(KRON) — The Contra Costa Mosquito and Vector Control District is reporting one chicken from its flock near Knightsen, and two chickens from a flock in Oakley, have tested positive for antibodies against West Nile virus. The district maintains flocks of sentinel chickens at four locations.

“When a chicken is bitten by an infected mosquito, the chicken develops antibodies to fight off the virus.  Samples that test positive for antibodies provide evidence of virus activity in specific areas of the county,” vector control officials wrote.

A group of mosquitoes trapped in Discovery Bay recently tested positive for West Nile virus, as well as a bird that died in Antioch. District employees will be conducting surveillance and control in areas where mosquitoes are found.

The district is advising all Contra Costa County residents to reduce their risk of mosquito bites.

“The risk of exposure to West Nile virus will continue until the overnight low temperatures start dropping below 55 degrees. It is important that residents of Contra Costa County continue to take precautions,” said scientific program manager Steve Schutz.

The district recommends residents use an insect repellent that is EPA-registered and includes one of the four most effective active ingredients: DEET, Picaridin, IR3535, or the repellent version of Oil of Lemon Eucalyptus.