San José, Costa Rica, since 1956
Science

It may sting, but mosquitoes just find some people more attractive than others

When mankind appeared about 7 million years ago, mosquitoes were already there, waiting to bite. They have been tormenting us ever since. It’s only the females who go after us. They feed on nectar and sugar, but they need protein from our blood to lay their eggs. And one female can produce several million offspring in a single season, in several separate sets, according to the American Mosquito Control Association.

Those fertile females are built to find and consume that meal. They have sensory structures (antennae with receptors) on their heads, used to cue in on our scent, which includes a combination of odorants. They bite with their proboscis, a mouthpart that males have, too, minus the mechanics to penetrate skin. So the “boys” simply use it to lap nectar.

Their several-stage life cycle spans days to a month, depending on mosquito type and weather. They start as eggs, laid one at a time or attached to form “rafts” that float on water. They hatch into larvae, spending their time hanging upside down on the water’s surface, shedding their skin several times. And finally they transform to adults, ready to take flight, traveling yards to miles from where they hatched, depending on species.

“Mosquitoes are attracted to a blend of human odorants that they detect with the receptors on their heads,” said Doug Norris, a microbiology professor at Johns Hopkins Bloomberg School of Public Health in Baltimore. They especially like the smell of carbon dioxide.

“As you exhale, you breathe out a CO2 plume that travels pretty far,” Norris said. “Mosquitoes go where they sense higher concentrations of it, looking for a blood meal.” The bigger or more active you are, he said, the more CO2 you put out, making you a prime target.

But mosquitoes detect other body scents, some known to be affected by bacteria on skin. “Research shows, for instance, that byproducts of bacteria that make socks and cheese stinky attract malaria mosquitoes,” Norris said. Sweaty people are more attractive to most mosquitoes, too; they like sweat’s lactic acid content.

L.J. Zwiebel, professor of biological science and pharmacology at Vanderbilt University in Nashville, said mosquitoes “see” the world through their nose, using complex sensors to pick up a combination of odor-emitting chemicals.

“When you smell roses, you detect a collection of chemicals,” he said. “Only when you put them together can you perceive scent as a rose. Mosquitoes have a similar sophisticated sense. They can tell two people apart based on human body odor produced by these chemical blends.”

Blood-feasting females are smart, and they’re fast, warns Zwiebel.

“She can be flying in one direction, turn on a dime and find you when she smells you. Some of them can find us from hundreds of yards away,” he said.

Our individual body chemistry is believed to be key in determining a mosquito’s attraction to us, which studies indicate is tied to genetics.

Some scientists also theorize that our diet may affect mosquitoes’ preference, though other research suggests not. Claims that garlic consumption repels mosquitoes do not hold up, but one study tied beer consumption to more bites.

You can make it harder for them to single you out as a menu selection. One way is to avoid the outdoors around dawn or dusk when mosquitoes feed. While they use smell to find you, they also notice movement and when the moon is full they are more likely to track you down by taking advantage of the night-time light, according to Joe Conlon, technical adviser at the American Mosquito Control Association.

If you would rather not plan your schedule around theirs, be sure to put up a good defense when you step outside. For instance, don’t give them as much exposed skin to come after.

“Wear long pants and sleeves. And wear tight-weaved, loose-fitting clothes, as tight weaves and clothes not pressed against skin are hard to bite through,” said Conlon.

Manufacturers make garments impregnated with a pesticide called permethrin for people living in heavily infested areas. Factory application of permethrin is approved by the U.S. Environmental Protection Agency for certain clothes and does not leach onto skin.

A simpler way to deter the bugs is to lather up with antibacterial soap and wash often. If you go camping, to lakes or other mosquito-infested areas, bring a bar of antibacterial soap, suggests Zwiebel. This will remove odor-producing bacteria.

Common mosquitoes like the Asian tiger and house mosquitoes fly only about 300 to 500 feet from where they live. So if you are getting bitten on your property, they are probably breeding in your yard, or your neighbor’s, said Peter Armbruster, a biology professor at Georgetown University.

Many of the folks he speaks to know that mosquitoes need water to multiply, aware that they lay their eggs on water and that larvae grow there.

“Residents tell us, ‘You won’t find any in my backyard. I got rid of conditions where they can breed.’ But my research team and I almost always find a bunch of them, whether in an old washing machine discarded in the back yard that filled with water, or in clogged rain gutters that collect water, or even in little bottle caps,” Armbruster said.

Eggs can lay dormant through freezing winters and hatch during the next spring and summer rains, though you would never know it, as they are microscopic. Pest controllers advise using a larvicide where and when mosquitoes breed to kill larvae before they transform to adults. They also suggest removing the insects’ habitats. Some people place mosquito-eating fish in ponds.

Despite precautions, you may find yourself with a bruiser of a welt that itches insanely.

“A big welt just means your body really did not like that bite,” Norris said. “You are reacting to the saliva they inject, but this is rarely dangerous.”

Slather on bite creams to reduce the itch, but call a doctor if you develop a fever within a few days of getting bit to rule out mosquito-transmitted infection, he said.

The most common such disease in the U.S. is West Nile virus. In 2014, the U.S. Centers for Disease Control and Prevention reported 2,122 cases, including 85 deaths.

A less common mosquito-borne virus is chikungunya. There were 200 cases reported in the United States as of June 30, carried in by travelers from tropical regions such as the Caribbean.

Read The Tico Times coverage of chikungunya in Costa Rica

“The fatality rate is very low, but the joint pain can be debilitating,” Norris said, adding that chikungunya symptoms appear in three to seven days and that the infection almost always resolves on its own within 10 days. The CDC advises people to see their doctor if they suspect they or a family member may have this virus.

As annoying and potentially dangerous as mosquitoes are, the illnesses they transmit did not originate with them, Armbruster said. Mosquitoes actually pick up disease by biting an infected person or animal. Then they bite an uninfected person.

Only a small minority of mosquitoes spread disease, he assures.

© 2015, The Washington Post

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