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Placer County has higher than normal levels of West Nile activity in 2018

By: Brody Fernandez Of Gold Country Media
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Nearly 200 mosquito samples in Placer County tested positive for West Nile virus, according to the Placer Mosquito Vector Control District. Twenty-four dead birds and two chickens have also tested positive for the deadly virus.

The western part of Placer County, including Roseville, Rocklin and Lincoln, according to the district, tends to see the highest levels of mosquito and West Nile virus activity during the summer.

In a press release Monday, the district describes what measures are being taken. The agricultural areas of western Placer County are scheduled for treatment of larval mosquito populations. Larval treatments involve an EPA-registered biorational mosquito larvicide applied by low-flying aircraft in an effort to prevent the emergence of adult mosquitoes and reduce the risk of mosquito-transmitted diseases. Treatments occur during daylight hours, most commonly in early morning through early afternoon.

“There are many factors that go into concentrated mosquito populations. Elevation plays a role along with the topography,” said Placer County Mosquito Vector Control District spokeswoman Ada Barros. “The valley is nice and flat and standing water is more prominent here in this part of Placer County as opposed to the foothills and the High Sierra, where gravity and topography pulls a lot of the standing water downhill and dissipates into small pools.”

Barros shared the top factors that lead to mosquito control.

“When humans change the landscape, it really plays a factor. We do see a lot of mosquitoes come in from agriculture. Rice fields especially. For a good part of the rice-growing cycle,” Barros said, “farmers need standing water so naturally we do a lot of our treatment out in the rice fields. It helps we have something we can see that we can treat like mosquito larva (in the standing water).”

Barros stressed temperature as a major factor.

“Our summer high temps and summer low temps in the valley are different in the foothills. Foothill lows can get lower in the summer as opposed to the valley where temps stay nice and warm (ideal for mosquitos),” Barros said. “Sometimes, we see triple-digit temps into the middle of August and that can play a role.”   

Barros was asked whether Placer County residents should be concerned.

“These cases of West Nile we are seeing are not an epidemic here in Placer County or in California. It will pop up every year,” Barros replied. “As far as infected mosquitos go, this year was pretty bad. Some years, it’s not as high.”

Awareness and caution should be highest during a relative time frame throughout the year, Barros said.

“The distinct time of year Placer County residents should be concerned is at the beginning of July to about mid-September,” Barros said.


“One of the best prevention measures of mosquito-borne disease is a shared responsibility,” Barros said. “We do our part but we can’t do it all. We rely on our residents to take precautions as well, fighting the bite together.”

Residents should dispose of unused standing water on their property, report any dead birds or large concentrations of mosquitos, and use mosquito repellant, according to Barros.

The district advocates that residents regularly check its website and map for up-to-date spraying locations at www.placermosquito.org.