JEFFREY BROWN: Later tonight, Dallas, Texas, begins an air war. The enemy is West Nile virus, and the immediate targets are the insects that transmit it from infected birds to humans.
On two feet and four wheels, crews in Dallas County, Tex., are already spraying insecticide at ground level to kill mosquitoes that carry West Nile virus.
ZACHARY THOMPSON, Dallas County Health and Human Services: We are talking about a Third World virus that we don't have a blueprint that says this works versus another plan.
JEFFREY BROWN: The county declared a state of emergency last week. And, yesterday, Dallas Mayor Mike Rawlings followed suit, clearing the way for aerial spraying for the first time since 1966.
MAYOR MIKE RAWLINGS, Dallas, Tex.: I'm in a tight window because we have people dying. We have to have a sense of urgency to get this done now. That's what's the most important thing.
JEFFREY BROWN: So far this summer, West Nile virus has caused 10 deaths in the Dallas area out of more than 200 cases.
Nationwide, the Centers for Disease Control and Prevention reports a total of at least 693 cases and 26 deaths. More than 80 percent of the cases have come in six states, Texas leading the way with 381 cases, California, South Dakota, Oklahoma, Louisiana and Mississippi.
Overall, it's the worst outbreak since West Nile virus appeared in New York City in 1999, the first time it had been detected in the U.S. Symptoms include fever and aches.
KATHARYN DEVILLE, West Nile Virus patient: It just makes me feel like somebody has an axe in my head, and it hurts really bad.
JEFFREY BROWN: In a small percentage of cases, the virus can be fatal, when it attacks the brain and spinal cord.
Officials say the aerial spraying that starts tonight in Dallas is the most effective way to kill the mosquitoes. Planes will spew out an insecticide, known as Duet, that's been used worldwide since the 1990s.
We take a closer look at the virus and efforts to combat it in Texas and elsewhere. Dr. Kristy Murray is principal investigator for the West Nile research program at the National School of Tropical Medicine at Baylor College of Medicine in Houston.
First, what's the thinking about why there are so many cases so early this year? This is unusual, right?
DR. KRISTY MURRAY, Baylor College of Medicine: This is unusual right now for Texas.
We did have a big outbreak in 2002 and 2003, when the virus first came into the area. But it's really not too surprising that we're having an epidemic. A lot is going to depend on what's going on in terms of the climate. Last year, we had a drought. This year, we had a lot of rain early on in the spring.
And so really it's not too surprising. We see outbreaks every three years. We see an increase in the number of cases. And I'm assuming that we're going to see a pattern like what we see with Saint Louis encephalitis virus, where we are going to have outbreaks every 10 to 15 years.
JEFFREY BROWN: And is it clear why it hits in a particular area, in this case Dallas, the Dallas area, as opposed to others?
DR. KRISTY MURRAY: It just depends upon what is going on locally in terms of transmission. Once you have transmission beginning, it starts to amplify.
Birds are the reservoir host for the virus, and so they get really high levels of virus in their blood. The mosquitoes feed on the birds and then it starts this complete cycle of transmission. Here in the Houston area, about 95 percent of our mosquitoes are testing positive right now.
But they started very early on with spraying and insecticides to get rid of larvae, as well as adult mosquitoes. And so that has helped to reduce a lot of our cases here. We only have 12 cases so far.
JEFFREY BROWN: Well, remind us a little bit more about this virus, particularly the symptoms. Some people, I gather -- most people experience rather mild symptoms, but others not.
KRISTY MURRAY: Correct.
About 80 percent of people actually are going to have no symptoms whatsoever. They will be infected with the virus, mount an immune response to it and develop no symptoms. About 20 percent of people are going to have flu-like illness, so they will have a fever, headache. They might have a rash.
Usually, people describe it as being one of the worst headaches they have ever had. And then about one out of 150 people are going to have the more severe form of illness, which is called neuroinvasive disease. And with that, they usually see encephalitis or meningitis.
