The Contra Costa Mosquito and Vector Control District on Friday warned of more mosquitoes and birds that have tested positive for West Nile virus around the county.

The district said dead birds that have tested positive for the virus were found in Clayton, Concord, Crockett, Danville and Walnut Creek, while Concord and Discovery Bay also had trapped mosquitoes that tested positive for the virus.

Sentinel chickens, which are placed around the county by the district to measure the spread of the virus, also tested positive recently for the virus in Oakley and Holland Tract, an island in the Delta in East Contra Costa County.

In all, 50 dead birds, 18 groups of trapped mosquitoes and seven sentinel chickens have tested positive for West Nile virus in the county this year, according to the vector control district.

With the season changing from summer to fall Friday night, the district sought to remind residents that mosquitoes will still be a problem until daily low temperatures consistently drop below 55 degrees in the region.

"The risk of disease transmission persists, and residents should continue to take precautions to avoid mosquito bites and eliminate sources of standing water on their properties," district scientific program manager Steve Schutz said in a statement.

Since 2003, 77 people in the county have been diagnosed with West Nile virus. In 2006, two people in the county died from the virus. Most people infected with West Nile do not feel sick, but about 1 in 5 develop a fever and other systems, according to the federal Centers for Disease Control and Prevention.

People can report dead birds -- often an indicator of the virus -- by phone at 877-WNV-BIRD or online at https://westnile.ca.gov/report.php. Contra Costa County residents can also request mosquito service for their residential properties or report neglected swimming pools that can spawn large quantities of mosquitoes by calling 925-685-9301 or going to this website.