Peak Season For W. Nile Infections

West Nile virus, now entering its peak season, appeared in Arizona for the first time as the number of human infections more than doubled to nearly 400 over the past week, federal officials said Thursday.

A new experimental screening process for the virus also flagged 163 infected blood donations, showing the virus can be effectively screened, the federal Centers for Disease Control and Prevention reported.

"As we had anticipated, West Nile is currently really picking up momentum within the United States," said Dr. Stephen Ostroff, deputy director of the CDC's National Center for Infectious Diseases.

"It certainly looks like we will be following a similar course over the next several weeks," he said.

Since the virus first entered the country through New York in 1999, late August through September has been its peak season. Last year, it infected 4,156 people and killed 284 in its largest U.S. outbreak yet.

The virus, passed to humans by mosquitoes that have fed on infected birds, has steadily been expanding West over the years.

Earlier this week, it was detected in a mosquito pool in Arizona for the first time, leaving only three states — Oregon, Nevada and Utah — that have yet to detect it.

Most of the 393 human West Nile cases reported so far this year, up from 153 last week — were in the high plains and Rocky Mountains, officials said. Colorado has reported the most cases, 195, followed by South Dakota, 51, and Texas, 39. Nine deaths have been linked to West Nile, in Colorado, Texas and Alabama.

West Nile rarely kills, and most who become infected do not develop symptoms. About one in 150 people infected will become seriously ill, with the worst cases developing potentially deadly encephalitis or meningitis.

Last year, 23 people were found to have been infected with the virus through blood transfusions.

To protect the blood supply this year, the Food and Drug Administration has started using an experimental test to screen blood donations for West Nile. Of 1.1 million donations screened, 163 have tested positive for the virus, officials said. The donations came from states including Colorado, Florida, Louisiana, Mississippi, New Mexico, South Dakota and Texas.

"The whole of the civilian blood supply is being tested," said Dr. Hira Nakahasi of the FDA. "We have already interdicted more than 150 samples that would have been transfused into people and may have caused the disease."