- The Washington Times - Monday, June 9, 2003

Mosquitoes have gone from annoying to slightly scary in the past few years as cases of West Nile virus have surfaced in the District and surrounding areas.

But don’t fret, local doctors say. Residents can take plenty of steps, ranging from eliminating standing water in the yard to using the most effective repellents to reduce the risk of mosquito swarms interfering with the joy of summer evening cookouts.

“The best thing you can do is use an effective repellent, one that contains DEET,” says Dr. Robert Vowels, supervisory medical officer at the Environmental Health Science and Regulation Office with the District’s Department of Health.

DEET, which has been around for 40 years, is a chemical compound referred to as N,N-diethyl-m-toluamide on product labels. It doesn’t kill mosquitoes, it just makes them unable to locate us as it covers up skin odor and the carbon dioxide on our breaths that attracts mosquitoes.

This is the way most repellents work, but they should not be confused with pesticides, which are used on inanimate things. Pesticides kill mosquitoes and other insects.

“For adults, you can use repellents with up to 30 percent DEET,” Dr. Vowels says.

Generally, the more DEET a product contains, the longer-lasting the repellent will be, says Dr. David Sullivan, a malaria researcher at Johns Hopkins Bloomberg School of Health.

“Studies have shown that [a product with] 23 percent DEET gives protection for five hours,” Dr. Sullivan says.

The increase in effectiveness that goes along with more DEET only holds true up to a certain point, however. Researchers have not been able to show any increased benefit in using products containing more than 50 percent DEET.

The repellent should be used on exposed skin and not under clothes, Dr. Vowels says. Many repellents can be applied to clothes, however, to prevent mosquitoes from biting through, he says.

Parents should use products containing less DEET when protecting their children, such as one brand-name product that contains 4.75 percent DEET, Dr. Sullivan says. With children, it’s more difficult to ensure they will use the product according to the instructions — avoiding eyes and mouth and using sparingly around ears.

“Children under the age of 10 have a tendency to rub it in their eyes or in their mouths,” Dr. Vowels says. “We recommend that adults spray it on their hands and then rub it on the kids.”

For children younger than 2, parents should consult their pediatrician, Dr. Vowels says.

While DEET is the most effective repellent, there are other plant-based, or “natural,” repellents that work but are not as long lasting, Dr. Sullivan says. Among these natural repellents are cedar oil, peppermint oil, lemon grass oil, citronella and soybean oil.

Citing a New England Journal of Medicine article from July 2002, Dr. Sullivan says soybean oil is the most effective of the natural repellents. A product containing 2 percent soybean oil lasts about three hours.

While concerns about possible toxicity have been raised about DEET, both Dr. Sullivan and Dr. Vowels say the chemical ingredient has been shown to be safe.

“The [Centers for Disease Control and Prevention] and [Environmental Protection Agency] have shown that DEET is very safe for humans,” Dr. Vowels says.

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Aside from repellents, there are many other ways of avoiding mosquito bites, Dr. Vowels says.

“One of the things that we’re doing early in the season is recommending that people do all they can to control water that’s close in to the home,” Dr. Vowels says. “People may not realize it, but even small containers like bottle caps can hold enough water for mosquitoes to breed.”

Birdbaths should be cleaned out and refilled once a week to prevent mosquitoes from breeding in the water. The cycle from egg to adult takes about 10 days.

“If there are any eggs that have hatched, when the larvae don’t have any water, they die,” Dr. Vowels says.

Dr. Vowels recommends using dunks — doughnut-shaped briquettes containing bacteria that kill mosquitoes when they’re in their larval stage — in fish ponds.

Dr. Vowels also recommends that residents inspect their window and door screens to make sure there are no rips or tears through which mosquitoes can get inside the house.

It’s also important to make sure gutters are clean, he says.

“They can get backed up with leaves and create a little pond for the mosquitoes to breed in,” Dr. Vowels says.

Feeding stations for pets outside the house also can become breeding grounds for mosquitoes.

Another, more expensive way to manage mosquitoes is a product, available for a few hundred dollars at home-improvement stores, that emits carbon dioxide. This attracts and subsequently traps and kills the mosquitoes.

Dr. Sullivan says this product works, but as with all other products, doesn’t provide complete coverage.

“It does attract them, but nothing is 100 percent,” he says. “None of these are 100 percent in preventing diseases like West Nile virus. But they knock down the risk.”

Dr. Sullivan says additional mosquito prevention methods include lighting citronella candles and keeping fans blowing as well as wearing long sleeves and long pants in the back yard.

Limiting exposure, however, especially at dusk and dawn, is also an important weapon, particularly for younger children, he says.

While West Nile virus is a relatively new phenomenon in the nation’s capital, disease-transmitting mosquitoes are not.

“The same mosquito that transmitted malaria a hundred years ago is still here,” Dr. Sullivan says.

Dr. Sullivan refers to the anopheles mosquito, which transmits malaria. The species that carries West Nile virus is called culex.

Worldwide, mosquitoes transmit disease to more than 700 million people annually, according to the New England Journal of Medicine. Fortunately, not all kinds of mosquitoes transmit malaria, West Nile virus or dengue fever.

“People should be aware that most mosquitoes that bite you don’t transmit disease,” Dr. Sullivan says. “Most mosquitoes are just nuisance mosquitoes.”

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