BERKELEY, Calif. -- Westward expansion of the West Nile virus has prompted the government to look to California as the possible epicenter of next year's virus season.

Officials with the Centers for Disease Control and Prevention believe the mosquito-borne illness will hit the West Coast hard next year, particularly California, said Dr. Lyle Petersen, acting director of the CDC's division of vector-borne diseases.

"If I had to make a guess, the biggest area of concern would be the Imperial Valley in Southern California," said Petersen, who spoke to reporters Wednesday at the University of California's Berkeley campus.

That's because the virus emerged in that area for the first time this season. Health officials believe human cases erupt one season after it first appears in the environment.

The West Nile virus has steadily moved westward and down the Eastern Seaboard since 1999 when it was found in New York City, the first time it was found in the Western Hemisphere.

Petersen said the CDC will be giving more money to California to improve virus surveillance and public education to prevent people from being infected.

The virus is passed to people by bites of mosquitoes that have fed on infected birds. Most often, it produces mild flu-like symptoms. In the most serious cases, it can induce a deadly inflammation of the brain.

Petersen said this year's outbreak -- causing nearly 2,000 cases in Colorado, an additional 1,000 in Nebraska and more than 800 in South Dakota alone -- likely will be of the same magnitude as last year's outbreak, which was the largest West Nile outbreak in the Western Hemisphere with 4,156 cases and 284 deaths.

Petersen characterized this year's outbreak as different from previous U.S. onslaughts because the virus has sharply affected rural areas. Previous outbreaks were spread by mosquitos that tended to live near homes and buildings.

The reason is the type of mosquito responsible for passing the virus in Western states -- Culex tarsalis -- lives among farmland, travels far and is "the most efficient vector of West Nile virus ever discovered," Petersen said.

Although treatments and vaccines against the virus are being worked on, they are still years away from public use, officials said. CDC officials estimate the 2002 outbreak cost the country at least $200 million in medical services.

"There is no elimination" of West Nile virus, Petersen said. "The only way to eliminate this is to get rid of every infected mosquito -- and that's impossible. It's here to stay."

Meanwhile, health officials in Colorado, the epicenter of this year's outbreak, announced Wednesday that the danger of new infections there is over for the year. Mosquitoes that carry the virus have switched from blood to nectar meals as they prepare for winter, epidemiologist John Pape said.

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