Deconstructing Dengue: How Old Is That Mosquito?

Mosquitoes like this one can carry the virus that causes dengue fever.

Mosquitoes like this one can carry the virus that causes dengue fever. James Gathany/CDC Public Health Image Library hide caption

itoggle caption James Gathany/CDC Public Health Image Library

Scientists can spend years working on problems that at first may seem esoteric and rather pointless. For example, there's a scientist in Arizona who's trying to find a way to measure the age of wild mosquitoes.

As weird as that sounds, the work is important for what it will tell scientists about the natural history of mosquitoes. It also could have major implications for human health.

Here's why. There's a nasty disease called dengue that is just beginning to show up in the United States. It's caused by a virus, and it's transmitted from person to person by a mosquito. A mild case of dengue is no worse than flu. A serious case can mean death.

Michael Riehle at the University of Arizona is trying to solve a curious puzzle about dengue: why there have been dozens of cases in nearby Texas and none, or virtually none, in Arizona. Riehle thinks the answer has to do with Arizona's geography.

"It's right on the edge of the range where these dengue mosquitoes are found," he says. "It's a fairly harsh environment, and we think that they might not be surviving long enough to efficiently transfer the disease to other people."

So to test his hypothesis, Riehle wants to be able to compare the life spans of mosquitoes in Arizona with those in Texas.

It's not easy to tell how old a mosquito is: It's not as if they carry around birth certificates or government-issued IDs. Right now the tools for measuring the age of mosquitoes are pretty crude. For example, you can look at a female mosquito's ovaries to see if they have produced any eggs. Riehle says if they have, that means the mosquito is at least five days old, since they can't produce eggs before that. "But that's all it can tell us — less than five days, or more than five days," Riehle says.

So Riehle has a new idea. He wants to see if he can use a mosquito's gene to tell its age.

Looking For Clues In Genes

There are ways to tell when a particular gene is switched on or off in a mosquito. Riehle is looking for genes that switch on or off when the mosquito reaches a particular age. He's found one so far. He needs more in order to make more age estimates.

To help in his search, he raises mosquitoes in a special climate-controlled room down the hall from his office. The insectary is about the size of a large closet with metal shelves floor to ceiling. This place is a Tupperware salesman's dream — the shelves are stuffed with plastic containers in a variety of convenient sizes. It's a level 2 containment facility, so the mosquitoes won't get out.

Since he knows exactly when mosquitoes are born in the lab, that gives him a precise starting point to see how, or if, different genes change over time. If he can find genes that change with age in the lab, Riehle can look for the same genes in wild mosquitoes and use them to estimate the wild mosquitoes' age.

This work has implications for a number of mosquito-borne diseases. Malaria mosquitoes need to live at least two weeks before they can transmit the malaria parasite. Knowing more about the age of wild mosquito populations could help prevent, or at least predict, outbreaks of disease.

But the search for age-related genes is going slowly.

That's not terribly surprising. In science, you usually have to go down a lot of blind alleys before you find what you're looking for, if you ever do. Riehle says it's a lesson students coming into his lab have to learn.

"You give them a project, and they just expect it to work, have no problems, get their results by the end of the semester, and they're on their way," he says. "It's always interesting to see them learn that, yeah, a lot of science is failure and building upon what doesn't work."

Failure is inevitable in science, so it's not failure itself that's bad, it's just not learning anything from your failures. "Actually, what we call failure a lot of times leads to new lines of discovery," says Riehle.

So Riehle will continue to search for genes that will reveal a mosquito's age. With persistence, and enough failures, he thinks he'll find them.

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