By Patricia Sullivan
Washington Post Staff Writer
Saturday, July 12, 2008
All Robin Levin has to do is step outside her Takoma Park home, and the hordes descend.
"Basically, if there's a mosquito within a seven-block radius, they find their way to me," she said. "I'm actually a good repellant for everyone else, because mosquitoes are attracted to me like a lightning rod."
She has the ruddy welts to prove it, evidence that mosquitoes have been piercing her fair skin, injecting saliva and anticoagulants and sucking out her blood for nourishment.
Levin, like so many other humans, is unconsciously attracting mosquitoes: Every time she breathes out, she expels carbon dioxide, a powerful siren song to mosquitoes. Her skin exudes hundreds of volatile organic compounds, such as acetone and lactic acid, as do all humans', but her particular combination sends out a come-hither wink to the neighborhood insects. In addition, she's a woman, and some scientific studies show that the biting mosquitoes, which are female, prefer their own gender, although no one is sure why.
What is clear is that to mosquitoes, some of us are smorgasbords, and others are as tasty as cardboard. As the mosquito season begins in earnest -- if it hasn't already in your microclimate -- those on the buffet line might dread its arrival more than the rest of us, but there's no escaping.
Virtually any place on Earth where the sun's long, warm rays coincide with the smallest bit of moisture, you will find some of the 3,700 species. Only 150 of them live in North America, and about 60 of those species call the Washington area home. Every time a new species bites you, your immune system goes into overdrive, often creating a big, angry welt; bites from more familiar mosquito species are smaller.
Just as not everyone attracts mosquitoes, not everyone who is bitten is bothered by the result.
"Some individuals are more sensitive to bites, so they notice them more," said Jonathan F. Day, a professor at the Florida Medical Entomology Laboratory in Vero Beach, Fla. "Some people produce more cues than others. They give off more carbon dioxide. Anyone who is really active or has a naturally high metabolism. It's more than carbon dioxide, though, or mosquitoes would waste a lot of time chasing cars."
In this area, most mosquitoes fall into three main genera: Culex, Aedes and Anopheles. All of them start their lives as eggs deposited in or near water, even in tiny pools that collect in the bottoms of planters or drainage pipes. A mother can lay up to 300 eggs at a time. The eggs hatch into a larvae, and after a few weeks, they turn into pupae. Within days, those pupae become adults, and if you walk into their path, you are their first meal. They usually live only a few weeks, so their interest in mammal blood is intense.
Mosquitoes find you by searching, the same way a bird dog hunts birds, Day said. Only the females bite, because, as egg-bearers, they are in search of the added nutrition that blood provides. As they zero in on their victim, sight comes into play. The best targets are people in motion, those who provide a large dark target against a light background, and those who provide meals close to the ground, where low wind and high humidity favors insects.
When she lands, she probes for a capillary, then inserts her very thin and sharp proboscis through the skin. She injects you with saliva, which numbs your skin and stops your blood from clotting. Then she drinks for one to three minutes. It's the saliva's proteins, not the bite, that irritate human skin and causes the itchy bump we call a mosquito bite.
The most common native mosquitoes, those in the Culex and Anopheles genera, feed mostly after dusk, when it's easier to bite birds and mammals as they're settling down to sleep. The Culex and Anopheles mosquitoes also are most likely the bugs you hear buzzing near your ear as you lay in bed on hot summer nights. They spend their days in the shade of bushes, grasses or leaf litter, near the twin food sources of moisture and microorganisms, and hidden from predators.
But the predominant mosquito in the area is the nonnative Aedes albopictus, or the Asian tiger mosquito, which slipped into our back yards 20 years ago. This black-and-white striped critter doesn't wait for dusk, as the other self-respecting skeeters do, but feeds all day long. All you have to do to trigger its wrath is stroll by the ivy or azaleas during the daylight hours and zing! Ouch! The Asian tiger targets the lower leg, between the knee and ankle, and scientists call it an aggressive daytime biter.
"It's the most common nuisance mosquito we have, and we found the West Nile virus in it last year for the first time," said Jorge Arias, the environmental health supervisor of the Fairfax County Health Department and an entomologist known as the mosquito hunter. His department's Web site, http://www.fairfaxcounty.gov/fightthebite, offers a comprehensive reference guide to mosquitoes and other disease-carrying insects.
Although the hysteria over West Nile virus has died down, it's still an active virus in the area. Between 2001 to 2006, Maryland alone had 146 human cases of West Nile virus , including 19 fatalities. Mosquitoes carry a wide variety of other dangerous diseases, including encephalitis.
The latest dread disease carried by the Asian tiger mosquito is chikungunya, which a Centers for Disease Control and Prevention online fact sheet notes is usually not fatal ( usually?) and "has been cited as the cause of numerous human epidemics in many areas of Africa and Asia and most recently in limited areas of Europe." It "can cause a debilitating illness, most often characterized by fever, headache, fatigue, nausea, vomiting, muscle pain, rash, and joint pain."
"If [chikungunya] comes here, we are going to be busy," Arias said. "It's a lot like dengue fever, but I don't want to scare everybody."
Too late, Dr. Arias. Mosquitoes transmit disease to 700 million humans every year, and the arrival of chikungunya in the United States is probably just a matter of time, other experts say.
So what good are mosquitoes? Setting aside the argument over whether humans should seek to destroy a species because we perceive it as a threat, the truth is that mosquitoes have a firmly established niche in the natural world. Their larvae and pupae are attractive food for fish and other aquatic insects, and the adults provide a tasty meal for birds, bats, spiders and dragonflies. The mosquitoes themselves feed on bacteria and algae from rocks, plants and water, as well as on birds, pets and humans.
Prevention comes in three versions: Rid the environment of the mosquitoes, block the insects from finding skin or repel them from the skin they do find.
Mosquito control districts try to eliminate the largest nesting areas for mosquitoes and aim to eliminate them when they're still larvae and stationary. On an individual level, most people know to drain standing water on their property every week, as the smallest bit of undrained rainwater can provide a lovely maternity ward for mosquito eggs.
You'll note that men in business suits are rarely plagued by mosquitoes. That's because socks, long pants, collared shirts and long sleeves are great avoidance strategies.
Finally, repellants work. Emily Zielinski-Gutierrez, a behavioral scientist with the CDC, said DEET-based sprays, lotions and wipes are most effective, although effectiveness does not increase after 50 percent of the solution is DEET. The agency also recommends another synthetic chemical, Picaridin, an odorless preparation good for short-term use.
On the non-toxic front, the CDC this year added to its approved list IR 3535, a biochemical functionally identical to naturally occurring beta alanine. IR 3535 is in such products as the old outdoorsman's remedy Avon's Skin-So-Soft lotion. There's also oil of lemon eucalyptus, which lasts two to four hours, with a "quite pungent" aroma, Zielinski-Gutierrez said. Oil of lemon eucalyptus is not approved for use by children younger than 3.
For Robin Levin, whose attractiveness to insects thwarts her enjoyment of the outdoors, there is good news. Maj. Kendra Lawrence, deputy director of the division of entomology at Walter Reed Army Institute of Research in Rockville, notes that there's evidence that as people age, they are less susceptible to mosquito bites as their body chemistry changes.
"They really itch, they're horrible," Levin said of the bites. "I don't sit outside that much in summer. But in the scheme of things in the world, there are worse things to deal with."