Invasive mosquito populations are spreading in Northern California, including the San Francisco Bay Area, according to health officials. The Aedes aegypti mosquito, known for transmitting diseases like dengue, yellow fever, chikungunya, and Zika, was recently detected in San Jose near Kelley Park. This marks the first sighting of A. aegypti in that area, as confirmed by Taylor Kelly, the scientific-technical services manager of the Santa Clara County Mosquito and Vector Control District.

"This year in particular, we've had so many detections throughout new parts of the county," Kelly said. A. aegypti is typically found in the southern United States, Puerto Rico, and various tropical and subtropical regions worldwide. The species was first identified in California in 2013 and has since been reported in 27 counties, primarily in Southern California and the Central Valley.

Earlier this month, Los Angeles County reported its first case of locally acquired dengue for the 2025 mosquito season, with A. aegypti likely responsible. Health officials noted that populations of this mosquito have been steadily increasing in Northern California since spring.

A. aegypti is known for its ability to travel with people, whether in vehicles or homes, and can breed in small containers. In Santa Clara County, it appears that A. aegypti has established a presence. Kelly stated, "I do think it's something that residents are going to notice because of the nuisance biting."

The California Department of Health has linked A. aegypti to local dengue virus transmission. Climate change may be contributing to the spread of mosquito populations and vector-borne diseases globally, as these insects thrive in warm, humid conditions. The World Mosquito Program noted that mosquitoes have even been found in Iceland, leaving Antarctica as the only region without them.

In California, climate has likely played a significant role in the spread of A. aegypti, but other factors, such as resistance to insecticides, may also be facilitating its growth in the Bay Area. A. aegypti is identifiable by its white bands on its legs and body, as well as stripes on its upper body. They are known to be aggressive daytime biters.

Officials warn that A. aegypti can lay eggs in various containers, including flowerpots and pet bowls, which can further the spread of the species. Residents are urged to eliminate standing water in their yards and clean containers that may collect rainwater. Kelly advised, "Everybody should dump and drain any standing water on their property at least once a week."

Roberto Barrera, lead entomologist at the CDC's Dengue Branch, described A. aegypti as a "top public health threat," stating, "These tiny troublemakers are responsible for spreading viruses that affect millions of people." California health officials encourage residents to report daytime mosquito bites immediately and to document and photograph mosquitoes with black and white stripes.