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Fighting West Nile: First time infected mosquitoes detected in county, officials responding aggressively
August 02, 2014, 05:00 AM By Samantha Weigel Daily Journal

Samantha Weigel/Daily Journal
Vector Ecologist Theresa Shelton sets a mosquito trap in San Mateo’s Central Park as a control site outside of where the San Mateo County Mosquito and Vector Control District fogged in response to West Nile Virus Wednesday night.

As reports of mosquitoes infected with the West Nile virus in San Mateo continue to rise, the San Mateo County Mosquito and Vector Control District is accelerating its efforts to get a grip on a statewide problem through tactical abatement treatments.

The most recent action, specific fogging on Wednesday night and Thursday morning was the largest effort so far and extended more than a square mile with an epicenter of the Sunnybrae neighborhood. Previous foggings took place in the North Central neighborhood.

Although deceased birds and squirrels have tested positive for the virus in the past, this summer is the first time the district has located infected mosquitoes and the first time it’s fogged since 2006, said the district’s Assistant Manager Brian Weber.

“The district is absolutely doing everything they can to combat West Nile virus and all the staff are working extra hard to try and help the community,” Weber said. “When it’s a matter of public health, you do everything you can.”

The district has made some changes since it first started fogging this summer, including giving the public more notice before treatment and using a different pesticide that is less harmful to humans.

Incidents are on the rise across the state and experts are trying to determine what’s leading to the increased accounts of birds, squirrels, mosquitoes and even humans testing positive, Weber said.

One possible theory is due to the lack of rain, the storm drain system isn’t being flushed out as often and could foster mosquito breeding grounds, Weber said.

“If you look at just the state’s statistics, there’s almost three times as many positive birds this year as there were last year and there could be a lot of factors. It’s not just the drought. But we’re not really sure why this is happening,” Weber said.

According to the state’s West Nile website, 19 people, 958 birds and 978 mosquito samples have tested positive.

Only about 20 percent of humans who contract West Nile virus show symptoms and less than 1 percent develop severe illness, Weber said.

The district acts when individuals call in dead birds or squirrels that test positive. It then sets up traps around the site and tests culex mosquitoes — the genus of mosquito that carries the West Nile virus, district Vector Ecologist Theresa Shelton said.

The traps use dry ice, which mimics the exhalation from an animal, and a light to attract mosquitoes. A small fan then sucks them into a net where they are collected and separated by genus, Shelton said.

If it finds a pool of West Nile infected mosquitoes, it typically treats around a half-mile radius of the detection site by driving a truck mounted with a fogging machine, Shelton said.

Wednesday night, it treated in the 94402 zip code area and expanded it to include the Shoreview neighborhood extending southeast to the Foster City Lagoon.

It was a little over 1-square mile and was the largest treatment area yet, Weber said.

“I suspect it’s going to spread to other cities for sure,” Weber said.

Another reason reported incidents are on the rise in the county could be attributed to new DNA testing equipment the district purchased about a year ago, Weber said. The district can now perform studies in-house instead of having to send samples to the state and wait for a response, Weber said.

Since its first treatment in June when it only gave a few hours warning before fogging, the district has adapted its process in response to public concern over short notice. The district is now aiming to give a 48-hour notice before fogging and is working in conjunction with the city of San Mateo and its police department.

The police department and city are sending out notices via and the San Mateo County Emergency Alert System, said Rebecca Zito, senior management analyst with the city of San Mateo.

“We want the community to be safe and our main concern is making sure that residents have the information they need and are aware that the spraying is taking place,” Zito said. “The comments and questions we’ve been getting from the community have primarily been around the chemicals that are being used.”

Since its last two abatement treatments, the district is now using the adulticide Zenivex, which the Environmental Protection Agency has determined is safe for urban use. Previously, it used Pyrethroid, which it had on hand. Zenivex is newer and considered to be less harmful, officials said.

Residents no longer need worry about bringing in children’s toys or pet bowls and the active ingredient breaks down within an hour, Shelton said.

In order to avoid being bitten by mosquitoes, the district recommends draining any standing water and wearing proper clothing or repellent during dawn and dusk when mosquitoes are most active.

The district also requests the public participate in its efforts to prevent more cases of West Nile virus by calling the district if it finds any deceased birds or squirrels or if they notice an increase in mosquito bites.

“One of the biggest things we really want to stress is people taking care of their own properties. Dumping out standing water, as generic as it is, and to call us when they see dead birds and are getting mosquito bites,” Weber said. “That’s why we’re here as an agency. And if people don’t call us, we don’t know there’s a problem.”

To report a dead bird or squirrel call (877) 968-2473. For more information about the district visit or call (650) 344-8592. For more information about West Nile visit To sign up for alerts visit or

(650) 344-5200 ext. 106



Tags: district, weber, mosquitoes, birds, mosquito, virus,

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