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West Nile virus, spread by mosquitoes, was first detected in the United States four years ago. Some experts have since downgraded how serious a public threat the disease is.
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A Look at the Numbers
Number of Human Infections in 2003: 1,355 cases, 19 deaths in 33 states
Number in 2002: 4,156 cases, 284 deaths in 44 states
Number between 1999 and 2001: 149 cases, 18 deaths in 27 states
Species most affected by the illness: humans, horses and birds
For most people, risk is low. Less than 1 percent of people who are bitten by mosquitoes develop any symptoms of the disease and relatively few mosquitoes actually carry WNV.
Greater risk for those outdoors a lot. People who spend a lot of time outdoors are more likely to be bitten by an infected mosquito. They should take special care to avoid mosquito bites.
People over 50 can get sicker. People over the age of 50 are more likely to develop serious symptoms of WNV if they do get sick and should take special care to avoid mosquito bites.
Risk through medical procedures is low. The risk of getting WNV through blood transfusions and organ transplants is very small, and should not prevent people who need surgery from having it. If you have concerns, talk to your doctor before surgery.
So far this year, health officials report nearly 1,300 human cases and 19 deaths from West Nile virus. Those numbers are certain to climb in the next few weeks; late summer is the time when people are most likely to encounter mosquitoes carrying the virus. Yet as NPR's Jon Hamilton reports, experts insist that the risk of dying or becoming seriously ill from West Nile is remarkably small.
Lois Levitan directs the Environmental Risk Analysis Program at Cornell University. She has the difficult job of explaining why people shouldn't be terrified of a virus that could kill them.
"Last year was the biggest known outbreak of any such disease in the United States," she says, "and the numbers of fatalities were somewhat fewer than 300 ... which is similar to the number of people who die every year in the United States from lightning strikes."
Levitan says it makes sense to take precautions, but Americans shouldn't consider West Nile a major threat to public health.
One reason West Nile generates fear is that early on, the only people known to have the infection were those who ended up in the hospital, often near death. Levitan says that made it look like every West Nile infection was life-threatening. With wider testing for the infection, the picture is changing.
"What I noticed in the statistics from this year is that about half of those (reported) are actually what are considered more mild cases," she explains.
And people who have acquired the virus but have no symptoms aren't tested at all. This means the vast majority of infected people will never show up in government statistics. Levitan says that nearly everyone in the United States gets bitten by mosquitoes from time to time. But only a very small percentage even get the virus. And less than one percent of those actually become sick enough to be included in the official tally kept by the Centers for Disease Control and Prevention.
Another reason West Nile frightens people is its rapid spread across the nation. That has produced a stream of alarming reports about new outbreaks. Philip Alcabes is an urban health expert at the City University of New York's Hunter College. He says it's comforting to look at places where West Nile has been around for several years.
"In New York City, where I live, there were 46 cases in 1999, and then 14 the following year, seven in 2001, 30 last year, and so far I think there's been one confirmed case, maybe it's two," he says. "That's not the picture you would get for an epidemic that's rapidly intensifying. That's not what we saw with AIDS 20-something years ago, for instance."
Alcabes says one major reason things haven't been worse is that West Nile isn't really a human epidemic at all.
"It's an animal epidemic," he says. "The human cases that occur are fallout, they're sort of accidents of the system that's trying to extend the infection of animals by the virus."
That's bad news for birds and horses, but good news for humans.
While West Nile deaths may remain almost as rare as deaths from lightning strikes, Lois Levitan says that doesn't mean people should ignore the risk. She points out that people take precautions against getting struck by lightning, and it makes sense to take steps to avoid West Nile.
"There are some smart things that we can do," she says. "Make sure that there's no murky standing water around, empty out the flower pots, try to control the outbreak of the mosquito early in the season while it's still in the larval stage."
And, of course, cover up or use insect repellent if you're in mosquito territory.