Environment & Science

Orange County won't spray for West Nile mosquitoes, for now

A field sample of mosquitoes that could carry West Nile Virus.
A field sample of mosquitoes that could carry West Nile Virus.
David McNew/Getty Images

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Orange County's Mosquito Vector Control District voted Thursday against aerial spraying to combat mosquitoes carrying the West Nile virus for the time being, but the board could decide later that it is necessary.

After a meeting that lasted over four hours, the Vector Control District board denied giving sole authority to the general manager to decide when to use aerial spraying to control the mosquito population in Orange County.

Orange County has experienced a West Nile wave in recent years, along with other pockets of Southern California. 

The disease first appeared in the area in 2004, said Dr. Matt Zahn, director of epidemiology at the Orange County Health Care Agency.

“We felt like we were setting into a bit of a pattern,” he said. “Every three to five years, we would have a bump in numbers and then they would go down.”

But the last two years have been distinct.

There were 280 West Nile infections in 2014; last year there were 97 cases. Over the last two years, 17 people have died in Orange County because of the disease.

Experts aren't sure exactly why Southern California is attractive to West Nile mosquitoes. The insect can fly lengthy distances. 

Zahn said there tend to be more illnesses in North County, which includes the cities of Santa Ana, Tustin, Anaheim and Fullerton, where the population is denser. He said drainage systems play a big role in allowing standing water to pool and mosquitoes to nest. 

Since Southern California has been in a drought for that past few years, it may seem backwards to think that there are pools of water attracting mosquitoes but UC-Davis epidemiologist Chris Barker said there are plenty of hidden opportunities.

“Water stagnates underground in urban areas,” he said. “In those areas, that’s one of the primary of the breeding grounds.”

Human activities like watering lawns affect the development of mosquito habits in Southern California communities, Barker said. 

A West Nile mosquito contracts the disease when it bites an infected bird. The disease replicates inside the mosquito's salivary glands. When that mosquito feeds, it injects the virus into the host—a human—and that’s when the risk for the virus occurs. 

Transmission is more intense during the summer time because when it's warm, a mosquito can start transmitting the virus to humans only seven days after ingesting it.

Barker said in California, experts prefer to eliminate mosquito larvae at their immature stages with bacterial larvicides and mosquito hormone regulators that stunt growth. 

Once mosquitoes mature and start feeding, they could start transmitting West Nile to humans. 

If they're not killed at the larvae stage, spraying can come next. 

“You’re only options really is to start killing the adult mosquitoes because those are flying around,” said Barker. “They’re actually infected at that point.”

A group of Orange County residents concerned about the environmental and health affects of aerial pesticides launched a petition to stop the county Vector Control District from spraying. They want the agency to focus on public education and awareness, instead, as a way to control mosquito populations.

A spokesperson for the O.C. Mosquito Vector Control District said the district manager would be working on different palatable ways to authorize aerial spraying including convening an emergency board meeting to vote on it or gathering a special advisory council to work with district staff.