Two nights of aerial spraying for mosquitoes in Davis yielded average kill rates of 48 percent one night and 56 percent the next in "exposed" sentinel cages, said Bill Reisen, director of field stations for the UC Center for Vector-borne Disease Research.

The entomologist led a team that set out the cages around the city on Aug. 8 and 9, in research carried out in conjunction with the Sacramento-Yolo Mosquito and Vector Control District.

The district contracted for the aerial application of insecticide in Davis and Woodland in an effort to break the West Nile virus cycle: Mosquitoes feed on infected birds and then transmit the virus to people and other birds.

As of 4 p.m. Aug. 18, the state had tallied 68 cases of human infection, including one death (Butte County). Human cases numbered 15 in Yolo County and four in Sacramento County, according to the state report.

Reisen's research team designated 21 sentinel cage placements as "exposed," even though some were under canopies of varying degree. The team placed 21 other cages under vegetation, designating them as "covered" cages. Each of the 42 screened cages held about 20 mosquitoes.

The aerial spraying in Woodland produced average kill rates of 92 percent the first night and nearly 70 percent the second night in exposed cages, according to preliminary data from the mosquito control district.

Kill rates in the covered cages ranged from 9.7 percent to 19.2 percent in Davis, and from 20.9 percent to 57.6 percent in Woodland, the preliminary data show.

Post-spray trapping showed Culex tarsalis and Culex pipiens populations mostly in decline, with the district's preliminary data showing control rates ranging from 25 percent to nearly 78 percent. To figure the control rates, the district counted trapped mosquitoes within the spray area and outside the spray area.

The Culex tarsalis population in Davis appeared to be a lone exception -- increasing 11.2 percent since the aerial spraying.

Also, since the spraying, three mosquito samples tested positive for West Nile virus. One of these samples came from the spray zone, just outside the Davis city limit, the mosquito control district reported. Another sample came from the rural area between Davis and Woodland, and the third came from Sacramento County.

Nevertheless, district Manager David Brown said in an Aug. 18 news release: "Our plan did what it was designed to do. We believe we were able to interrupt the disease transmission cycle of West Nile virus through our comprehensive and integrated approach."

The district used EverGreen Crop Protection EC 60-6 for the aerial spraying. Pyrethrins, derived from chrysanthemums, are the active ingredient in the insecticide.

The aerial spraying drew vocal opposition in Davis. The district continues to spray from the ground and to urge the public to drain standing water that can provide mosquito habitat.

The news release states that "a full evaluation" of this year's control efforts will be conducted after the mosquito season. This evaluation will include such variables as geography and weather, and "further research provided to the University of California, Davis," according to the news release.

As of Aug. 18, the district had confirmed West Nile virus in 75 samples of multiple mosquitoes trapped in Yolo and Sacramento counties. Also, the district had detected West Nile virus in 74 dead birds, seven sentinel chicken flocks and one horse.

Most people who are infected with West Nile virus do not develop any disease at all, state and federal health authorities say. According to the California Department of Health Services, up to 20 percent of people, about one in five, will show mild symptoms such as fever, headache, body aches, nausea, vomiting and sometimes swollen lymph glands, or a rash on the chest, stomach and back.

About 80 percent of people who are infected will not have any symptoms, state health authorities say.

In severe cases, affecting less than 1 percent of people who are infected -- about one in 150 -- symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent.

Among severe cases, fatality rates range from 3 percent to 15 percent, with the highest rate applying to elderly people, according to the U.S. Centers for Disease Control and Prevention.

Yolo County reported 12 human infections and no deaths last year, while Sacramento County reported the most infections in the state, 177, and one death among 19 statewide.