Mosquito tests for West Nile virus done last week came back negative on Thursday, after a dead American crow in South San Francisco was found to have the virus.

The bird signaled the first indication of West Nile virus in San Mateo County since 2018, according to a statement by the San Mateo County Mosquito and Vector Control District.

Following the report, the district deployed mosquito traps in the neighborhood where the dead bird was collected.

West Nile virus is primarily transmitted between birds through mosquitoes, which can then transmit the virus to humans. The virus cannot be spread person-to-person. This makes the risk relatively low, said Megan Sebay of the district.

However, the district said residents should be vigilant and exercise best practices to avoid being bitten by a mosquito infected with the virus. Precautions include removing any standing water, which provides breeding ground for mosquitoes, and wearing protective clothing at dawn and dusk when mosquito activity is highest and applying a DEET-based repellent.

Residents are encouraged to report fresh carcasses of birds to the California West Nile Virus Hotline at or by phone at 877-WNV-BIRD (877-968-2473).

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