CONCORD, CALIFORNIA - The Contra Costa Mosquito and Vector Control District (District) reports two groups of mosquitoes have tested positive for West Nile virus (WNV) in Contra Costa County. The mosquitoes were collected from a trap in the town of Discovery Bay. These are the first mosquitoes to test positive for WNV so far this year in Contra Costa County.

West Nile virus is transmitted by the bite of an infected mosquito. As mosquitoes may transmit WNV with a single bite, it is important for Contra Costa County residents to take precautions to reduce the risk of mosquito bites, particularly now, as the summer heat increases.

"This week's increasing temperatures can increase both mosquito production and West Nile virus risk because mosquitoes can develop from egg to adult more quickly in hot weather, and West Nile virus can grow more efficiently when temperatures are consistently warmer than 55 degrees. With the increasing risk of West Nile virus, the District recommends Contra Costa County residents wear insect repellent and report mosquito activity to the District," said Steve Schutz, Ph.D., Scientific Program Manager.

According to the Centers for Disease Control and Prevention (CDC) when used as directed, Environmental Protection Agency (EPA)-registered insect repellents are proven safe and effective. The District recommends using EPA-registered insect repellents with one of the following active ingredients:
  • DEET
  • Picaridin
  • Repellent version of Oil of Lemon Eucalyptus
  • IR3535

Always follow the instructions on the label when using repellent.

Another way to reduce the risk of mosquitoes is to dump out containers holding any amount of standing water and scrub the inside of the containers to remove mosquito eggs. Containers can include buckets, cans, toys, plant saucers, bird baths, rain barrels, and any other items that may hold water for an extended period of time. It is important to dump out any amount of water because mosquitoes can develop from egg to biting adults in as little as a bottle cap full of water. Also, make sure window and door screens do not have any rips or tears and that screens fit properly to prevent mosquitoes indoors.

Contra Costa County residents can report dead birds by phone at (877) WNV-BIRD (968-2473) or online. County residents can also request mosquito service for residential property by calling (925) 685-9301 or online.

Since 2005, 75 people in Contra Costa County have been diagnosed with West Nile virus. In 2006, two people died from the disease. For human case information, please visit the California Department of Public Health Vector-Borne Disease Section online.
Contra Costa Mosquito & Vector Control District, an independent special district and public health agency, is located at 155 Mason Circle in Concord.