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The mosquitoes were trapped March 27, nine weeks earlier than the first positive mosquitoes last year, district spokeswoman Deborah Bass said.
Bass said it is believed that the unusually warm weather in January could have brought female mosquitoes carrying the virus out of hibernation early.
West Nile virus was first detected in Contra Costa County in 2004. Since then, 27 people have been diagnosed with the virus and two have died from it, Bass said.
The district is urging people to use insect repellent when there is a chance they will be exposed to mosquitoes, drain sources of standing water where mosquitoes could lay their eggs and check door and window screens for tears.
Anyone who finds a dead bird is asked to report it to the California Department of Public Health's Dead Bird Hotline at (877) WNV-BIRD.
The department uses the data to identify areas that have high populations of infected mosquitoes so mosquito abatement crews know where to focus their efforts.
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