A mosquito found in West Santa Rosa tested positive for West Nile virus, according to the Marin/Sonoma Mosquito and Vector Control District.

As a result, vector control technicians will check all known mosquito sources, search for new sources and place additional traps in surrounding areas.

"We encourage residents to report mosquito problems and/or potential mosquito breeding areas so that we can do everything possible to prevent the spread of West Nile virus," district manager Jim Wanderscheid said in a statement.

West Nile virus activity has been mild in the region this year, with no humans, horses or other animals reported infected. The last time West Nile virus was found in either Marin or Sonoma counties was in 2008, when it was detected in two pools of mosquitoes and 12 birds.

"In general, West Nile virus activity is typically fairly low in our coastal counties compared to the Central Valley region and Southern California," said Dr. Vicki Kramer, chief of the Vector-Borne Disease Section of the California Department of Public Health. "I think it's important that people recognize that there is some West Nile virus activity in Sonoma County, and people should take precautions to repel mosquitoes."

Statewide, more than 2,000 cases of mosquito pools infected with West Nile virus have been found this year. The virus also has infected 115 humans this year, mostly in Los Angeles and Kern counties, where five people have died. A similar number of mosquito cases were found in the state in 2008.

The disease is maintained in nature in a cycle between mosquitoes and birds, so the state has a program to track the virus in dead birds, Kramer said. However, the Marin/Sonoma Mosquito and Vector Control District is not participating this year, while virtually all other local districts are, she said.

Representatives of the Marin/Sonoma Mosquito and Vector Control District did not return phone calls seeking comment.

Residents are reminded that mosquitoes are most active between dusk and dawn. The district encouraged residents to wear long-sleeved clothing and to apply insect repellent that contains DEET, picaridin or oil of lemon eucalyptus.