Sacramento County is not alone in trying to beat back increasing swarms of mosquitoes carrying the West Nile virus.
To the north and south, mosquito control districts are aerially spraying their rice fields and other crops with chemicals identical or similar to those used over northern Sacramento County.
Epidemic or near-epidemic levels of infection have been noted in at least 12 districts statewide, out of 40 that are doing mosquito trapping, according to data released Tuesday by the state Department of Health Services.
Among them are mosquito control districts in Placer and the Sutter-Yuba areas.
Sign Up and Save
Get six months of free digital access to The Sacramento Bee
Officials with several nearby districts said they would consider spraying over cities if that's what it takes to defeat a sometimes-dangerous disease - although none is planning such spraying now.
The actions taken in Sacramento fall in line perfectly with state guidelines on how to attack a mounting outbreak, said Charlie Dill, manager of the Placer Mosquito Abatement District.
One Sacramento-area West Nile "hot spot," which includes Citrus Heights, Elverta and parts of Orangevale, bleeds over into the older sections of Roseville, crossing district lines, Dill said.
He sees no need for aerial treatments there, though, partly because it is such a small part of his entire district that he can send in crews to target storm drains and give residents door-to-door education. He also flat out couldn't afford it.
"Even if my health officer declared a state of emergency, my response would have to be, we have no money to put an airplane up," he said.
Across the state, many mosquito districts track West Nile by trapping live mosquitoes, separating them by gender and species, then testing "pools" or batches of up to 50 insects at once.
If a pool of 10 mosquitoes tests positive, data gatherers presume there's a minimum infection rate of at least one mosquito in 10, but no one is really comfortable with such small numbers.
"The more the better," said Vicki Kramer, chief of the vector-borne disease program for the state Department of Health Services. As pools are grouped together for statistical analysis, "anything under 100 (mosquitoes) is going to be somewhat biased. Anything above 500 is a little more solid."
The state compares districts by adding pools together and doing other calculations to come up with a figure that shows how many infections there are, at a minimum, for every 1,000 mosquitoes. Anything over a minimum infection rate of five per 1,000 is seen as "epidemic conditions or the potential for an epidemic," Kramer said.
According to Tuesday's statewide tally, the Sacramento-Yolo Mosquito and Vector Control District's worst problem species showed up at 11.4 out of 1,000.
In spots, it could easily be worse than that.
Kramer stressed that state data can sometimes mask smaller, regional spikes when they're part of a larger, less troubled area. Local officials have said that in the northern Sacramento area, the number has gone as high as 15 in 1,000, and one federal official who analyzed the same data said in a letter that some Sacramento levels could actually be closer to 23 or 27 per 1,000.
Sacramento "certainly is among the highest" in the state, said Kramer.
Others nearby are also having serious West Nile problems.
Placer reports 6.1 per 1,000, and the Sutter-Yuba Mosquito and Vector Control District comes in at 5.7. El Dorado County isn't measuring infected mosquitoes, but testing could begin if large numbers of dead birds show up in specific areas.
Farther south, the San Joaquin Mosquito and Vector Control District recently found some minimum infection rates of 10 mosquitoes per 1,000.
The San Joaquin, Sutter-Yuba and Sacramento-Yolo districts have long sprayed agricultural areas aerially to combat mosquitoes, although locally only Sacramento has also flown over cities.
While some had suggested that Sacramento held the dubious title of first in the state for urban aerial spraying, at least one other California city has been sprayed previously.
The Merced County city of Atwater, with a population of about 30,000, was treated by air with a product called Pyrenone last week and will be sprayed again tonight, said Allan Inman, manager and entomologist for the Merced County Mosquito Abatement District.
"We had no complaints from anybody," Inman said, although residents were notified through a newspaper article.
About the writer:
- The Bee's Carrie Peyton Dahlberg can be reached at (916) 321-1086 or firstname.lastname@example.org.