Experts try to crack the code behind why mosquitoes like some humans more than others.
By Elizabeth Heubeck, M.A.
Reviewed By Brunilda Nazario, MD
You're flipping burgers for the neighborhood barbecue, and the mosquitoes have already begun their feast -- on you. As you swat madly at the pests, you notice other folks seem completely unfazed. Could it be that mosquitoes prefer dining on some humans over others? This may clear up the mystery.
It's true. Mosquitoes do exhibit blood-sucking preferences, say the experts. "One in 10 people are highly attractive to mosquitoes," reports Jerry Butler, PhD, professor emeritus at the University of Florida. Incidentally, it's not dinner they're sucking out of you. Female mosquitoes -- males do not bite people -- need human blood to develop fertile eggs. And apparently, not just anyone's.
Who Mosquitoes Like Best
While researchers have yet to pinpoint what mosquitoes consider an ideal hunk of human flesh, the hunt is on. "There's a tremendous amount of research being conducted on what compounds and odors people exude that might be attractive to mosquitoes," says Joe Conlon, PhD, technical advisor to the American Mosquito Control Association. With 400 different compounds to examine, it's an extremely laborious process. "Researchers are just beginning to scratch the surface," he says.
Scientists do know that genetics account for a whopping 85% of our susceptibility to mosquito bites. They've also identified certain elements of our body chemistry that, when found in excess on the skin's surface, make mosquitoes swarm closer.
"People with high concentrations of steroids or cholesterol on their skin surface attract mosquitoes," Butler tells WebMD. That doesn't necessarily mean that mosquitoes prey on people with higher overall levels of cholesterol, Butler explains. They simply may be more efficient at processing cholesterol, the byproducts of which remain on the skin's surface.
Mosquitoes also target people who produce excess amounts of certain acids, such as uric acid, explains entomologist John Edman, PhD, spokesman for the Entomological Society of America. These substances can trigger the mosquitoes' olfactory sensations, or sense of smell, causing them to launch their "landing" onto unsuspecting victims.
But the process of attraction begins long before the landing. Mosquitoes can smell their dinner from an impressive distance of up to 50 kilometers, explains Edman. This doesn't bode well for people who emit large quantities of carbon dioxide.
"Any type of carbon dioxide is attractive, even over a long distance," Conlon says. Larger people tend to give off more carbon dioxide, which is why mosquitoes typically prefer munching on adults to small children. Pregnant women are also at increased risk, as they produce a greater-than-normal amount of exhaled carbon dioxide. Movement and heat also attract mosquitoes.
So if you want to avoid an onslaught of mosquito bites at your next outdoor gathering, stake out a chaise lounge rather than a spot on the volleyball team. Here's why. As you run around the volleyball court, the mosquitoes sense your movement and head toward you. When you pant from exertion, the smell of carbon dioxide from your heavy breathing draws them closer. So does the lactic acid pouring from your sweat glands. And then -- gotcha.
Where Mosquitoes Lurk
Even if your body chemistry doesn't attract mosquitoes, where you're located might.
Some of the worst mosquito populations exist along coastal areas, Conlon tells WebMD. And being several miles inland does not guarantee your safety from the pests. "They'll fly 40 miles for a meal," Conlon says.
While any water source is potential breeding grounds for mosquitoes, they much prefer stagnant water. So if you crave a mosquito-free water oasis on your property, forego the backyard pond and seek out a babbling brook instead.
"Even in a desert area, mosquito biting tends to be intense around a water source," Conlon says.
Can you find respite high in the mountains? Don't count on it. Although they're generally not active below 55 degrees Fahrenheit, mosquitoes have been sighted in the Himalayan Mountains, Conlon tells WebMD.
How about cold climates in places such as Alaska? You're safe for most of the year. But, says Conlon, mosquitoes flock there for a brief, three-week period between July and August. "The Arctic National Refuge is one big bog," Conlon explains, making the mosquito population there second only to that in the Florida Everglades.
