Note: The District's sentinel chicken surveillance program is on hold for 2025. Below information is about the history of the program, but the District will not have sentinel chicken flocks in 2025.

Each summer, surveillance for West Nile virus ramps up as the weather warms. The District has historically used three surveillance techniques to guard against West Nile virus (WNV) outbreaks: trapping adult mosquitoes, collecting dead birds, and monitoring sentinel chicken flocks in locations throughout the County.

Chickens are useful for WNV surveillance because while they can be infected with WNV, they do not become sick and die, which means they can be tested regularly for evidence of exposure.

District vector ecologists have monitored District-owned flocks in locations throughout the County over the years. From April until October each year, laboratory staff took blood samples from the chickens every other week. These samples were tested for antibodies against West Nile virus, western equine encephalitis virus, and Saint Louis encephalitis virus. If present, these antibodies in the chicken blood would inform us that these viruses are circulating in the area and allow us to take further steps to protect the public

To date, no West Nile virus antibodies have been detected in sentinel chickens in San Mateo County.

Page last reviewed: February 26, 2025