Spells of warm weather in April and May coupled with late-spring rains are blamed for the early arrival of West Nile virus in Sacramento and Yolo counties this year.
Citing high infection rates of birds and mosquitoes, the Sacramento-Yolo Mosquito & Vector Control District announced that it will conduct aerial spraying of 30,000 acres in south Sacramento County over two nights next week.
The area scheduled for treatment from 8 p.m. to midnight Monday and Tuesday extends from Elder Creek Road south to Elk Grove Boulevard and from Bruceville Road on the west to Excelsior Road on the east and bordering with Grant Line Road.
So far this year, 29 dead birds and 42 mosquito samples have tested positive for West Nile virus in Sacramento County, with two dead birds and two mosquito samples in Yolo County.
"We usually don't see it in Yolo County till much later," said Luz Maria Rodriguez, spokeswoman for the Mosquito & Vector Control District.
Last year, the first infected mosquito samples in Yolo County were found July 21, and the first bird wasn't found until Aug. 23.
With the exception of a dead bird found in February 2011, and considered an anomaly, the virus was first detected in birds in Sacramento County last year on June 21 and in mosquito samples on July 6.
"If the current trend continues," Rodriguez said, "it will probably be a long and intense mosquito season."
The aerial spraying and ongoing ground treatments are attempts to quickly reduce adult mosquito populations, interrupt the virus transmission cycle and protect public health, officials said.
West Nile virus is transmitted to humans, birds, horses and other animals by infected mosquitoes.
The mosquitoes contract the disease from infected birds while taking blood and can pass it on when they bite animals or humans.
Most people who are bitten by infected mosquitoes show no symptoms of the virus, but about 20 percent of people who are infected develop West Nile fever.
The symptoms typically are similar to the flu, but severe symptoms may include fever, seizures, weakness, change in mental status, paralysis and death.
As in recent years, the majority of West Nile virus activity to date has been found in the southern portion of Sacramento County, particularly in the areas of Calvine and Bradshaw roads.
Rodriguez noted that the area includes agricultural lands as well as a large number of abandoned swimming pools, creating prime breeding grounds for mosquitoes.
The 30,000 acres scheduled for aerial spraying next week are comparable to the 32,500 acres 21,700 acres in the Elk Grove area and 10,800 in Wilton treated over two nights last year, she said.
Either a pyrethroid or an organophosphate insecticide registered for use in mosquito control by the U.S. Environmental Protection Agency and California EPA will be used in the treatments.
Insecticides may vary from year to year so mosquitoes don't become resistant to them, but Rodriguez said all have been used in communities nationwide for mosquito control.
Although most of the West Nile virus activity so far this year has been centered in south Sacramento County, residents throughout the region are urged to eliminate mosquito breeding conditions such as standing water and avoid mosquito bites.
Officials stress that virus activity has been detected in other areas of Sacramento County.
Updates on West Nile virus activity and aerial spraying are available at: www.FighttheBite.net
Residents can sign up on the website to receive notification of spraying via email, Facebook or Twitter. Updates also are available by calling (800) 429-1022.