Clean up began Wednesday on the Pacoima Wash in Panorama City, considered the county's most fertile breeding ground for mosquitoes carrying the West Nile virus.
The majority of West Nile cases detected in the county this year have been in San Fernando Valley, said Michael Shaw, operations director for the Greater Los Angeles County Vector Control District.
"It's not just a Panorama City issue, but a countywide one," he said, noting that the birds carrying the virus can travel many more miles than the mosquitoes that transmit it to humans by biting them.
Mosquitoes become infected when they feed on birds carrying the disease.
Of the 32 mosquito samples that tested positive for West Nile in Los Angeles County this year, 29 were in the Valley cities of Panorama City, Granada Hills, Northridge, Encino and Van Nuys, county vector control officials found.
Fourteen of the 17 samples found in Panorama City were traced directly to the Pacoima Wash and a one-mile stretch of the wash near Parthenia Street.
The results prompted Shaw's office to call in the county Department of Public Works to use its larger equipment to clean up the area by Saturday.
The request marked the only instance this year where vector control needed help cleaning up mosquito pools, said Arthur Vander, an area engineer with public works.
As bulldozers uprooted tall grass at the Pacoima Wash late Wednesday afternoon and dump trucks waited to haul off the debris, Supervisor Zev Yaroslavsky, whose district includes the wash, and officials with the county public health, vector control and public works departments urged residents to keep an eye out for stagnant water that could provide a breeding ground for mosquitoes.
"This is a potential emergency and there's no need to be timid about this," Yaroslavsky said. "It's a serious public health concern in the county."
As of Wednesday, the virus has been detected in 19 dead birds and 33 mosquito pools in Los Angeles County, according to the California Department of Public Health.
Human cases of West Nile virus in California are noticeably up this year, with 108 humans infected compared with 68 in 2006. There have been five fatalities in the state this year, with two in nearby Kern County.
Only one human case has been reported in Los Angeles County: a San Fernando Valley resident who was briefly hospitalized with West Nile-related meningitis last week.
In its milder form, West Nile virus causes flu-like symptoms; many people are infected without suffering any symptoms, Shaw said.
But in its more severe form, it can cause encephalitis, meningitis and death.
While California health officials have said West Nile, first detected in the state five years ago, will be an ongoing problem, people can better protect themselves by wearing long-sleeved clothing, applying insect repellent and notifying vector control when a stagnant pool of water attracts mosquitoes, Shaw said.
County residents can contact the vector control district at (562) 944-9656 to report stagnant water pools.
"We are in the height of the season," Shaw said. "And with the heat and dry weather, it can lead to places being neglected where water pools accumulate. Reports of West Nile happen every year, but is the worst in the Valley this time around."