With a long track record -- mosquitoes have been around for 170 million years -- and more than 175 known species in the U.S., these shrewd summertime pests clearly aren't going to disappear any time soon. But you can minimize their impact.
Keeping the Bite at Bay -- Chemical-Based Repellants
Plenty of mosquito repellants line the shelves of drug stores and supermarkets each summer, but they're not all created equally.
The majority of available mosquito repellants derive their effectiveness from chemicals. Protecting the public from mosquitoes since 1957, DEET continues to be the chemical of choice used in repellants. In repeated studies, it's been proven the most effective chemical repellant on the market. Repellants with 23.8% DEET (most formulas contain between 10% and 30%) protect wearers for about five hours, according to a recent study led by Mark Fradin, PhD, researcher with Chapel Hill Dermatology. The American Academy of Pediatrics and other experts suggest that it is safe to apply repellent with low concentrations of DEET (10% or less) to infants over 2 months old.
The chemical IR3535, better known as Avon's Skin-So-Soft, has also been marketed as a repellant in the U.S. in recent years. To date, research shows it's much less effective than DEET.
Safety of Chemical Repellants
Just how safe is it to coat yourself in a chemical-based product like DEET just to keep from getting bit by mosquitoes?
"[DEET] has been in use for over 40 years and has a remarkable safety record. Only few hospitalizations have been reported, mainly due to gross overuse," Conlon tells WebMD.
The Environmental Protection Agency (EPA), after extensively assessing the safety of DEET, concluded that "as long as consumers follow label directions and take proper precautions, insect repellents containing DEET do not present a health concern." The agency does, however, offer the following safety strategies for DEET use:
Follow label directions and precautions. Use sparingly. Avoid spraying on or near open skin, eyes, mouth, and nose, under clothing, or near food. Wash treated skin with soap and water.
The American Academy of Pediatrics (AAP) provides these additional recommendations for DEET use on children:
Select the lowest concentration effective for the amount of time spent outdoors. Avoid use on infants under 2 months of age. Avoid repeated applications, which may increase the potential toxic effects of DEET.
Want to avoid chemical-based repellants altogether? Alternatives do exist, with one or two showing promise.
"Of the products we tested, the soybean oil-based repellant was able to protect from mosquito bites for about 1.5 hours," Fradin reports. He and fellow researchers found other oils -- citronella, cedar, peppermint, lemongrass, and geranium -- provide short-lived protection at best. Oil-of-eucalyptus products, however, may offer longer-lasting protection, preliminary studies show.
Hate to spray or slather yourself with any product, either chemical- or plant-based? Mosquito traps, a relatively new product, may be the answer. They work by emitting substances that biting mosquitoes find attractive -- such as carbon dioxide, heat, moisture, and other mosquito-friendly byproducts. They attract, then trap or kill female mosquitoes. When placed strategically near breeding spots "they have knocked [mosquito] populations down," Conlon tells WebMD.
So, is it worth the effort it takes to prevent mosquitoes from nipping at your ankles? Yes, if you don't want to be bothered by bouts of mosquito-induced itching all summer long. Certainly, if you are one of the few unfortunate souls in whom mosquito bites result in severe allergic reactions. And most definitely if you believe you're likely to be exposed to potentially fatal mosquito-borne diseases, some of which are becoming increasingly common. Take the mosquito-transmitted West Nile virus, for instance which Conlon says "is probably here to stay." And with it, the age-old, ever-adaptable mosquito.
Published July 12, 2004.
SOURCES: Jerry Butler, PhD, Professor Emeritus, University of Florida. Joe Conlon, PhD, technical advisor, American Mosquito Control Association. John Edman, PhD, Entomological Society of America; and Center for Vector-Borne Disease Research, University of California-Davis. Mark Fradin, PhD, Chapel Hill Dermatology, North Carolina. Environmental Protection Agency web site. The American Academy of Pediatrics Committee on Environmental Health web site.
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