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Worst-Ever West Nile Epidemic: What Happened?

WebMD Health News
Reviewed by Louise Chang, MD

Nov. 19, 2012 -- This week or next, 2012 officially will become America's worst year ever for death and brain damage from West Nile virus.

More than 2,600 Americans already suffer paralysis or other lingering neurological damage from the virus. At least 229 have died.

With reports still coming in and new cases continuing -- infected mosquitoes continue to bite in southern states -- the CDC expects the toll to pass 2002's record 284 deaths and 2,946 cases of neuro-invasive (brain/spinal cord) disease.

So how many people got a West Nile infection this year?

The best estimate comes from this year's 2,601-and-counting cases of neuro-invasive infection. This very serious illness puts virtually every patient in the hospital. For each such case, the CDC figures that 30 to 70 more people were infected. Some had severe fever lasting weeks; others with mild infections hardly felt ill at all.

"We estimate 78,000 to 182,000 cases so far this year," says Lyle Petersen, MD, MPH, CDC's director of vector-borne diseases. "Cases are still being reported. And states are still following up. So this year will be probably pretty close to or over the record."

As case counts rose every week this summer, they far surpassed counts for the same weeks of the large 2002 and 2003 epidemics.

Why Was 2012 a Record West Nile Season?

Birds carry West Nile virus. Mosquitoes bite the birds and then bite people. If there's enough virus in the mosquito -- that is, if conditions are right for the virus to multiply inside the insect -- people get infected.

Birds, bugs, and bites seem to happen every year. This year's big difference was the weather.

"The smoking gun is the abnormally warm spring and summer this year. In many parts of the country, it was the hottest year on record," Petersen says. "We know that when conditions are right, increasing temperatures promote virus growth in mosquitoes. This makes it easier for them to transmit the virus."

More West Nile Epidemics to Come

As temperatures drop and mosquitoes stop biting, the 2012 West Nile season finally is ending. Does this mean we've dodged the West Nile bullet?

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