California’s warming climate fuels surge in mosquito, tick and flea-borne illnesses

A series of recent outbreaks—including tularemia on Long Island, flea infestations in a Marin County park, and a rare Rickettsia infection in Sonoma County—highlight a growing vector-borne disease problem in California. Researchers attribute the rise to a combination of factors: an unprecedented March heat wave spurred a boom in native Culex mosquitoes that spread West Nile virus, while warming temperatures extend flea seasons and enable invasive species like Aedes aegypti to thrive, raising dengue and other mosquito-borne risks.

Data show West Nile virus-positive mosquito samples in Los Angeles County are ten times higher than last year, and dead bird reports have surged to 373 statewide. Climate-driven range expansions also increase exposure to diseases such as typhus, with Los Angeles County reporting 220 cases in 2025. Some officials warn that bans on rodenticides, intended to protect wildlife, may inadvertently boost rodent populations that serve as disease reservoirs. Experts caution that human encroachment into new habitats and ongoing climate shifts will likely introduce additional pathogens in the future.