As the West Nile virus marches across America, valley health officials are marshaling their forces for a pitched battle against the disease this summer.

The mosquito-borne virus can lead to encephalitis and meningitis. It has been unstoppable since it first appeared in North America in 1999, spreading through 46 states.

The battle in the Northern San Joaquin Valley has already begun.

Merced County's public health officer, Dr. Tim Livermore, said, "There's just no doubt that it's going to affect California in a big way and Merced on the way.

"We have the ideal ecological setting for an outbreak; lots of water, lots of birds, lots of humans."

Unseasonably warm spring weather is already spawning mosquitoes, and Merced is closest to the current Southern California infestation, where three infected birds have already been found. A horse was detected with the disease last year.

Allan Inman, manager of the Merced County Mosquito Abatement District, reported the district's airplanes began spraying in the second week of March.

"It's unprecedented in the history of the district to begin aerial spraying in March," Inman said, adding that spraying usually begins in late April or May.

The district bought a third airplane last year in anticipation of the West Nile virus. The twin-engine plane is designed to spray a mild pesticide over the cities and urban areas of the county.

"This is the biggest challenge anybody has had in their mosquito abatement careers," Inman said.

The pesticide used is pyrethrum, which is derived from geraniums. Inman said the public will be notified before any spraying is done.

Livermore, the public health officer, spoke with reserved optimism about the overall prognosis for the valley.

"The main thing I think we in public health want to emphasize is that we want widespread awareness. There's no need for panic, but the best thing anyone can do is prevent bites by mosquitoes to themselves and family members."

In severe cases, the virus can lead to encephalitis or meningitis. Encephalitis is inflammation of the brain, and meningitis is inflammation of the lining around the brain.

But statistics from the national Centers for Disease Control and Prevention in Atlanta indicate that most people infected will not show any symptoms. About 20 percent will experience fever, headaches and body aches similar to those caused by the flu. A rash on the lower trunk is also common for West Nile virus fever.

Of those suffering such symptoms, one out of every 150 will die, according to the CDC.

Stanislaus County Health Officer Dr. John Walker said people over 65 are most at risk. While acknowledging there is no vaccine and no known specific treatment, Walker said most people recover.

Cases reported in 46 states

Since West Nile virus was first found along the East Coast, human cases have been reported in 46 states, including California. Only Oregon, Washington, Alaska and Hawaii have been spared.

The central United States was the hardest-hit region in 2003. Of 9,389 cases nationwide, 6,000 came from four states: Colorado, Nebraska and North and South Dakota. There were 246 deaths in 2003.

The virus affects animal populations first, especially birds and horses. Blue jays, crows and magpies have been susceptible.

For horses, there's a silver lining. There were 15,000 equine cases in the United States in 2002 and 5,000 in 2003. The drop is attributed to an equine vaccine. About 30 percent of horses who show symptoms usually die or are put down due to equine encephalitis.

There is no vaccine for humans.

Leticia Rico, a public information officer with the California Department of Food and Agriculture, said the state is hoping most horse owners avail themselves of the vaccine.

"People have so much time, money and emotion invested in their horses," she said, "we hope they'll get their animals vaccinated."

She said time is crucial since it takes two shots and a month before horses are fully immunized.

She said that for people and horses alike, mosquito control is important.

An urban mosquito

The mosquito that is the culprit and carrier of West Nile virus is Culex tarsalis. Officials call it the backyard mosquito because it is found in urban areas and breeds in less polluted sources of standing water.

Jerry Davis, manager of the Turlock Mosquito Abatement District, which includes the southern half of Stanislaus County, compares Culex tarsalis to a Stealth fighter jet.

"This is the mosquito that buzzes you and when you turn on the light he's disappeared," he said. "When you turn off the light, he's buzzing you again. They're crafty."

Lloyd Douglass, manager of the East Side Mosquito Abatement District, which includes Modesto and the northern half of Stanislaus County, said with this variety the public must join the fight.

The East Side district checks all complaints about mosquitoes. Douglass said he would rather look into a dozen false alarms than miss one infestation. Even small things matter when it comes to mosquitoes.

"Just one old tire standing on end with a couple of quarts of water can produce 200 to 300 mosquitoes a day," he said.

The districts use air and ground spraying. They even enlist the 2-inch mosquito fish in the fight against the insects. A map in Douglass' office shows hundreds of red pins around Stanislaus County, as well as green and yellow. Red means complaints and evidence of mosquitoes while yellow means none was found. Green pins represent places where fish have been planted.

In Merced, Inman has been out trapping mosquitoes and has already noticed an increase in numbers.

"We used to run a trap all night long and get 700 overnight," Inman said, "Now we're getting close to 500 in 30 minutes."

Striking after sunset

But he's spotted an interesting trend. The mosquitoes that tend to carry the virus hit their peak of activity about 30 minutes after the official sunset, then there is a significant dropoff in activity.

Inman will continue monitoring the timing because it will be critical if aerial spraying is necessary over the cities. There wouldn't be any point in spraying too long after sunset if the virus-bearing mosquitoes aren't active.

"We're probably more at risk than other counties. We have a unique blend of breeding habitat," Inman said. Merced County has about 100,000 acres of wetlands that are perfect breeding grounds for the Culex tarsalis.

Then there are 330 dairies and their lagoons that are preferred by the Culex pipiens, often called pasture mosquitoes. They do not carry the disease.

Adding to the problem are the urban areas, with standing water serving as breeding grounds.

Spraying early and often could significantly reduce the mosquito count. Inman said a University of California study showed that if the district can eliminate 6 percent of each generation, it could see the mosquito population reduced by 80 percent to 90 percent when it hits the peak season in the fall.

Since chickens can be infected but do not die from the disease, they are used as sentinels, offering early warning when the diseases makes an appearance. The chickens are checked every other week for the disease. The tests do not harm them.

Merced County will put out its sentinel chicken flocks on Wednesday. Stanislaus and San Joaquin counties will put flocks out the following day.

"We went from four sentinel chicken flocks to six this year," said Aaron Devencenzi, the community education specialist for the San Joaquin County Mosquito and Vector Control District.

"We had our people out early, checking for sources of mosquitoes," he said. "Our primary goal is to reach them before they become adults."

Bee staff writer Mike Conway can be reached at 667-1227 or mconway@modbee.com.

Bee staff writer Roger Hoskins can be reached at 578-2311 or rhoskins@modbee.com.

Reprinted by permission of The Modesto Bee

For more information on West Nile Viras, visit the Public Health's Wes Nile Viras page.