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\nCalifornia’s warming climate is making the state more hospitable to dengue virus, and to the Aedes aegypti mosquitoes that carry it. Jentavery via Flickr<\/p>\n
Climate change may increase local transmission of \u201cbreakbone fever\u201d in the state\u2019s mosquito-friendly cities.<\/p>\n
\nCalifornia’s warming climate is making the state more hospitable to dengue virus, and to the Aedes aegypti mosquitoes that carry it. Jentavery via Flickr<\/p>\n
Break-bone fever. The nickname alone attests to the pain that dengue fever can cause. Though about half the people who get the virus are asymptomatic, for the rest, an infection can be excruciating, even life-threatening, and there is no treatment.\u00a0<\/p>\n
Historically, dengue fever has been rare in California. But according to new research published in\u00a0The Lancet Regional Health Americas<\/a>\u00a0this May, a warming climate is making parts of the state more hospitable to the illness \u2014 and the mosquitoes that carry it.<\/p>\n A growing global threat, dengue thrives in more than 125 countries. It\u2019s mainly spread by the mosquito\u00a0Aedes aegypti, an urban bloodsucker that preferentially feeds on humans. The dengue virus has four strains, and a second infection with a different strain can be much more dangerous than the first.\u00a0<\/p>\n \u201cIt\u2019s just a horrible disease,\u201d said Lisa I. Couper, a postdoctoral scholar in environmental health sciences at UC Berkeley and the primary author on the new study. \u201cEven mild cases are described as having a really high fever, joint pain, muscle pain and intense fatigue. More severe cases can involve vomiting, bleeding and being out of commission for weeks, and can be fatal.\u201d\u00a0<\/p>\n California health officials first documented\u00a0Aedes aegypti\u00a0in the state in 2013. Within about a decade, the insect had been found in more than half of California\u2019s counties. Then,\u00a0in 2023<\/a>, a person in Pasadena was hospitalized with dengue; in a sweep of neighborhoods near that person\u2019s home, public health officials found one other person who tested positive for dengue but was asymptomatic. Neither had traveled recently, leading the officials to conclude for the first time that the disease had been contracted locally.\u00a0<\/p>\n So far, dengue is not\u00a0endemic<\/a>, or consistently present, in California. It has rarely been detected in the state, with only about 20 cases of local transmission to date. Instead, it overwhelmingly originates in people who have recently returned from places where the virus is endemic.\u00a0<\/p>\n Couper said this suggests that dengue is repeatedly reintroduced by travelers, but isn\u2019t sustained locally \u2014 in part because the climate has been too cool for the virus to persist in much of the state. However, that may be changing in many of California\u2019s human-dominated landscapes.<\/p>\n \u201cCalifornia is on the edge of temperature suitability for dengue,\u201d Couper said. \u201cThe optimal temperature for the virus to reproduce is around 29 degrees Celsius (or 84 degrees Fahrenheit). For a lot of the year, and for most places, California is below that optimum temperature. But as the climate warms in California, we\u2019re getting closer to it.\u201d<\/p>\n From Southern California to the Central Valley, warming temperatures mean potentially more illness<\/p>\n In the new study, researchers at UC Berkeley and colleagues from UC Santa Barbara, Stanford University, Arizona State University, Santa Clara County and Los Angeles County modeled the current and future habitat suitability in California for dengue based on predicted warming.\u00a0<\/p>\n They looked for places with three overlapping qualities: likely presence of host mosquitoes, likely human travel from places where dengue is endemic, and temperatures warm enough for the virus to spread. They confirmed that parts of the Central Valley and Southern California already are well-suited for the virus to circulate locally.<\/p>\n \u201cWe found that 18.2 million people currently live in an area where they have those necessary conditions for local dengue transmission,\u201d Couper said. Collectively, that represents 46% of Californians,\u00a0according to 2020 U.S. census data.<\/a>\u00a0That number may increase by more than 4 million in the future.<\/p>\n Right now, there is a dengue \u201cbottleneck\u201d in cooler parts of the state, including the north and coastal regions, because of how the virus replicates. Once inside a mosquito, the dengue virus needs time to reproduce before that mosquito\u2019s bite can then infect the next person.\u00a0<\/p>\n \u201cIt\u2019s not as if an\u00a0Aedes aegypti\u00a0could bite one infected person and then immediately go bite and transmit dengue to another person,\u201d Couper said. \u201cThe mosquito takes in the virus, but that virus takes awhile to develop inside of the mosquito and become infectious to another person.\u201d In cooler temperatures, the virus replicates more slowly, extending how long its buzzing pest host must survive for the virus to be passed on. But as California warms, those bottlenecks may lessen.