As fears grow that an unseasonably warm winter could lead to a severe West Nile virus season in the Bay Area, statewide vector control agencies are facing a huge new obstacle in their fight against the mosquito-borne illness.

Mosquitoes already have awakened from their hibernation, and a late rain could create perfect breeding conditions for West Nile, just as a federal court ruling imposes strict regulations on the use of mosquito-abatement pesticides.

The ruling, which took effect in the fall, requires pesticide use to adhere to the Clean Water Act, meaning seasonal fogging may cease in parts of the Bay Area. That will increase the chances of humans getting infected with the potentially fatal virus, experts say. A bill that would free vector control agencies from these rules is stuck in Congress.

Meanwhile, the agencies are watching closely an infestation of the Asian tiger mosquito in Southern California. This dangerous mosquito can carry not only West Nile but also dengue fever, known as "break-bone fever" because of its accompanying joint and muscle pains. This mosquito was found in Santa Clara County six years ago but was quarantined before it could spread.

Most people bitten by a mosquito carrying West Nile experience no symptoms. Twenty to 30 percent, however, will contract West Nile fever and flu-like symptoms, according to the Contra Costa Mosquito & Vector Control District. Since 1999, the U.S. has had more than 30,000 reported human cases, with 1,220 deaths.

"We could definitely see an increase in the number of human cases of West Nile virus," said Steve Schutz, scientific program manager for the Concord-based agency.

Scientists from the state's 65 vector control agencies are meeting at their annual conference in Burlingame this week to discuss the issues.

Leaders of the mosquito-abatement agencies in Alameda, Contra Costa, Santa Clara and San Mateo counties all said they believe the restrictions on fogging are unnecessary and could complicate long-term efforts to combat West Nile.

In 2001, an herbicide spill into an Oregon creek killed 92,000 steelhead, sparking a lawsuit over pesticide use in waterways. Since then, the Environmental Protection Agency has sought to develop new rules.

A 2009 federal appeals court ruling finalized the regulations and gave states two years to prepare. The court order went into effect Nov. 1.

Vector control agencies use two chemical types -- pyrethroids and organophosphates -- to kill adult mosquitoes. The new rules forbid the agencies to use either type near a waterway designated as seriously polluted, or "impaired," by the federal government if the chief pollutant in the waterway is from the same chemical family.

In Contra Costa, Kirker Creek in Pittsburg is impaired by pyrethroids, and numerous creeks in the West Nile hotbed of East Contra Costa, including Marsh Creek, are polluted with organophosphates. Contra Costa mosquito abaters will not be able to rotate the chemicals, meaning the mosquitoes will have a higher chance of building resistance, said Deborah Bass, spokeswoman for Contra Costa Mosquito & Vector Control.

West Nile is typically less common in Alameda, Santa Clara and San Mateo counties, where officials don't foresee an immediate effect on their fogging operations from the court ruling. But they worry about the long-term consequences as more waterways are declared impaired and monitoring requirements increase.

"Then it can become a big problem," said John Rusmisel, district manager of the Alameda County Mosquito Abatement District. The district typically fogs for mosquitoes two to three times a year, he said, typically in the Tri-Valley region around Pleasanton, Dublin and Livermore.

Dana Perls, Northern California community organizer for Pesticide Watch, said, "There are many ways of targeting West Nile virus without doing widespread fogging, and we believe nontoxic practices are more effective and healthier," and fogging can cause a far greater medical risk.

"There are a lot of ingredients we don't know about in the fogging cocktail," Perls said. "We don't want that mixed into our groundwater."

With months before the fogging season begins, mosquito control agencies are crossing their fingers and lobbying legislators to halt the regulations.

"It's the only known way to reduce adult mosquito populations," said Gary Goodman, assistant manager for the Sacramento-Yolo agency, stressing that the products are sanctioned by the EPA.

House of Representatives Bill 872, which would reduce the regulatory burdens, passed the House but has stalled in the U.S. Senate. The bill has enough votes to pass, but Sen. Barbara Boxer, D-Calif., has put a hold on it, said Sacramento-Yolo manager David Brown.

Staff writer Craig Lazzeretti contributed to this report. Contact Matthias Gafni at 925-952-5026. Follow him at Twitter.com/mgafni.

West Nile activity

Statewide human cases*
2004: 779 (29)
2005: 880 (19)
2006: 278 (7)
2007: 380 (21)
2008: 445 (15)
2009: 112 (4)
2010: 111 (6)
2011: 155 (8)

( )= fatalities


Human cases by county
in 2011*
Contra Costa: 3
San Joaquin: 5
Santa Clara: 1
Rest of the Bay Area: 0

* Many less serious West Nile cases go unreported because symptoms often mimic the flu.
Source: California Department of Public Health