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.