NEW ORLEANS (AP) -- West Nile virus is an "emerging, infectious disease epidemic" that could be spread all the way to the Pacific Coast by birds and mosquitoes, the director of the Centers for Disease Control said Sunday.

The Northeast and the South have been hardest hit by the virus since it was first identified in the United States in 1999, but Dr. Julie Gerberding said birds and mosquitoes infected with West Nile are now in most states east of the Mississippi River and some to the west of it.

West Nile is "a problem that is having an unusually high human toll this year. So it is serious, and we have to continue our public health action to combat it," Gerberding said on CBS' "Face the Nation."

Seven people with West Nile virus have died in Louisiana this year, and Mississippi officials are investigating a death they say appears to be linked to the virus. The virus has been detected in 35 states and Washington, D.C.

In Louisiana, state and local workers are spraying insecticide in residential areas where the Asian tiger mosquito and the Southern house mosquito typically lay eggs, under the assumption that the two species are the most likely carriers of West Nile.

"We have made an assumption about which species are involved in transmission of the disease here based on what has happened in other parts of the United States," said Dawn Wesson, a medical entomologist at the Tulane University School of Public Health and Tropical Medicine.

Gerberding said Louisiana's experience last year with the deaths of four people from St. Louis encephalitis, a mosquito-bourne virus similar to West Nile, has helped officials deal with this year's outbreak.

"I think the investments that we've made over the past several years in this kind of public health response have really paid off," she said.

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