11-08-2025
- Health
- San Francisco Chronicle
Could chikungunya virus spread in the Bay Area? Here's what to know
An outbreak of chikungunya virus this summer has sickened more than 7,000 people in southern China, particularly in the city of Foshan.
The virus is spread by Aedes aegypti and Aedes albopictus mosquitoes, which also transmit dengue and Zika viruses. These invasive mosquitos have expanded worldwide, including to California.
Experts say that there's a low chance of chikungunya-infected mosquitoes spreading the virus to people in California, also known as local transmission. But it's not impossible — especially as climate change makes more of the state amenable to Aedes aegypti and Aedes albopictus.
'Global warming has enabled the spread of both of these mosquito species into new areas,' said Lark Coffey, a professor of pathology, microbiology and immunology at UC Davis, by email.
Chikungunya virus typically causes mild symptoms, including fever and joint pain. But in some cases, this joint pain can be severe and persist for months. Through July, the European Centre for Disease Prevention and Control reported approximately 240,000 chikungunya cases and 90 chikungunya-related deaths worldwide in 2025.
The virus' most likely path into California is via an infected traveler returning from an area with chikungunya virus transmission, Coffey said. U.S. health officials have issued a travel health notice for parts of China due to the chikungunya outbreak.
All previously reported cases of chikungunya in California, including two this year as of July 1, have been associated with travel. But that hasn't been the case for dengue virus, which is spread by the same mosquitos.
'We have seen local transmission of dengue in California in the last couple years. Which, to me, implies that there's no reason we couldn't also have local transmission of chikungunya,' said Erin Mordecai, an associate professor of biology at Stanford University and a senior fellow in the Woods Institute of the Environment. 'The more chikugunya cases there are, the more likely it is.'
Local transmission would involve multiple steps: an Aedes aegypti or Aedes albopictus mosquito biting a chikungunya-infected person, chikungunya virus incubating in the insect and then the mosquito going out and biting more people.
The Bay Area may generally be a little too cool for transmission to readily occur. 'Temperature affects the entire life cycle of the mosquito and the virus,' Mordecai said. The cases of local dengue transmission occurred in the warmer climes of Southern California in 2023 and 2024.
But climate change has made California warmer, and scientists expect the state to continue getting even warmer in the future.
'We expect mosquito development to increase (and) mosquito survival to increase,' said Lisa Couper, a disease ecologist and postdoctoral research fellow at UC Berkeley.
An additional consideration is urbanization, as Aedes aegypti is 'really well-adapted to urban systems,' said Andy MacDonald, a disease ecologist and assistant professor in UC Santa Barbara's Bren School of Environmental Science and Management. 'It prefers breeding in, say, a small amount of water and some trash that's been discarded on the street.'
In ongoing work, Couper and MacDonald have found that climate warming and urban expansion have increased the risk of local dengue transmission in California.
'We would expect these same factors to promote the suitability of chikugunya transmission,' Couper said.
In the Bay Area, Aedes aegypti mosquitoes are present in Santa Clara and Solano counties.
'While there have been no local transmissions of chikungunya occurring in (Santa Clara) County as of now, the possibility remains, which is why we ask for the public's help in stopping this mosquito from spreading,' said Taylor Kelly, scientific technical services manager with the Santa Clara County Mosquito and Vector Control District, by email.
That includes reporting black and white striped mosquitos, as well as mosquito bites that occur during the daytime. Experts also recommend using mosquito repellent and dumping standing water.
