Lake County reports signs of West Nile virus

West Nile virus has been detected in three mosquito samples collected in Lake County over the past week, the first signs of the virus’ arrival in the North Bay region this year.

Lake County is now one of at least 25 California counties to report evidence of the potentially fatal mosquito-borne virus in 2024. Many of the cases have been in central and southern parts of the state, according to the California Department of Public Health. So far no human cases have been reported.

Sonoma, Napa, Marin and Mendocino counties have not reported the virus this year, though that doesn’t automatically mean it is not present.

First detected in the United States in 1999 and in California in 2003, West Nile virus spread to all 58 counties by 2004 and has remained a fixture throughout the state and the country, resulting in at least 25 human cases in 12 U.S. states so far this year.

Carried by several species of mosquito, particularly those out at dawn and dusk, the virus causes low-level symptoms or no symptoms at all in 70% to 80% of those infected, according to the federal Centers for Disease Control and Prevention. Many of those cases go undetected.

When symptoms are present, they typically include fever, headache, body aches, gastrointestinal symptoms or rash, experts say. In about 1% of cases, the virus can cause neuroinvasive disease including meningitis, encephalitis and acute flaccid myelitis, which can result in paralysis, the CDC says. About 10% of cases involving neuroinvasive disease result in fatality.

Lake County last year had six human cases, including one fatality.

Human cases also have been reported over the past two decades in Marin, Napa and Mendocino counties, as well as in Sonoma County, though a Petaluma man who fell ill in 2005 was believed to have been infected while traveling in Sacramento.

Between 2003 and 2023, California reported 8,116 symptomatic human cases, including 4,979 cases of neuroinvasive disease.

All five counties have had numerous cases of West Nile virus in songbirds, mosquito samples and in sentinel chicken flocks, which are monitored to detect the virus. Horses in the area have been infected, as well.

Lake County’s recent virus-positive mosquitoes were trapped July 12 in Kelseyville and July 16 in Lower Lake, the Lake County Vector Control District said.

Residents are advised to restrict spread of the virus by eliminating standing water where mosquitoes may breed, wearing EPA-registered insect repellents and long-sleeved shirts and pants outdoors, installing screens, maintaining swimming pools and using mosquito fish in ornamental ponds or other water sources that cannot be drained.

In Sonoma and Marin counties, residents experiencing mosquito problems can request services from the Marin/Sonoma Mosquito & Vector Control District at www.msmosquito.org.

You can reach Staff Writer Mary Callahan (she/her) at 707-521-5249 or mary.callahan@pressdemocrat.com. On X (Twitter) @MaryCallahanB.