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June 2023 Newsletter

Winter Rainfall

Photograph of a person in a uniform, safety glasses, and glove outdoors. The foreground is tall green grassy plants, the midground is an area of mud and low water, the background is sky with electric wires and towers.
Operations Supervisor Ryan applies larvicide to an impound in San Mateo to control mosquito larvae and prevent the emergence of adult mosquitoes.

Water in a confined area is referred to as an 'impound.'  Impounds can form in low-lying areas or when man-made structures like dams, levees, and gates prevent the flow of water. Impounds of standing water can be a significant source of mosquitoes. Much of our work in the rainy months is focused on these seasonally-formed impounds. Usually by May and June, many of our natural freshwater mosquito breeding habitats would begin to dry up. However, the heavy rains this past season resulted in many of these areas continuing to hold water into June. Staff manage these large impounded areas by inspecting for mosquito larvae and applying biorational larvicides that control mosquitoes in the juvenile larval stage before they can become adults.

Report Dead Birds

Would you like to help us protect against mosquito-borne illnesses? The California Department of Public Health’s West Nile virus call center is now open with live operators. Residents who find a dead bird in good condition (head and eyes intact and not infested with flies or ants) should call 1-877-WNV-BIRD (1- 877-968-2473) or file a report online at If suitable for testing, a District staff member will bring the bird back to the lab and test the carcass for the presence of mosquito-borne disease.

West Nile Virus 2023 Season

San Mateo County

To date, West Nile virus (WNV) has not been detected in any dead bird, mosquito, or sentinel chicken samples in San Mateo County in 2023.


Thus far, in 2023, WNV has been detected in nine counties statewide, including two in the coastal region, Alameda County and Santa Clara County. These detections include fifteen mosquito samples and sixteen dead birds. Statewide, 1,993 birds have been reported, 350 have been tested, and sixteen have tested positive for WNV (5%). No sentinel chickens have tested positive for WNV, and there have been no human cases reported in California. For additional information, visit

Preventing West Nile Virus

Photograph of a person in a white Jeep.  The Jeep has a yellow hose along the side and a photo on the back window about "Something bugging you?"
Seasonal driver Ric is one of several employees helping control mosquitoes in catch basins during the summer.

The peak months for West Nile virus detections in our County have historically been between late July and October, but the work begins before then. In late April,  our seasonal catch basin drivers begin treating most of the catch basins and storm drains throughout San Mateo County. A single catch basin can produce thousands of mosquitoes if untreated. A spring start helps us keep the adult population low and reduces the chance of West Nile virus being detected in our County.

Also, starting in April, district staff begin treating standing water in the horizontal pipes connecting storm drains. This larvicide application system can treat water standing in pipes  that our catch basin drivers cannot reach with their equipment. This program usually targets neighborhoods built on historic marshland, and the shifting of the substrate has caused sags in the lateral pipes and resulted in pockets of standing water where mosquitoes can grow.

Graphic of a street with houses and a view underground. One house is overwatering their lawn, and the water is entering the storm drain and creating a little puddle of water in the pipes under the street. In the puddle, there are little mosquitoes growing. Text reads "During the summer, small amounts of water are breeding sites for mosquitoes. Water from overwatering can stand in drains and breed mosquitoes. Sagging of broken pipes or aging infrastructure can cause pools of water where mosquitoes can breed."
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