SAN MATEO COUNTY -- Two dead birds tested positive for the West Nile virus on Wednesday in Redwood City and Atherton, the first time the disease has been detected this year in the county.

The birds, which were American crows, were collected in West Atherton and in the Eagle Hill-Mount Carmel area of Redwood City, according to the San Mateo County Mosquito & Vector Control District.

Summer is the peak season for West Nile. The virus has already been detected in adult mosquitoes several times in San Jose since early June.

Replication of the virus in mosquitoes occurs more quickly during hotter temperatures, which is "why there tends to be more in the South Bay than the north," according to district spokeswoman Megan Caldwell. The native Culex pipiens mosquito is the insect primarily responsible for the spread of West Nile virus in the Bay Area.

Last year, the virus was detected in San Mateo County as early as April. Caldwell said the later detection this year was no indication this season would see less virus activity.

"It doesn't mean we'll have an easier season," she said. "The hotter, the better for mosquitoes."

The good news is that the type of mosquito that can carrry the Zika virus - the non-native Aedes aegypti - hasn't been detected locally since May 2015, according to the district, and Zika has never been transmitted by mosquitoes in the U.S. That means the district can put all its resources toward fighting West Nile.

However, Caldwell warned, given that there is a community of Aedes in other sections of the state, the closest of which is Fresno, the species could be reintroduced locally at any time.

"If we have both going on at once, that's a real strain on us as an agency," she said.

The district said in a news release that while the risk of human infection remains low, county residents are asked to report fresh carcasses of birds or tree squirrels to the California West Nile Virus Hotline at www.westnile.ca.gov or by phone at 877-968-2473. The recently found dead crows were reported by residents.

The district is going back into the two neighborhoods with mosquito traps to collect adult mosquitoes for testing, and plans to issue the test results early next week. It is also continuing a pest management program to "control mosquitoes in their immature stages ... and treat mosquito-breeding sources throughout the county," according to a news release.

Over the hot summer months, residents are asked to help reduce the risk of West Nile by draining any standing water on their property; making sure their homes' doors and windows have tight-fitting screens to keep mosquitoes out; and applying insect repellent containing DEET, picaradin, oil of lemon eucalyptus or IR3535 while outdoors between dawn and dusk.

Email Kevin Kelly at [email protected] or call him at 650-391-1049.



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