LAKE COUNTY – For the first time this year West Nile Virus has been detected in Lake County.

On Wednesday, Lake County Vector Control received confirmation of a positive test result on a dead bird, according to District Manager Dr. Jamie Scott.

Scott said the positive finding was made on a dead crow collected in Lucerne on July 1.

“The holiday extended the time it took us to get the results,” said Scott.

Dead birds and tree squirrels are necropsied at the California Animal Health and Safety Laboratory laboratory at University of California, Davis, said Scott. Samples taken during the necropsies are forwarded to the UC Davis Center for Vectorborne Diseases for West Nile virus testing using the singleplex RT-PCR Taqman assay and confirmed with a second primer set.

So far this year, 22 dead birds had been reported around the county, but none had tested positive for West Nile Virus, which first appeared in Lake County in 2004, according to Vector Control records.

Vector Control keeps two sentinel chicken flocks – one in Upper Lake and a second near Anderson Marsh, between Lower Lake and Clearlake.

Those chickens have been sampled seven times this season – once every two weeks – for West Nile Virus antibodies, St. Louis encephalitis and Western equine encephalomyelitis, and have been clear on all counts, Vector Control reported.

In neighboring Yolo County, they've reported their first West Nile Virus positive chicken, said Scott. The only other West Nile Virus activity reported in neighboring areas were two dead birds founds in Colusa County.

As of Wednesday, West Nile Virus had been detected in 31 California counties this year, seven more than this time last year, according to the state's West Nile Virus Web site, www.westnile.ca.gov .

So far, no human cases of the virus have been reported in California, the state reported. Neither have horse cases been reported in 2009 thus far.

State records show there were 445 human West Nile cases last year, including 15 fatalities. The peak year for the virus in the state's human residents was 2005, with 880 cases and 19 deaths.

Equine cases numbered 32 in 2008, with 2004 being the peak year, with 540 cases, the state reported.

Scott said West Nile Virus has been particularly active is the Fresno area. “It's an unusual amount of activity for that part of the valley so early in the season,” she said.

Weather, water and temperatures conditions could account for fewer West Nile numbers in Lake County this year.

Cooler temperatures control virus activity, said Scott. Prime West Nile conditions include a combination of high mosquito numbers and very high temperatures.

Scott said cooler temperatures – specifically, nighttime temperatures cooler than 60 degrees – tend to kill West Nile Virus.

However, she cautioned that West Nile Virus is “still so new” to the United States, with this being the virus' 11^th season here. While it's settling into the country's ecology, the only thing scientists have been able to identify as impacting the disease is temperature.

The lake's level is two feet below the 88-year average for this time of year, the district reported, which could affect other mosquito species.

“The low water has helped us out somewhat,” said Scott. She explained that mosquito species that hatch in flood water or shallow pools have been lighter in population this year.

However, the three mosquito species that carry West Nile Virus and are most prevalent locally – Culex tarsalis, Culex stigmatosoma and Culex erythrothorax – are “very opportunistic,” said Scotts.

“They will develop in any water standing for more than five days,” said Scott.

While some areas have no standing water due to lack of rain, irrigation is prevalent in other places, said Scott. The lake's weedy edges also provide good habitat for the mosquitoes.

Scott and a group of colleagues have just published a paper in the Journal of the American Mosquito Control Association on a recently introduced mosquito species, Aedes japonicus japonicus. The mosquito, which carries West Nile Virus, is now in 20 states, but isn't in California, having come as far as Michigan since in appeared in the US in 1998.

Besides West Nile, the district is keeping an eye on other diseases as well.

It collected 530 ticks from 14 sample sites, and have shipped all of them to the California Department of Health Services, and will be forwarded to the Centers for Disease Control and Surveillance so they may be tested for Rickettsia 364D and other tick-borne diseases.

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