This year is on track to be the worst ever for West Nile virus in the United States. Here are the latest numbers from the Centers for Disease Control and Prevention:
- 1,590 reported cases, nearly 500 more than a week ago for a rise of 44 percent.
- 889 cases, or 56 percent, involve severe neurological disease.
- 66 deaths, compared to 41 last week.
Every state except Alaska and Hawaii has reported West Nile virus in people, birds or mosquitoes. "We expect the number of cases will rise through October," says Dr. Lyle Petersen, director of the CDC's division of vector-borne infections.
In Texas, the worst-hit state, cases jumped by nearly 200 over the past week and deaths reached 31, up 10 from last week. "As I look at the data, I'm not convinced we have peaked," Texas health commissioner Dr, David Lakey told reporters on a CDC conference call Wednesday.
As the reported numbers have mounted, many NPR listeners emailed us with questions about West Nile. We put them to to Dr. Erin Staples, a medical epidemiologist in CDC's arbovirus diseases branch, and Dr. Robert Haley of the University of Texas Southwestern Medical Center in Dallas. Haley wrote a commentary on the situation in the current JAMA, the Journal of the American Medical Association.
Here's a selection of the most frequently asked questions and answers from them and other sources.
Q: Where did West Nile virus come from? And how did it get into the United States?
A: It was discovered in the West Nile District of Uganda in 1937, and later found in Africa, Asia, the Middle East and Europe.
The West Nile virus now found across the continental United States, Canada, Central America and the Caribbean is an Israeli strain introduced into New York City in 1999. It was likely carried by an infected stowaway mosquito that arrived by plane or cargo ship. Some think an off-course bird might have carried the virus to North America, but Haley thinks this is unlikely. "Bird infections last only a few days," he says, "and the bird would have been sick and unable to fly long distances."
Haley says WNV is related to mosquito-borne viruses that cause St. Louis encephalitis, dengue fever, yellow fever and Japanese encephalitis.
Q: Why is West Nile disease so bad this year? Is global warming a factor?
A: "That's a good and complicated question," Staples says. "It's not clear why we're seeing more West Nile Virus activity." One prominent theory is that an unusually mild winter last year allowed more WNV-infected mosquitoes to survive, giving the virus a head start when spring arrived.
Then there's the unusually hot weather many areas suffered this summer. Haley says the Culex pipens mosquito, the urban-dwelling primary carrier of WNV, "breeds a little differently than others. They like a hot, dry summer punctuated by a little rainfall here and there – and a lot of stagnant water produced by humans."
Haley says this summer "was just right" from Culex's point of view – especially in Dallas, which has been hardest hit by WNV of any US city. "We've had several very strategic rainfalls — large thunderstorms that swept over Dallas," Haley says.
But Staples says it's too soon to attribute this year's unusual WNV activity to climate change. "Some of our largest years were 2002 and 2003, and more recent years have been low by comparison," the CDC epidemiologist says. "We see waxes and wanes, not a steady increase."
Q: Health officials in Texas and other areas are ordering aerial spraying of insecticides to kill mosquitoes, hoping that will quell outbreaks. But won't that kill beneficial insects such as bees and butterflies? And what's the risk for people?
A: The CDC and Environmental Protection Agency say there's virtually no risk to people from aerial spraying to kill mosquitoes, and very small risk to "good" insects or the environment.
The insecticides used for this purpose are pyrethrins, synthetic versions of a natural insecticide found in chrysanthemums. They have a good safety record. The insecticides excite mosquitoes and eventually paralyze the insects' breathing muscles.
These insecticides are used in very low concentrations for aerial spraying — about a teaspoon for an area the size of a football field. This kind of spraying produces microscopic droplets about the size of a red blood cell. So nobody standing underneath gets more than a negligible exposure.
And most of the spray, Haley says, doesn't penetrate the tree canopies where Culex mosquitoes hang out.
Moreover, aerial spraying is done at night, when the mosquitoes are active but beneficial insects aren't. Pyrethrin insecticides break down quickly when exposed to sunlight and air, so there is environmental buildup isn't an issue.
One problem: Too much spraying can produce insecticide-resistant mosquitoes. So public health officials generally restrict spraying to a couple of applications within a two-week period. But that's been shown to squelch WNV outbreaks.
