SAN JOSE -- In a record West Nile virus season that has seen infected birds literally dropping out of trees, officials on Friday announced what they said was no surprise: Five human cases have cropped up countywide in recent weeks.

Two women, ages 60 and 79, were hospitalized when the disease manifested itself in its potentially deadly, neuroinvasive form. Both developed encephalitis, and one remains in the hospital in serious condition. Another case involves an 84-year-old man who was treated for the less serious but still debilitating West Nile fever, while the other two cases -- men aged 59 and 60 -- never exhibited any symptoms at all and were only discovered through a routine blood donor screening.

"What's surprising is that we haven't seen more," said Dr. Sara Cody, health officer for Santa Clara County. "We haven't seen a lot of human cases in several years, but this year is different; we've had really quite an extraordinary level of the virus in birds. Given that, we expect human infection."

Russ Parman, acting director of the Santa Clara County Vector Control District, said that to date they've collected about 700 dead birds testing positive for the virus -- nearly half of the total logged statewide -- making the county a hot zone.

"Eighty to 85 percent of the birds we collect are testing positive," he said. "That's really unusual. People are seeing them fall out of trees. They're seeing them acting sick before they die."


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Officials did not specify what cities the human cases originated from, but said all of them live in areas that had been fogged earlier this year after infected mosquitoes were found nearby. That includes parts of West San Jose, Santa Clara, Sunnyvale, Cupertino, Campbell, Los Gatos and Saratoga. The county plans to fog an area of Sunnyvale and Mountain View centered around South Mary Avenue and El Camino Real on Tuesday.

So far this year, there have been 57 human cases statewide with four fatalities. In the Bay Area, Alameda and Contra Costa counties have each had a single case.

But officials said that doesn't mean there haven't been more.

"For every case we know about, there's probably 20 to 50 that we do not," said Parman.

That's because the virus is often asymptomatic. It sometimes produces flu-like fatigue and fever, and only 1 percent of the time becomes neuroinvasive.

But when it does, it's serious.

"Most people recover, but for some there's going to be long-term problems that will last a lifetime," Parman said. "And a small percentage die."

Santa Clara County had one fatal case, in 2011. The last time five human cases were detected was in 2006. Officials do not know a specific reason why the virus is so active in the South Bay this year, but have said that warm temperatures and the drought are possible catalysts.

Cody said that despite all of the county's cases being older adults, age only plays a factor in the chance that the virus will go neuroinvasive.

"The risk of being infected doesn't have to do with age, it has to do with how much time someone spends in an area of increased activity," she said. "It's the risk of getting bitten by a mosquito, and that's equal opportunity at any age."

But those over 50 with diabetes or hypertension are at greater risk for complications and serious illness as a result, she said.

Cody said there's no specific treatment for the virus, and thus the thrust of their outreach is to urge prevention. She said that's particularly important for residents of areas where infected mosquitoes have been found, who might benefit from insect repellent and long sleeves when outdoors during bug-biting hours.

"People need to be aware that there's a lot of West Nile activity in the county, and living in areas with really intense activity means a greater chance of exposure," she said. "The primary way to prevent exposure is keeping the mosquito population under control, but you can't eliminate every infected mosquito. You need to have other layers of prevention."

Contact Eric Kurhi at 408-920-5852. Follow him at Twitter.com/erickurhi.

VIRUS PREVENTION
Health officials said that because there is no specific treatment for West Nile virus, they urge residents to take measures to avoid being exposed in the first place, particularly those who live in fogging areas where infected mosquitoes have been found. A map of the areas can be found at http://bit.ly/Y9MWLM
Preventive measures include:
Use insect repellents when outdoors. Repellents containing DEET, picaridin, IR3535, and some oil of lemon, eucalyptus and P-menthane diol products provide longer-lasting protection.
Dress in long sleeves and pants when outside from dusk through dawn.
Install or repair screens on windows and doors. Use air conditioning if available.
Empty standing water from containers such as flowerpots, gutters, buckets, pool covers, pet water dishes, discarded tires and birdbaths.
Report any mosquito breeding sources to the Santa Clara County Vector Control District at
www.SCCvector.org or (408) 918-4770.
More information and tips are available at http://westnile.ca.gov, www.cdc.gov/westnile and www.sccvector.org
Source: Santa Clara County Department of Health