Rules of attraction

Scientists are figuring out why mosquitoes love to feast on some humans, but not others, as well as ways to make them stop

June 14, 2007|By Frank D. Roylance Frank D. Roylance | Frank D. Roylance Frank D. Roylance,Sun Reporter

Many of us learn it by bitter experience. Now scientists say it's true.

Some people, thanks to their genetics, behavior, diet or some poorly understood combination of factors, have body chemistry that draws mosquitoes like linebackers to a loose football. Others just seem invisible to the bugs.

"I am irresistible to mosquitoes," said Michele Karanzalis, 33, a research project manager from Overlea. "I just try to stay inside a lot ... I start to get panic atacks after a while when I feel like I'm getting bit too much."

She's worn jeans and carried citronella torches in a futile bid to ward them off. But "mosquitoes bite through the jeans and fly through the flames. Yes, I am that irresistible."

Such bite victims are further tormented when people such as Randi Brown, a 33-year-old technical writer from Cockeysville, just shrug and say, "I don't know what it is, but I never get bitten."

She doesn't do anything special - no repellents, no particular foods. She just can't remember ever being bitten. "It seems to be only me," she said. "Every other member of my family is a mosquito magnet. It does drive them crazy. They say it's not fair."

Researchers have identified more than 300 chemical compounds that a human emits, chiefly from the skin, and they're trying to figure out which ones most influence mosquito behavior.

Some claim they've isolated a few ingredients that appear to make us undetectable to mosquitoes. They're working to formulate them into a new way of protecting us from biting insects.

What are they?

"I can't tell you those yet. We're still working on patenting," said Ulrich R. Bernier, a research chemist at the U.S. Department of Agriculture's Mosquito and Fly Research Unit in Gainesville, Fla.

They're not repellents, which are analogous to something that smells bad to the mosquito. Rather, they're more like a Klingon warship's cloaking device. making us undetectable, he said.

The hope is to publish findings next year, then seek a manufacturer to license the discovery, develop and market a product. He envisions an aerosol device that would release cloaking compounds into the air.

"It's looking pretty promising so far," Bernier said.

Mosquitoes can be dangerous. In the United States, they can carry potentially deadly West Nile virus, which killed 177 Americans last year. They can also transmit various encephalitis viruses.

Public health officials say the best way to stop the biting is to take precautions. Curtail the breeding by eliminating standing water around your home, from gutters to dog dishes. Avoid going out when mosquitoes are biting. When you must, use repellents with DEET and wear long sleeves, long pants and socks.

But persistent mosquitoes will find the most irresistible of us.

As dry as this spring was, "those little biters are rockin' out there. They're ready to go," said University of Maryland entomologist Mike Raupp. Besides the familiar dawn and dusk biters, Asian tiger mosquitoes arrived in the 1980s to add daytime bites to our worries.

It's the females that torment us. They need blood protein from birds or mammals to produce eggs. And those that feed on people have evolved elaborate mechanisms to find us.

"As human beings move through the world they leave behind a vapor trail," Raupp said. A major component is carbon dioxide [CO2], a byproduct of our respiration. Another is lactic acid, a breakdown product of muscular activity. It's exuded from our skin as a component of sweat.

Once they're downwind of us, mosquitoes' antennae, and mouth parts called palps, can detect our CO2 plume and other compounds. The American Mosquito Control Association says our body chemistry attracts them from nearly 40 yards away.

"And as they move out of the vapor concentration, they turn back in. They turn and correct and keep working their way upwind until they find you," Raupp said.

"Once they're upfront and personal, they're sensing moisture and a whole complex of chemicals," he said. They also can home in on our body heat, our movement and color.

Compound attraction

Your individual chemical complex is the key to how attractive you are to mosquitoes. Bernier's research captured between 300 and 400 compounds from the oily, waxy residue from human sweat. One, dimethyl disulfide, is a byproduct of bacterial activity. It appeals strongly to at least one species - Aedes aegypti - best known for transmitting yellow fever and dengue fever.

The sorts of bacteria and fungi common on human feet have also been shown to attract some mosquitoes, including Anopheles gambiae, the type that spreads malaria. So the Limberger-cheese odor of smelly feet, socks and shoes may make you a target. And it makes evolutionary sense.

"A foot is a great place to bite a human," Raupp said. "It's as far as you can get from your arms - arms and hands are lethal to mosquitoes."

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