Sweltering temperatures may have slowed human activity lately but potentially deadly West Nile virus-carrying mosquitoes have been more active than usual.

"Heat drives a lot of the West Nile virus activity," said Jamesina Scott, district manager and research director of the Lake County Vector Control District.

Vector control officials statewide are reporting high numbers of infected mosquitoes earlier in the year than usual.

Heat boosts transmission of the disease both by reducing the time it takes for a mosquito to mature from an egg to a flying biter and by increasing virus replication within the insect, Scott said.

In Lake County, two batches of mosquitoes captured since June 19 have tested positive for the virus, she said.

"This has been a really early year for us," Scott said.

No infected mosquitoes have been reported in Sonoma County so far this year, but the Marin/Sonoma Mosquito and Vector Control District has detected the virus in two dead birds found in Marin County.

The virus has been detected in 24 California counties this year, according to the state's West Nile website. The infected include a Sacramento man, 106 birds, one chicken and one squirrel. The man died, but he had other medical conditions that could have contributed to his death, state health officials said.

About half of the West Nile cases found in California have been in Sacramento and Los Angeles counties.

State officials said the incidence of West Nile virus may rise further with global warming.

Less than 1 percent of people infected with the virus develop serious illness, officials said. About 20 percent will experience milder symptoms that can include body aches, nausea, rashes, swollen lymph nodes and vomiting. Most people who become infected will not have symptoms.

The disease is transmitted to humans through mosquito bites. Mosquitoes pick up the virus when they feed on infected birds.

Vector control agencies are working to reduce mosquito populations, but residents needs to do their part to limit spread of the disease, officials say.

There is no cure for the virus, so prevention is the best medicine, Scott said.

That includes eliminating stagnant water — whether in an irrigation ditch, swimming pool or bucket of water — in which mosquitoes breed; avoiding going outside at dawn and dusk when mosquitoes are most active; and wearing mosquito repellent, she said.

You can reach Staff Writer Glenda Anderson at 462-6473 or glenda.anderson@pressdemocrat.com