Texas health commissioner Lakey says officials there should know soon if spraying in Dallas County has eradicated WNV-infected mosquitoes. Because of a lag time in reporting human cases, it will take several weeks to know if spraying has been effective in stopping the outbreak there.
Q: How serious is WNV disease? Can people suffer long-term effects?
Four out of five people infected with WNV don't even know it. They have no symptoms. The rest experience an unpleasant flu-like illness marked by fever and chills, headaches, fatigue and pain in muscles and joints. Symptoms often last three to six days, but some people don't recover for weeks.
About 1 in 150 people infected with the virus develop infections of the brain (encephalitis) or surrounding tissue (meningitis). Those can cause severe headache, fever, stiff neck, confusion, convulsions, coma, tremors, muscle weakness and paralysis. Around 10 percent of people with WNV encephalitis die. Many with neurologic involvement require weeks of hospitalization and some are left with permanent effects, such as muscle weakness and paralysis.
In July Dr. Kristy Murray and her colleagues at Baylor College of Medicine in Houston published a report on 139 people who had recovered from WNV infection. Forty percent had chronic kidney disease. While some of these had kidney problems before their WNV infection, the Houston researchers think many developed kidney disease as a result of the virus.
Staples says the CDC is aware of the Houston report, but it's too small a study to determine if WNV causes long-term kidney disease. "It's definitely something that needs further study," she says.
Curiously, WNV disease was largely a milder disease before it arrived in North America 13 years ago. Staples says no one knows why so-called "neuroinvasive" disease is more common these days. Petersen of the CDC says there's no evidence so far that neuroinvasive disease has become more common since 1999.
Q: Is West Nile Virus a threat to my pet or livestock?
A: Among mammals, humans and horses are the only ones known to die of WNV infection – almost always from the neurologic form of the disease. Staples says experiments with dogs showed they developed no symptoms at all, and cats had only mild symptoms. There is no evidence of animal-to-person transmission or animal-to-animal infection. The mosquito is always the bridge.
Dr. John Glisson of the U.S. Poultry and Egg Association says all birds are susceptible to WNV to some degree, but many species don't get sick from it. "Chickens can become infected but show no ill effects," he says. "It has never been a problem for the poultry industry."
Among wild birds, the corvids — especially crows and blue jays — are most likely to die from WNV. But Haley says robins are most important in amplifying the amount of virus in the environment because they have several broods in a season, and baby robins are especially vulnerable to the bites of infected mosquitoes.
Q: If I get infected with WNV, will I be immune from future infections?
A: Yes. Doctors think infection produces lifelong immunity. The CDC estimates about 3 million Americans have been infected with WNV, though most of them don't know it.
Q: Can I get tested to see if I'm immune?
A: There is a test, called an ELISA, for WNV antibodies. But the CDC doesn't encourage people to get it unless a doctor recommends it for a particular medical reason. The test can be prone to false positives. "It's best to assume that you're not immune unless you know you've had WNV," Staples says.
The best way to protect yourself is to avoid mosquito bites. That means using insect repellant, wearing long sleeves and pants when outside at dawn or dusk when mosquitoes are most likely to bite, and keeping window screens in good repair. The agency also stresses emptying any standing pools of water — from bird baths, wading pools, tire swings or any other source — to foil mosquito breeding.
Q: Since people become immune after infection, why isn't there a vaccine for WNV?
A: There is a vaccine – actually four different marketed vaccines – for horses. And there has been some research on a human WNV vaccine, including some successful early trials in humans. But the effort to develop a human vaccine has hit a wall.
"We're not currently aware of any ongoing studies to move a human WNV vaccine towards licensure," Staples says.
One big problem is how to test a candidate vaccine in large-enough numbers of human volunteers — some of whom would get the real vaccine and others a placebo. "To do that we'd have to be able to predict where WNV will occur," Staples says.
And if a human vaccine were developed, it's not clear how it would be deployed. A 2006 CDC study concluded universal vaccination against WNV wouldn't be cost-effective. It's conceivable that a vaccine would be targeted to older individuals and those with particular risks from WNV infection, such as those with kidney disease or compromised immune systems. But that market might not be large enough to justify the costs of development.
The calculus could change, however, if WNV outbreaks continue to increase.
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. Transcript provided by NPR, Copyright NPR.