Worst-Ever West Nile Epidemic: What Happened?

By Daniel J. DeNoon
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?

Probably not. Some mosquito-borne diseases run in cycles in the U.S. West Nile virus arrived in New York only in 1999 and took until 2004 to spread from coast to coast -- so it's too soon to say whether natural cycles will appear.

One good thing about West Nile virus is that once you've had an infection, you're likely immune for life.

"We've never seen WNV twice in the same person," Petersen says.

It might seem that with so many infections this year, there wouldn't be many vulnerable people left to infect. But the CDC says many Americans remain vulnerable.

"Most people are still going to be susceptible to being infected in subsequent years," Petersen says. "Studies we have done in North Dakota -- the most heavily affected state since West Nile has been in the U.S. -- even there with very high incidence from year to year, only about 15% of the population has been infected. So in Texas [and other states hit hard this year], most people still are susceptible."

Stopping West Nile: Lessons Learned

There's no vaccine against West Nile virus. There's no treatment. The best advice for avoiding infection is CDC's "Fight the Bite" campaign. This means wearing protective clothing and insect repellent, avoiding the outdoors at dawn and dusk, and ridding your home of places mosquitoes breed.

One controversial measure has been the use of aerial spraying of insecticide to kill adult mosquitoes. While such spraying is considered safe, it's not totally without risk. And there have been questions about whether it works.

There's still no definitive answer to those questions. However, data strongly supporting aerial spraying comes from Texas, which saw some 30% of the nation's severe cases this year.

"In areas of Texas subjected to aerial spraying, the spray reduced the number of vector mosquitoes by more than 90%. Areas not sprayed had an increase in mosquitoes," Petersen says. "Once we analyze all the data, we expect that these control measures were effective -- probably highly effective -- in stopping the outbreak in those areas."

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SOURCES: CDC web site. Lyle R. Petersen, MD, MPH, Director, Division of Vector-Borne Infectious Diseases, Centers for Disease Control and Prevention. Texas Department of Health web site.

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