Combating the worst outbreak of West Nile virus infections ever recorded in Orange County, officials Monday will begin spraying targeted neighborhoods with insecticide in an attempt to stop the increasing number of residents infected by the disease.

From 3 to 5 a.m. Monday through Thursday, vector control trucks specially equipped with foggers will begin spraying four selected neighborhoods in Santa Ana, one on the northeast side of town and the other three on the western side.

“We’re trying to cover as much area as quickly as possible,” said Jared Dever, a spokesman for the county’s vector control district. “One of the final tools in our arsenal is to go to truck-mounted fogging.”

County officials have been particularly alarmed by the amount of mosquitoes infected with the virus this year. On average, 10% to 20% of the mosquito samples collected in Orange County test positive for West Nile. This year, the rate has reached 80%, Dever said.


The number of people infected with the West Nile virus in Orange County this year has increased to 91 as of Thursday, compared with a total of 12 cases in 2013. A Seal Beach woman in her 80s and a Huntington Beach man in his 70s were the first reports of people in the county who died from the virus this year.

At least nine people have died from the virus in California this year.

The virus is usually transmitted to humans from a bite by an infected mosquito and can cause flu-like symptoms and, in rare instances, death. For most people, the risk of serious illness is low. People 50 years of age or older have the greatest risk of developing serious complications.

Santa Ana was selected as the first city to fog because “they have tested consistently positive for the virus over and over again,” Dever said. Officials are waiting to see how effective this week’s fogging will be before trying it in other hard-hit areas such as Anaheim, Fullerton and Orange.


Mosquitoes become infected with the West Nile virus after feeding on birds that had the disease.

Fogging is the only known way to control the population of adult mosquitoes, Dever said. This is the first year county officials have gone to such an extreme in a dense, urban population, he said.

Residents have been instructed to stay inside during and at least half an hour after the fogging. Doors and windows need to be closed and non-recirculating air conditioners shut down until half an hour after the spraying. Pet food bowls and outdoor fish ponds should be covered.

The active ingredient in the pesticide, AquaANVIL, poses a minimal risk to human health and the environment, according to the California Environmental Protection Agency. It will not stain stucco or damage cars or house paint, Dever said. The county has used the fogging trucks routinely on wetlands and other open spaces in the past.


Officials this summer have tried many other ways to break the cycles of virus transmission, but “it hasn’t helped to the point that we wish it would,” Dever said.

West Nile reports are also up across the state. Ron Chapman, director of the California Department of Public Health, said this week that the proportion of mosquitoes infected with West Nile is at the highest level ever detected in the state.

So far this year, 37 California counties have detected the virus. At least 181 cases have been reported to CDPH — a significant increase compared to the 101 cases reported by this time last year. The five-year average at this time of year is 56 reported cases.

“We expect to see more people become infected as this is the time of year when the risk of infection is the highest,” Chapman said in a statement.


Los Angeles County this year has reported 25 cases, including one death, with the largest numbers in the San Fernando Valley and the South Bay, according to the Los Angeles County Department of Public Health.

To date, vector control districts across the state have detected the virus in 1,827 dead birds, 274 sentinel chickens and 2,660 mosquito samples this year.

Health officials called on residents across California to take precautions, such as wearing insect repellent, limiting activity at dusk and dawn and draining any “standing water” in flower pots, buckets and pools, which attract egg-laying mosquitoes.

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