The Santa Clarita Valley’s year-to-date West Nile virus numbers doubled in a single week as a summer thunderstorm provided fertile breeding grounds for infected mosquitoes.

As many dead crows and mosquito samples tested positive for the virus in the past seven days as had been confirmed all year for the SCV, according to a report Friday from the Greater Los Angeles County Vector Control District. (A vector is any arthropod, insect, rodent or other animal capable of transmitting diseases to humans.)

The SCV’s totals for crows that died from the virus jumped from four to seven thanks to one new discovery each in Newhall, Canyon Country and Valencia. All four of the previous crows were in Valencia.

Crystal Brown of the Vector Control District said the three new casualties were discovered in the 23600 block of White Oak Court off of Calgrove Boulevard in Newhall, the 19000 block of Wellhaven Street in Canyon Country and the 25600 block of Yucca Valley Road in Valencia.

Meanwhile, virus-positive mosquito samples – a collection of multiple insects from a single location – more than doubled. The six discoveries in the past week eclipsed the five found up to that point in 2011. Three samples were found in Valencia (including two on the South Fork Trail), two in Canyon Country and one in Castaic.

It was the first time the virus was reported in Castaic this year.

Year-to-date, Valencia is the local leader in infected mosquito samples with four, passing the previous leader, Newhall, which is now tied with Canyon Country at three apiece. Castaic has one, and Saugus is mysteriously virus-free. So far.

Vector watchers say it isn’t over.

“West Nile virus activity persists this fall as warmer temperatures follow this week’s rainstorm,” according to an agency statement. “The weather patterns and influx of new water sources are contributing to a prolonged mosquito breeding season and continued risk for West Nile virus transmission.”

With 19 new mosquito samples and 13 dead birds confirmed countywide in the past week, Los Angeles County’s year-to-date totals are 162 dead birds, 399 mosquito samples and 13 sentinel chickens  showing West Nile virus antibodies. Statewide totals are 587 dead birds, 1,896 mosquito samples and 252 sentinel chickens.

West Nile virus is a seasonal epidemic that causes signs of sickness in about 1 of 150 infected people. It is typically transmitted through mosquito bites.

The Vector Control District urges residents to employ the three “D’s” of virus prevention:

Dump/Drain: Eliminate standing water on your property because that’s where mosquitoes breed.

Dusk/Dawn: Avoid outdoor activities when mosquitoes are most active.

Defend: Wear insect repellent containing EPA-registered ingredients such as DEET, IR3535, Picaridin or Oil of Lemon Eucalyptus and long-sleeve shirts and pants when outdoors. Use properly-fitted screens free from tears on windows and doors around your home to keep mosquitoes out.

To report a dead bird or squirrel, visit www.westnile.ca.gov or call 1-877-WNV-BIRD.