperature plays a central role in dengue transmission because the virus must develop inside a mosquito before that mosquito can pass it to another person.
Lisa I. Couper, a UC Berkeley postdoctoral scholar in environmental health sciences and the primary author of the study, told Berkeley News that California sits near the edge of temperature suitability for dengue. The optimal temperature for dengue virus reproduction is around 29 degrees Celsius, or about 84 degrees Fahrenheit.
For much of the year, and in many parts of California, temperatures have often remained below that point. The Berkeley study suggests that warming may reduce some of those temperature barriers, especially in areas that already have suitable mosquito habitat and frequent human travel connections to places where dengue is present.
The study examined three overlapping factors: the likely presence of Aedes aegypti mosquitoes, likely human travel from areas where dengue is common, and temperatures suitable for transmission. Researchers found that parts of Southern California and the Central Valley already show conditions that could support local circulation.
The report cited an estimate that 18.2 million people currently live in areas where those necessary conditions for local dengue transmission exist. That equals 46% of Californians based on 2020 U.S. Census data. The number could rise by more than 4 million under future warming scenarios.
Southern California and the Central Valley Draw Focus
The study gives particular attention to Southern California and the Central Valley because these areas combine human density, warming temperatures, and mosquito suitability in ways that could matter for public health planning.
Urban environments can be favorable for Aedes aegypti because the mosquito prefers to feed on humans and can breed in small amounts of standing water. Buckets, plant saucers, containers, and other water holding items around homes can create breeding sites.
The Central Valley also drew attention because of population growth. Couper noted in the Berkeley article that the model may underestimate future risk if more people move into mosquito suitable areas. She specifically described the Central Valley as the fastest growing region in California.
The model also included travel related considerations, since dengue is often introduced when someone returns from a place where the virus is already circulating. Researchers acknowledged uncertainty in those assumptions, including how travel patterns may change and how dengue seasonality may shift globally as temperatures change.
That caution is important. The study does not present a fixed forecast of future case counts. It highlights areas where conditions may become more favorable and where monitoring could help public health officials respond earlier.
What Californians Can Watch For
For the public, the Berkeley research places emphasis on awareness rather than alarm. Dengue risk in California remains much lower than in regions where the virus is consistently present. Couper told Berkeley News that people in California are currently more likely to get dengue through travel to an endemic region than through local exposure.
Still, symptoms can be serious. Fever, body aches, joint pain, fatigue, vomiting, bleeding, and prolonged illness may occur. Because some people do not show symptoms, travelers can unknowingly bring the virus into areas where Aedes aegypti mosquitoes are present.
The California Department of Public Health recommends that people returning from areas where dengue is common continue using mosquito repellent and protecting themselves from mosquito bites for three weeks, even if they feel well. The goal is to reduce the chance that a local mosquito bites an infected traveler and later spreads the virus to someone else.
Common prevention steps include removing standing water around homes, wearing long sleeves and pants during mosquito season, and using repellents that contain DEET. These steps are not specific to dengue alone. They can also help reduce exposure to mosquitoes linked to other diseases.
Disclaimer:
This article is for informational purposes only and does not provide medical advice, diagnosis, or treatment guidance. Readers should consult qualified health professionals or local public health agencies for health related concerns.
