AUDIE CORNISH, HOST:
Almost 1,600 Americans are known to have gotten sick from West Nile so far in 2012, but a lot of cases don't even get reported. So the CDC thinks the real number of infections for the year is closer to 133,000.
On Monday we asked you to send us your questions about West Nile. With me today from Boston is NPR health and science correspondent Richard Knox to answer some of them.
And, Richard, to start, one of the most common questions: Where did West Nile virus come from and how did it get into the United States?
RICHARD KNOX, BYLINE: It was discovered in the West Nile district of Uganda 75 years ago. And it didn't really cause a whole lot of problems. People thought it was just a mild fever. But in 1999, it somehow or other got into the United States. There were some quick cases in Queens in New York City. Don't really know why. Probably an infected mosquito stowed away on a plane or a cargo ship.
CORNISH: Another big question for this year is why is West Nile disease so bad this season?
KNOX: Don't really know. According to the experts, they think that it may have something to do with the mild winter last year, which allowed some of the infected mosquitoes to winter over and to give the virus a kind of a head start in the spring. And also, the famously hot summer that we've been having favors the breeding of this mosquito. So we have more of the virus around in the environment.
CORNISH: Now, Richard, health officials in Dallas and other areas have begun aerial spraying to kill mosquitoes, hoping that will quell West Nile virus outbreaks. And that got a lot of our listeners concerned about the effect on people and on beneficial insects, like bees, butterflies and dragonflies. What's the risk?
KNOX: Yeah, people - a lot of people really have expressed concern about spraying. The risk, according to the CDC and the EPA, is really very small. There's a long track record with the kind of insecticides called pyrethrins, or pyrethroids, that are used to kill these particular kind of mosquitoes. They are used in tiny amounts. I mean, for aerial spraying it's like a teaspoon to cover an acre, which is about the size of a football field.
And so, anybody underneath is not likely to get a whole lot of exposure. Also, much of the spray stays in the treetops where the mosquitoes are. The spraying is done at night when most beneficial insects - bees, butterflies, dragonflies - are not flying around but the mosquitoes are. And so, they're supposed to be really minimal risk to most people or to good insects.
CORNISH: Richard, we got a host of questions that people had about what happens if they get infected. One of which was, how serious is West Nile disease? And can people suffer long-term effects? And, I think, to add to that, can people be immune from future infection if they are infected?
KNOX: Yeah, all good questions. It can be very serious. And that's one reason why people are taking it so seriously. But I think it's important to remember that most people who get infected, maybe four out of five, don't even know that they were infected. They don't have any symptoms. About the remaining 20 percent, they can have a pretty unpleasant illness, like flu; headaches, muscle pain, joint pain, chills, fever. And that can go on for days and even weeks, but they recover. There's no treatment for it. Most people recover.
The really serious problem comes to - one out of every 150 people get infected have brain involvement, either infection of the brain itself or the tissues around the brain. That can be deadly. About 10 percent of the people who get West Nile encephalitis die from it. Many of the others can have lifelong effects; paralysis, muscle weakness, and so on.
Yes, you can become immune from an infection. In fact, it's thought that you'll be immune for life or for a long time, unless you have something wrong with your immune system after you've had an infection.
CORNISH: One question following that is since people become immune after infection, why isn't there a vaccine for West Nile virus?
KNOX: There is if you're a horse. There are four vaccines that have been developed for horses. And horses are - along with humans, are the other mammals that die from West Nile. So that's pretty widely used among horse owners. For a human vaccine, there's been work on it and there's been some successful early trials of a human vaccine. But there's a big problem.
In order to really test the effectiveness of a vaccine, you need a fairly sizable group of human volunteers. Some of them get the test vaccine, others get a placebo vaccine or, you know, a non-active thing. You don't know how to recruit large numbers of people who are going to get exposed to West Nile, because from one year to the next you don't know how big an outbreak you're going to have. And you don't know where it's going to be.
I mean, nobody knew a year ago that Dallas was going to be heavily infected. So that's kind of hit a wall in terms of doing further human trials.
CORNISH: NPR health and science correspondent Richard Knox, answering your questions about West Nile. Richard, thank you.
KNOX: Any time, Audie.