JEFFREY BROWN: And do we know why some...
KRISTY MURRAY: And of course that's much more severe.
JEFFREY BROWN: Yes.
I'm wondering, do we know why some people get hit harder by this? And who is most vulnerable?
KRISTY MURRAY: Well, the most vulnerable are people who are older. Usually, we see people that are over age 50 being at higher risk for severe disease, although we can see it at any age.
It's not just that group. It's just certainly the older people are, the more at risk they are. A lot of that is thought to be due to immune systems. As we age, our immune systems age with us and so it puts us at greater risk.
Other things that we have seen are high blood pressure being a risk factor, as well as immunosuppression and conditions such as diabetes.
JEFFREY BROWN: So, in Dallas, they're -- as they're telling us they're about to start this aerial spraying, are there risks involved? There's been some nervousness there, I gather.
KRISTY MURRAY: And I can understand that.
Whenever I was with the CDC -- I was part of the original outbreak in 1999 in New York City, and it was the exact same thing where people who are not used to having any kind of aerial spraying or insecticiding, it's nerve-racking. I can understand that.
Here in Houston, we are always spraying. As soon as there's an area that comes up positive for West Nile, then they are immediately out there with the trucks spraying those areas. And again, I think that's what attributing to the fact that we have so few cases here in the Houston area, even though we have so many mosquitoes positive.
And so from a public health perspective, the spraying is really going to be effective at helping to knock down those mosquito populations and protect people from becoming infected.
JEFFREY BROWN: Well, so -- but in Houston, I think, you're talking about on-the-ground spraying or is that aerial as well? Explain the differences here of how different regions, different cities respond to this.
DR. KRISTY MURRAY: Well, here in Houston, we have a combination of both aerial and ground spraying.
In the urban areas, we have ground spraying where they have the trucks that come through. And then in areas that are a bit harder to reach, they approach those areas through aerial spraying, and so there is a combination of both here.
JEFFREY BROWN: So, in terms of the risks, again, getting back to what people are worried about, do they have -- are the studies definitive about this? Do people have any reason to take precautions?
DR. KRISTY MURRAY: Well, the thing is, with any kind of insecticide, you want to protect yourself.
EPA has cleared the insecticides that are being used for this type application. They do break down quickly in the environment. They usually try to do the spraying at nighttime, pretty late. At least here in the Houston area, they do. And so it reduces the amount of exposure that would be to pets or people.
With the spraying, it's important to disperse it. It's important to get it out at a time when the Culex mosquito is most active. That is the mosquito that is mostly responsible for the transmission of West Nile virus. And really it's most active between dusk and dawn. And so that is when you are going to want to try and do what you can to reduce those numbers.
JEFFREY BROWN: Well, and just finally coming back to the outbreak itself, is there any way to predict how this will go or the rest of the season, whether we're at the peak now or whether there's a lot more to come and it will spread to other places?
DR. KRISTY MURRAY: If you had my opinion, it's going to continue.
Right now, historically speaking, we're just about to enter the peak time for it; 80 percent of our cases in Texas occur in August and September. And so really we're still at the beginning of what's probably going to be a much larger effect.
When we look at this -- here we are in the middle of August. It typically takes about a week to two weeks before cases even get reported to the local public health authorities. There's time for the person to develop symptoms. They go into the hospital. The physician draws typically spinal fluid for the test. And then that gets submitted to the laboratory. And when they test it, it comes up positive. Then, it gets reported.
So we have this lag period before cases are even identified. And really right now, when we look at historically what's happened in Texas, this is right at the time when onset of illness for the majority of our cases is occurring. So, in the next couple of weeks is where we're probably going to see the most cases come up.
JEFFREY BROWN: Dr. Kristy Murray of the Baylor College of Medicine, thanks so much.
And one more note on this story: We spoke with the mayor of Dallas by phone earlier today. You can hear what he had to say on our website.