West Nile virus: State set for spike in cases PUBLIC HEALTH

David Wexler, vector control inspector Contra Costa Mosquito & Vector Control District, dons a respirator and gloves before he treats an area using vectobac 12AS in a channel which was sampled and found to contain a number of mosquito which was above the districts threshold on Friday, June 15, 2012 in Concord, Calif. less David Wexler, vector control inspector Contra Costa Mosquito & Vector Control District, dons a respirator and gloves before he treats an area using vectobac 12AS in a channel which was sampled and found to ... more Photo: Lea Suzuki, The Chronicle Photo: Lea Suzuki, The Chronicle Image 1 of / 4 Caption Close West Nile virus: State set for spike in cases 1 / 4 Back to Gallery

Public health experts are bracing for an especially active West Nile virus season resulting from an unusually warm winter combined with a new federal regulation that is hampering efforts to control virus-spreading mosquito populations.

State vector control agencies have identified eight times as many cases of West Nile virus in dead birds and almost 20 times as many cases of the virus in mosquitoes this year compared with last. West Nile has been detected in 15 California counties, including Contra Costa, Solano and Santa Clara in the Bay Area.

"Last year our first bird was on June 29. This year was on April 9," said Deborah Bass, a spokeswoman for the Contra Costa Mosquito and Vector Control District. "We're obviously out working diligently on this."

Bass and other vector control officials advise residents to start taking precautions against West Nile virus, which means draining any standing water on residential properties - even a bottle cap filled with an inch of rainwater can draw mosquitoes - and reporting dead birds. Residents also are encouraged to report standing water that isn't on their property - for example, a neglected swimming pool behind an empty house - to their vector control district.

West Nile virus is spread to humans by mosquitoes that acquire it from birds. After a human has been bitten by a mosquito, it can take two to 12 days for symptoms to show up. Symptoms include fever, headache, body aches, skin rash and swollen lymph glands.

Most people fully recover, but the virus can cause life-threatening swelling of the brain or partial paralysis. Last year 158 human cases of West Nile virus were reported in California, and nine of those people died. No human cases have been reported yet this year.

But cases of West Nile in birds and mosquitoes are already much higher than usual for this time of year. This year, 95 cases of West Nile have been found in dead birds and 91 samples from mosquito pools have tested positive.

Thriving in warm air

The unusual winter combined with a wet spring probably contributed to the increase in cases because mosquitoes thrive in higher temperatures - they reproduce and mature from larva to adult faster. Plus, the virus replicates faster at higher temperatures.

But vector control agencies also have been hampered this year by regulations that went into effect in January, requiring extra permitting procedures to spray pesticides on water supplies, said officials with the Mosquito and Vector Control Association of California, a nonprofit group that represents state vector control agencies.

A bill that would repeal the new regulations is before the U.S. Senate. Environmental groups oppose the bill because they say it would weaken the federal Clean Water Act. But the new regulations may actually lead to more pesticide use, said officials with the vector control association.

The extra permit rules mean it takes longer for vector control officials to visit possible mosquito sites, and they're missing opportunities to deal with larval mosquito populations in relatively small pools of water, according to the association.

More pesticides

If they can't control the larvae, vector control officials instead have to kill an adult mosquito population, which means using more pesticides over a wider territory, said David Brown, manager of the Sacramento-Yolo Mosquito and Vector Control District, which has reported more West Nile cases in birds and mosquitoes than any other part of the state.

"We're a little frustrated because we think the regulation has had some unintended consequences," Brown said. "We all want to control mosquitoes in the larval state. It's much preferred over controlling them when they're flying and can travel miles."