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Summer is here, and you're probably finding yourself swatting away a mosquito or two trying to drill a hole in your ear. Mosquitoes can be more than annoying. They transmit diseases like malaria and West Nile virus. In Latin America and Asia and even in a few places in the U.S., another mosquito-borne disease called dengue fever is growing more common.

Today, we have the first of two reports on the problem. Reporter Dan Charles traveled to the Amazon region of Peru where an outbreak turned deadly. Scientists there are trying to figure out how to stop the mosquito that carries dengue.

DAN CHARLES: Earlier this year, an epidemic of dengue fever broke out in Iquitos, Peru. Almost a thousand feverish patients filled the regional hospital. Dr. Stalin Vilcaromero says temporary cots lined stairwells and hallways.

So, this area was full of cots, patients...

Dr. STALIN VILCAROMERO: Si, full of patients. I remember the third floor and the fourth floor, full of patients.

CHARLES: Most of the time, dengue is no worse than the bad case of the flu. But sometimes, especially when a person gets re-infected, it turns into something much worse: dengue hemorrhagic fever. In Iquitos, at least 15 people died.

The epidemic passed. On this evening, only two dengue patients remain in the hospital. Vilcaromero takes me to see one of them.

Dr. VILCAROMERO: Finally, he's leaving the intensive care unit. Now, he's in this area. He's one of the survivors.

CHARLES: Can you tell me your name?

Mr. JAVIER ALMINDO GARCIA: Javier Almindo Garcia.

CHARLES: Did you know it was dengue? Did you know what it was?

Mr. GARCIA: (Through translator) Well, I had my tests done, and they came out positive.

CHARLES: Do you have any idea where you might have gotten it?

Mr. GARCIA: (Through translator) No, I have no idea. It just suddenly hit me.

CHARLES: Most people in Iquitos do know dengue is caused by mosquitoes, but there's a lot about these mosquitoes they don't know. In fact, there's a lot that scientists don't know, either, about how best to keep dengue from spreading.

(Soundbite of crowd chatter)

CHARLES: Every morning, a dozen young men go door to door down sun-baked streets in Iquitos that are sometimes paved, sometimes just dirt, carrying out a mosquito census.

(Soundbite of knocking)

CHARLES: Two-by-two, the men knock on doors. Usually, they're invited in.

Unidentified Man: (Spanish spoken)

CHARLES: Inside, walls of rough, wooden planks divide the space into rooms. Clothing is piled on tables. One mosquito hunter pokes around in dark corners with a miniature vacuum cleaner that traps insects in a wire-mesh cage. His partner counts water containers where mosquitoes could lay their eggs.

My guide here is a scientist named Amy Morrison. She leads the U.S. Navy's medical research unit here in Iquitos. She's also field director for the Mosquito Research Laboratory at the University of California Davis.

Dr. AMY MORRISON (Field Director, Mosquito Research Laboratory, University of California Davis): I'll give you a good example of kind of ideal resting site. You know, the clothes, under beds.

CHARLES: And in one corner, the mosquito hunters find their query: tiny specks in a bucket of water. To me, they look just like bits of dirt, but they're actually mosquito larvae.

Ms. MORRISON: I'd probably miss it, too.

CHARLES: Where did we find it?

Ms. MORRISON: So, they found this bucket in the bathroom. (Spanish spoken)

Unidentified Man: Si, senora.

Ms. MORRISON: It was precisely under a location where there is a roof leak. And that's a very common practice here for people to put a bucket or a container under a roof leak. You know, it's just there collecting water.

CHARLES: These larvae are the specific kind of mosquito that spreads dengue fever. The species is called Aedes aegypti.

Ms. MORRISON: What's fascinating to me about aegypti is it's probably the mosquito vector most closely associated with human beings and the most adapted to human beings.

CHARLES: Aedes aegypti is a kind of unwelcome house pet. No other mosquito species is quite so comfortable laying its eggs inside people's homes. It thrives in tropical cities. And more than most mosquitoes, the female of this species has a particular taste for blood.

Half a century ago, public health officials unleashed chemical insecticides on Aedes aegypti and actually wiped it out in large parts of Latin America. By the late 1970s, it was gone from all of Peru. Dengue fever disappeared, too. But it survived elsewhere. Scientists now say it was bound to. And 10 years later, inevitably, it came back.

Ms. MORRISON: And it was first detected in Iquitos. That was the first place that the reinvasion or reemergence of Aedes aegypti was detected.

CHARLES: The dengue virus soon followed, and scientists decided this city in the middle of the Amazon would be an ideal place to monitor how the virus spreads. So, along with a mosquito census, teams of nurses monitor people who live in selected neighborhoods, going door to door to see who's running a fever, testing to see if it's dengue.

Hundreds of people give blood samples every six months. All this information gets recorded in a computerized map of the city. It's a picture - a movie, really - of the virus's migration.

Amy Morrison and her coworkers are hoping they will help them understand the spread of mosquito-borne diseases everywhere, even in the United States.

Mr. ROGER NASCI (Centers for Disease Control and Prevention): It not only could happen here, it has happened here.

CHARLES: Roger Nasci monitors mosquito-borne diseases for the Centers for Disease Control and Prevention.

Mr. NASCI: We've had yellow fever epidemics in the United States as recently as 100 years ago, in 1910. We've had dengue epidemics as recently as 50 to 60 years ago.

CHARLES: These seem long ago, almost forgotten. They ended when we put barriers between ourselves and the insects - window screens and air conditioning. But West Nile virus was a reminder that mosquito-borne diseases aren't completely under control here, either. In just a few years, West Nile spread from New York to California.

Dengue cases exist in a few places in the U.S. now, too, and other diseases can follow, Nasci says. For instance, there's the Chikungunya virus, which has spread from East Africa to South Asia. Recently, a traveler from India brought it to Italy, and suddenly mosquitoes there picked it up and started spreading it.

Mr. NASCI: We have infected travelers coming to the United States, both dengue and Chikungunya virus, that could hypothetically and very easily set up this similar type of a transmission cycle.

CHARLES: In Iquitos, spraying campaigns the city carries out do have an impact, but they don't stop dengue entirely. Amy Morrison admits the mosquito is a really tough adversary.

Ms. MORRISON: I always say we keep discovering things that turn out to be sort of bad news to the overall goal, which is finding more clever, interesting ways to combat the mosquito or to control the mosquito.

CHARLES: The mosquito probably cannot be wiped out in Latin America, she says, but she's convinced it can be controlled well enough to prevent really big outbreaks like the one this year. And she's hoping what they learn in Iquitos will help prevent mosquito-borne diseases everywhere.

For NPR News, I'm Dan Charles.

WERTHEIMER: Tomorrow, we'll hear about recent cases of dengue fever in Florida.

(Soundbite of music)

WERTHEIMER: This is NPR News.

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