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Worst-Ever West Nile Epidemic: What Happened?

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More West Nile Epidemics to Come continued...

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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