\u00a0<\/p>\n Questions remain about dengue\u2019s future in California<\/p>\n There are a few caveats to the model. One is that the model may underestimate how many people move to the mosquito-suitable areas it identifies, possibly leading to an underestimate of future dengue cases.<\/p>\n \u201cWe may be underestimating risk, because the Central Valley is the fastest growing region in California,\u201d Couper said.\u00a0<\/p>\n Additionally, the model assumes that some regions \u2014 those with more people who self-reported ethnicities from places where dengue is already endemic \u2014 would see more local cases, because community members would be more likely to visit and bring infections back home.<\/p>\n \u201cOf course, that\u2019s a pretty big assumption,\u201d Couper said.\u00a0<\/p>\n There\u2019s also a potential connection between wealth and travel that the study was not able to tease out, such as the likelihood of wealthier Californians traveling to Southeast Asia or other dengue-endemic vacation hotspots and bringing the virus back. And it\u2019s hard to say at this point how the seasonality of dengue may be changing around the world along with climate, or how people\u2019s travel patterns may shift in the future. Nonetheless, suitable habitat for\u00a0Aedes aegypti\u00a0is increasing in the state.<\/p>\n Though this study focused on dengue risk, Couper said that local transmission of other mosquito-borne tropical diseases also may increase in California in the future, as is happening around the world.\u00a0<\/p>\n \u201cClimate warming is increasing the transmission of these environmentally-mediated diseases in temperate regions that are at the margins of suitability, like in California,\u201d she said. \u201cWe could also see increasing risks for things like chikungunya and zika, which are vectored by the same mosquito.\u201d<\/p>\n For now, Couper encourages Californians to educate themselves on the symptoms of dengue, especially if they live in the Central Valley or southern cities. There are ways to control mosquito exposure, such as by removing the standing water they need to breed, wearing long sleeves and pants during mosquito season, and using products containing the insect repellant DEET. The California Department of Public Health recommends that upon\u00a0 returning to California, people who travel to areas where dengue is common should\u00a0keep using mosquito repellant and protecting themselves from bites for three weeks<\/a>\u00a0even if they don\u2019t have dengue symptoms, to avoid spreading the disease locally.<\/p>\n Couper also encourages people not to panic. \u201cAt present, people in California are, by and large, more likely to get dengue from traveling to a dengue-endemic region than they are locally,\u201d she said. \u201cBut the risk here does seem to be increasing.\u201d<\/p>\n","protected":false},"excerpt":{"rendered":"Climate change may increase local transmission of \u201cbreakbone fever\u201d in the state\u2019s mosquito-friendly cities. California’s warming climate is…\n","protected":false},"author":2,"featured_media":355886,"comment_status":"","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[34],"tags":[139452,143,145,144,139450,139451],"class_list":["post-355885","post","type-post","status-publish","format-standard","has-post-thumbnail","category-oakland","tag-faculty-excellence","tag-oakland","tag-oakland-headlines","tag-oakland-news","tag-research-berkeley","tag-uc-berkeley-research"],"_links":{"self":[{"href":"https:\/\/www.newsbeep.com\/us-ca\/wp-json\/wp\/v2\/posts\/355885","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.newsbeep.com\/us-ca\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.newsbeep.com\/us-ca\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us-ca\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us-ca\/wp-json\/wp\/v2\/comments?post=355885"}],"version-history":[{"count":0,"href":"https:\/\/www.newsbeep.com\/us-ca\/wp-json\/wp\/v2\/posts\/355885\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.newsbeep.com\/us-ca\/wp-json\/wp\/v2\/media\/355886"}],"wp:attachment":[{"href":"https:\/\/www.newsbeep.com\/us-ca\/wp-json\/wp\/v2\/media?parent=355885"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.newsbeep.com\/us-ca\/wp-json\/wp\/v2\/categories?post=355885"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.newsbeep.com\/us-ca\/wp-json\/wp\/v2\/tags?post=355885"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"Headshot](\"https:\/\/www.newsbeep.com\/us-ca\/wp-content\/uploads\/2026\/06\/LisaCouper_HeadShot-768x768.jpg\")
\nLisa I. Couper, a postdoctoral scholar in environmental health sciences at UC Berkeley, modeled how California\u2019s warming Central Valley and southern cities are becoming more hospitable to disease-bearing mosquitoes. Courtesy of Lisa I. Couper<\/p>\n![\"The](\"https:\/\/www.newsbeep.com\/us-ca\/wp-content\/uploads\/2026\/06\/tom-schumann-hIjKeg5brU-unsplash-768x512.jpg\")
\nHistorically California cities such as Los Angeles have not been warm enough for dengue fever to circulate. With a warming climate, more locally-acquired cases of dengue are turning up in the LA area. Tom Schumann via Unsplash<\/p>\n