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West Nile Up 35%, but CDC Says Peak Has Passed

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WebMD Health News
Reviewed by Louise Chang, MD

Sept. 12, 2012 -- Illnesses in the ongoing West Nile epidemic increased 35% since last week, but the CDC says this worst-ever season probably has peaked.

"The worst part of the epidemic is behind us," Lyle Petersen, MD, director of the CDC's Division of Vector-Borne Diseases, said at a news teleconference. "We continue to believe this year's outbreak is the most serious to date since West Nile virus was discovered in the U.S."

West Nile Virus 2012

Read the latest news and information about the outbreak.

West Nile Virus Special Report

While in the middle of a West Nile epidemic, the number of severe illnesses is the best indicator of how bad the season is. That's because nearly everyone with severe symptoms of "neuroinvasive" disease -- sudden encephalitis, meningitis, or polio-like paralysis -- is tested for the West Nile virus.

So far, there have been more than 1,400 cases of severe disease this year -- the most ever recorded by the second week of September. The CDC estimates that 150 to 250 times more people have actually been infected with the virus: 210,000 to 350,000 infections, some 20% of which will result in illness.

The majority of these illnesses are West Nile fever, but Petersen says only 2% or 3% of these cases are tested and reported to CDC. West Nile fever can be mild, but many cases last for weeks with lingering symptoms of fatigue.

In the worst previous West Nile seasons, 2002 and 2003, there were about 3,000 cases of severe disease, suggesting 450,000 to 750,000 infections and 112,500 to 187,500 illnesses. This year is expected to far surpass those totals.

Even though case counts may double or even triple before the 2012 West Nile epidemic is over, Petersen says the season likely peaked at the end of August. That's because it can take weeks for the CDC to get a case report.

Peaked, but Not Over

And the end of August is the usual seasonal peak for the mosquitoes that carry West Nile virus.

"As long as it stays warm, more West Nile transmission will occur," Petersen says. "In some areas of the country the outbreak may not have peaked yet. But in most areas, the epidemic probably peaked around the end of August."

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