Mosquitoes collected last week near Kelseyville have tested positive for West Nile Virus, a possible indication that the 2013 season will be a bad one, Lake County Vector Control officials said.

"This is the earliest date that we have detected WNV activity in Lake County, and it may be a warning that this will be a very active year for WNV," Jamesina Scott, manager and research director for the Lake County Vector Control District, said in a written statement. "Last year we saw more WNV activity in Lake County than we had since 2005, and had the first human case since 2006."

West Nile Virus is carried by mosquitoes and can infect humans. It causes symptoms only in about one in five people but in rare cases can lead to severe illness, permanent neurological problems or even death.

As of Wednesday, the virus had turned up in 13 California counties so far this year, with one human death reported in Sacramento.

Thirty-six birds and one sentinel chicken have been found dead with the virus, including one in Marin County, according to the state Department of Health.

Most dead birds were found in the Central Valley and Southern California. None has been found in Sonoma, Napa, Mendocino or Lake counties.

Residents are urged to take precautions by ensuring there are no still water sources or puddles around their homes where mosquitoes can breed and by avoiding being bitten through use of insect repellants with EPA—registered active ingredients like DEET, Picaridin or oil of lemon eucalyptus.