Disease-Carrying Aedes Aegypti Mosquitoes Confirmed in Brentwood, Contra Costa County

Key Takeaways

• Invasive Aedes aegypti mosquitoes, capable of transmitting Zika, dengue, yellow fever and chikungunya, were confirmed in Brentwood, Contra Costa County.
• The discovery followed a resident service request, and additional trapping found more mosquitoes nearby.
• The Contra Costa Mosquito and Vector Control District has expanded surveillance to determine how far the mosquitoes have spread.
• County officials released sterile male mosquitoes in July as part of a control program.
• Residents should remove standing water and report daytime-biting mosquitoes to the district.

Contra Costa County has a new mosquito detection. Invasive Aedes aegypti mosquitoes have been confirmed in Brentwood, adding another East Bay community to the map of this disease-carrying pest.

What We Know About the Brentwood Detection

According to the Contra Costa Mosquito and Vector Control District, the initial discovery came after a resident requested service. Additional trapping in the area found more invasive mosquitoes, prompting the district to expand surveillance. Additional trapping has since confirmed that invasive mosquitoes are present across multiple areas of the city, indicating a larger footprint than initially known. Specifically, positive traps have been identified in three zones: the area between Veterans Park and McClarren Park, neighborhoods north of Liberty High School, and the vicinity around Central Boulevard between Fairfield Avenue and Minnesota Avenue. “The District is working to control the spread of this mosquito, but we cannot do this alone. Aedes aegypti can develop in very small amounts of water around homes, so we need every Brentwood resident to check their front and back yards, dump out standing water, and eliminate places where these mosquitoes can develop,” said David Wexler, Operations Manager.

County officials confirmed the Brentwood detection on Aug. 15, 2026, after mosquitoes trapped in the city were identified as Aedes aegypti.

The district says Aedes aegypti mosquitoes breed in small amounts of standing water commonly found in bird baths, plant saucers, buckets, untreated spas, unmaintained swimming pools and other containers. Beyond common household items, officials urge residents to look carefully for water in buckets, containers, watering cans, toys, outdoor equipment, tarps, plastic covers, clogged drains, gutters, wheelbarrows, garden equipment, and any other container that can hold water. Even a small amount of water can be enough for these mosquitoes to reproduce. As Scientific Programs Manager Steve Schutz noted, “Something as small as a bottle cap or plant saucer can hold enough water for mosquitoes to develop.”

Why Aedes aegypti Is a Health Concern

Aedes aegypti is a daytime-biting mosquito that prefers to feed on humans. Unlike many native mosquitoes, it can transmit viruses from person to person, including Zika, dengue, yellow fever and chikungunya.

These mosquitoes are particularly tricky to spot because they are small and quick, often approaching, biting, and leaving rapidly. They frequently target the ankles and lower legs when feeding.

Female Aedes aegypti are aggressive biters of humans and can pester hosts more than other types of mosquitoes, even in small numbers, according to county health officials.

County health officials have made eradicating this species a priority. Because female Aedes aegypti are aggressive biters, even small populations can cause significant nuisance and public health risk.

What Officials Are Doing

In July, county officials launched a program to release male Aedes aegypti treated with bacteria. When these males mate with females, the eggs do not hatch, helping to reduce the population over time. Male mosquitoes do not bite, so the program does not add risk to humans.

In response to the expanded detections, the District plans to conduct truck-mounted larval mosquito control in portions of Brentwood within the next week. The goal of the treatment is to reduce the number of immature mosquitoes before they can grow into biting female adults.

District officials are emphasizing the need for resident cooperation. "We need every resident’s cooperation. The previous success in Martinez was only possible because the public did their part in reporting biting mosquitoes, dumping any amount of water, and letting us check their properties for the presence of this mosquito. We now need the help and cooperation of the residents of Brentwood," said Dr. Paula Macedo, General Manager.

The district is also asking residents to inspect properties for standing water and to report any daytime-biting insects. Reports can be made online at contracostamosquito.gov or by calling (925) 685-9301.

For residents who spend time outdoors at any time of day or evening, wearing mosquito repellent reduces mosquito bites and the chance of becoming infected with West Nile virus or other mosquito-borne bacteria or viruses. Repellents with DEET, picaridin, oil of lemon eucalyptus, or IR3535 are most effective. Always follow the instructions on the label. Contra Costa County residents pay for District services through property taxes, so there is no additional charge for county residents to request a District service.

Local California Context

Brentwood is the latest Contra Costa County city to detect Aedes aegypti mosquitoes. The species has been present in Antioch for the past three years. The district previously eliminated infestations in Martinez and Concord, but Antioch has been harder to control.

According to Steve Schutz, Scientific Programs Manager, "These mosquitoes have been spreading throughout California for several years. Previous findings in Martinez and Concord were limited to fairly small areas and, with the cooperation of local residents, we were able to eradicate these mosquitoes and prevent them from spreading. In Antioch; however, by the time residents first reported them to us three years ago, they had already occupied a fairly large area, and conventional residential inspections and treatments have so far not been sufficient to eliminate them."

This new detection shows that the invasive mosquito continues to spread across the county, despite ongoing control efforts.

Background

Aedes aegypti is not native to California. Once established, it is difficult to eliminate because it thrives around homes and requires very little water to breed. Public health agencies across the state have been working for years to slow its spread.

In Contra Costa County, surveillance and public reporting are the first lines of defense. The July sterile male program is one of the newer tools being used to suppress local populations.

Conclusion

Brentwood residents should check their yards for standing water, empty containers that could hold water, and report any daytime-biting mosquitoes to the Contra Costa Mosquito and Vector Control District. Quick action can help prevent this invasive species from becoming established in another part of the county.