The year?s first signs of West Nile virus on the North Coast have been found in mosquitoes and birds in Sonoma, Lake, Mendocino and Marin counties in the past week, health authorities reported.
Tests confirmed Wednesday that mosquitoes trapped last week in marshes southeast of Petaluma were infected with the virus as was a house finch found dead in San Rafael, said Jim Wanderscheid, manager of the Marin/Sonoma Mosquito & Vector Control District.
?They?re the first two cases this year in either county,? he said.
Mendocino County authorities also received confirmation of their first known case of West Nile virus on Wednesday ? a songbird found dead in Ukiah and dropped off at the Environmental Health Department.
Mendocino County does not have a vector control district and, unlike Sonoma and Lake counties, does not regularly test mosquitoes or strategically placed ?sentinel? chickens for the virus.
In Lake County, mosquitoes trapped last week near Lakeport were the first this year found to be carrying the virus, said Lake County Vector Control District Manager Jamesina Scott.
West Nile virus is spread by the bite of infected mosquitoes.
Of about two dozen mosquito species in the region, the Culex pipiens, or house mosquito, is the most likely to transmit the virus.
Human cases have been reported this year as far north as Solano County, where a Vacaville man tested positive for the mosquito-borne virus last week after donating blood, Solano County authorities reported. Like an estimated 80 percent of people who become infected, he had no symptoms of the disease, officials said.
Symptoms can include headache, fatigue, fever, skin rash, swollen lymph glands, nausea and eye pain. No fatalities have been reported by state health officials this year. Of 380 people who tested positive last year in California, 21 died.
At least 46 Californians have tested positive for the virus so far this year, most of them in Southern and Central California, according to the California Department of Public Health. Nationwide, there have been at least 62 human cases.
The virus ? common in Africa, west Asia and the Middle East ? was first reported in the United States in New York City in 1999 and has since made a steady march west.
The first North Coast case was diagnosed in 2004 in Lake County.
Two other people in Lake County were reported infected with the virus in 2006.
Sonoma County has had had two human cases, one in 2005 and one in 2007.
Mendocino County?s two human cases were reported in 2007.
No fatalities have been reported on the North Coast.
The virus primarily affects birds. More than 800 dead birds in 27 counties have tested positive for the virus this year, the state health department reported.
Horses are also susceptible to West Nile, and veterinarians recommend the animals be vaccinated against the virus.
Two horses were infected in Sonoma County in 2007, and Sonoma and Marin counties each reported one equine death from West Nile in 2006, according to the vector control district.
The positive West Nile virus tests are a reminder that people need to take precautions against mosquito bites, Scott said.
They should eliminate standing water ? which serves as mosquito breeding grounds ? around their homes and wear mosquito repellent when outside at dawn or dusk, she said.