CONTRA COSTA COUNTY (BCN) — A group of mosquitoes on Bethel Island in unincorporated Contra Costa County tested positive for the West Nile virus, officials with the Contra Costa Mosquito & Vector Control District confirmed on Friday.

“Mosquitoes are still present and the rain may help more thrive if the weather stays warm,” vector control district scientific program manager Steve Schutz said in a statement.

Schutz is encouraging residents to dump all standing water and clean gutters since they can breed tens of thousands of mosquitoes.

Mosquitoes cannot breed without water. Residents are also urged to use mosquito repellent containing DEET, picaridin or oil of lemon eucalyptus when they are outside.

Though residents should try to stay indoors at dawn and dusk when mosquitoes are most prevalent, vector control officials said.

Residents are also urged to maintain their swimming pools because just one can breed more than 1 million mosquitoes and affect people up to five miles away.

Dead birds may indicate that the West Nile virus is active in the area. So vector control officials are asking residents to report any dead birds to the state hotline at (877) 968-2473 or to public health officials at http://www.westnile.ca.gov/.

Three human cases of the virus have been reported this year in Contra Costa County. Since 2005, 56 people in the county have been diagnosed with the disease and in 2006 two people died from it.