Signaling the tightening grip of West Nile virus in California, Sacramento County logged its first human cases of the disease Thursday, and state officials announced the first death in the state this year.
The virus, which is spread by the bite of infected mosquitoes, killed an elderly man from Kings County in the San Joaquin Valley, the state Department of Health Services said.
In Sacramento County, two women were found to be infected with the virus. Their experiences exemplify the sharply different effects the virus can have:
A 58-year-old woman from the northern part of the county was hospitalized with neurological disease, according to county public health officer Dr. Glennah Trochet.
An 18-year-old woman from the southern end of the county wasn't sick at all. Her infection was discovered after blood she donated last Friday underwent a routine screening, said Dr. Patricia Kopko, medical director at BloodSource.
A third Northern California case also announced on Thursday illustrates yet another manifestation of the virus.
In that instance, a Butte County youth had a headache, rash and fatigue that cleared up within a few days, said Dr. Mark Lundberg, public health officer in that county.
The teen's discomfort was so mild that he didn't see a doctor. It wasn't until he - like the young woman in Sacramento County - donated blood that his infection was detected.
The majority of individuals who are exposed to West Nile virus won't even know it, but scientists can't predict who those people are. The virus has tended to hit hardest those who are older or have weakened immune systems, but young people also have become gravely ill.
About 20 percent of those infected come down with West Nile fever, symptoms of which mimic the flu.
Less than 1 percent develop neurological disease such as meningitis or encephalitis.
The Kings County victim was 65 or older and had underlying medical conditions when he died, according to the state health department.
Department spokesman Ken August said that to protect the man's identity, he could not divulge details such as when the man died or his exact age.
Last year, 830 Californians contracted West Nile virus, with three cases in Sacramento County. The disease was fatal to 27 people, ranging in age from 26 to 94.
Sacramento County public health officer Trochet said the onset of human cases locally this summer is expected, and that there likely will be many more before the season is over.
"I think we are on the brink of an explosion of human cases throughout Northern California," Trochet said. " ... I suspect we will be getting reports of positive blood, whether or not people are seriously ill. I would expect large numbers of those."
BloodSource's Kopko, whose agency collects donations from the Oregon border to Merced County, and from Vacaville to Lake Tahoe, said the center screens every unit of blood it receives, amounting to more than 500 samples a day.
Because West Nile virus can be transmitted via transfusions of infected blood, units that test positive cannot be used.
Furthermore, BloodSource asks individuals who have tested positive to refrain from donating for six months.
Leslie Botos, a BloodSource spokeswoman, said the agency is concerned that widespread occurrence of West Nile virus could markedly reduce the number of eligible donors. "If this escalates, the impact on the blood supply can be significant," she said.
She also is concerned that people will avoid donating blood out of misguided fear that they might have West Nile virus.
"If you get a mosquito bite, please don't think you can't give blood," she said. "Don't pre-screen yourself. Let us help you make the decision."
David Brown, general manager of the Sacramento-Yolo Mosquito and Vector Control District, said the district this month began spraying pesticides in selected public sites in the northern half of the county, based upon a spike in the number of dead birds in that area. Birds are especially vulnerable to West Nile virus.
The places that have been treated include C-Bar-C park and Rusch Community Park in Citrus Heights and Cherry Island Golf Course in Elverta, he said.
Brown said the district has not sprayed residential neighborhoods. He said the agency has targeted parks and golf courses because it's harder for visitors there to retreat from mosquitoes.
"If you're around your home and you're being bothered, it's a lot easier to go inside, as opposed to in a park or a golf course," Brown said.
Brown said that although the district is working overtime to control mosquitoes, the severity of West Nile virus in the region may depend on how well individuals protect themselves.
At every opportunity, the district urges people to eliminate stagnant water on their properties, such as in unmaintained swimming pools, saucers under potted plants or children's toys that catch water from the sprinkler. Mosquitoes lay their eggs in standing water.
District and health officials also stress the importance of using mosquito repellent when outdoors, especially during dawn and dusk when mosquitoes are most active.
The Bee's Edie Lau can be reached at (916) 321-1098 or email@example.com. Bee Medical Writer Dorsey Griffith contributed to this report.