CDC: West Nile outbreak still biggest in US history

"This is the highest number of cases reported to the CDC through the last week in August since West Nile virus was first detected in the U.S. in 1999," said Lyle R. Petersen, director of the CDC's Division of Vector-Borne Infectious Diseases.

He added that contrary to some guesses, Hurricane Isaac will have "no noticeable effect" on the outbreak.

Several Gulf Coast states have been hit hard by West Nile virus since the start of the year. Petersen encouraged residents to use insect repellent, wear long sleeves and pants and avoid standing water.

"The number of cases continues to rise," Peterson said, "and will get higher throughout October. ... We are working closely with state and local government to contain the outbreak."

Only two states — Alaska and Hawaii — have not reported any cases of West Nile virus in people, birds or mosquitoes this year, Petersen said.