The nation is heading toward the worst outbreak of West Nile disease in the 13 years that the virus has been on this continent, federal health authorities said Wednesday.

But it is still unclear where and how far cases will spread. Dallas declared an emergency last week, and West Nile deaths have been concentrated in Texas and a few nearby states, including Louisiana, Mississippi and Oklahoma, as well as South Dakota.

So far this year, there have been 1,118 cases and 41 deaths reported to the Centers for Disease Control and Prevention, Dr. Lyle R. Petersen, director of the agency’s division of vector-borne diseases, said Wednesday in a telephone news conference.

“That’s the highest number of cases ever reported to the C.D.C. by the third week of August,” he added. “And cases are trending upward.”

Because it takes some time for symptoms to develop and cases to be reported, those people were probably infected by mosquitoes two to three weeks ago, he said. The agency expects cases to increase through the end of September. In 2003, there were 264 deaths.

It takes three days to two weeks after a bite for symptoms to come on, but they may then be rapid and overwhelming.

“I was out for a jog, and within one mile I went from feeling normal to where I could barely walk,” he said.

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