The local mosquito control agency says it has seen the first Sonoma County birds infected with West Nile virus this year.

The Marin/Sonoma Mosquito and Vector Control District confirmed Monday that two crows from Santa Rosa and one barn owl found in Cotati have tested positive for the virus.

Two birds in Marin County tested positive earlier this year, one each in May and June, spokeswoman Nizza Sequeria said.

By this time last year, no birds had tested positive in either county, though that is not necessarily an indication that this will be a worse year for the virus, she said.

Nearby Napa County has reported no cases this year, said Wes Maffei, manager of the Napa County Mosquito Abatement District.

Most cases of West Nile virus in humans are reported in the late summer and early fall, he said, though infection can occur during any of the warm months when mosquitoes are active.

West Nile virus is not usually serious in humans; the Centers for Disease Control reports that only about 20 percent of those infected will show any symptoms, usually a mild flu-like illness.

In about 1 percent of cases, however, the virus can turn dangerous, causing a severe, even fatal, neurological illness. That has led health officials to keep a close eye on the virus, which has been found in 19 states so far this year.

There have been 14 reported cases of the virus in humans this year in California, including two deaths, according to the California Department of Public Health Services. The deaths were reported in Sacramento and Los Angeles counties.

The virus is transmitted to humans by certain types of mosquitoes. Birds are a good indicator of the presence of the virus, since the virus transmits back and forth with them and mosquitoes.

Sequeria said residents should not be complacent because this has been a cool, dry year, which would seem to be unfavorable for mosquitoes. The insects are quick to take advantage of even small puddles of standing water. Even old, unsealed or cracking septic tanks provide good habitat.

"Mosquitoes are very opportunistic," she said.

The district has treated the Laguna de Santa Rosa with insecticides twice this year since its slow-moving waters provide good breeding grounds for mosquitoes, she said, but mosquitoes can breed almost anywhere there is standing water.

Health officials encourage homeowners to eliminate breeding grounds for mosquitoes by emptying containers that hold water for longer than about five days, such as buckets, old fountains and discarded tires.

They also encourage people to reduce their exposure to mosquitoes by limiting activity at dawn and dusk, when the insects are most active, wearing protective clothing or insect repellent when outside, and keeping screens on windows to keep the insects from coming indoors.

They also ask residents to report dead birds to the local mosquito control district so the bodies can be tested for the virus.

For information on the virus or to report dead birds, call (877) WNV-BIRD (968-2473) or www.westnile.ca.gov.

You can reach Staff Writer Sean Scully at 521-5313 or sean.scully@pressdemocrat.com.