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West Nile Virus Season and Surveillance Efforts

Surveillance for West Nile virus begins in California each year in April. The District protects residents from this mosquito-borne disease with control that targets mosquitoes in the larval stage, mosquito population monitoring, disease surveillance and public education. The laboratory staff conduct surveillance to detect the presence of mosquito-borne disease in three main ways:


A blue background with brown soil ground. Three dark brown chickens peck at the ground.

Sentinel Chickens

The District manages two sentinel chicken flocks in the cities of East Palo Alto and San Mateo. These chickens arrive to their District coops in April. Chicken blood is tested every two weeks for antibodies to West Nile virus, Saint Louis encephalitis, and Western equine encephalitis from April to November.


Dead Bird Surveillance

Black icon of a bird lying on its back with feet in the air (meant to represent a dead bird)

The District collects dead birds that are reported by residents. Dead birds should be in good condition without signs of decay or trauma. Birds except water fowl, poultry or pigeons are collected and tested.

To report a dead bird, residents should call the state West Nile virus hotline at 1-877-WNV-BIRD (1-877- 968-2473), which opens with live operators in mid-April each year. Online reports can be made at


Mosquito-borne Disease Testing

The District laboratory tests mosquito samples for West Nile virus, Saint Louis encephalitis and Western equine encephalitis.  Samples tested include whole mosquitoes for the presence of these viruses.


Page last reviewed: November 6, 2023

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