You've probably heard by now that the summer of 2003 will likely be the summer of the mosquito, particularly in the Northeast. Thanks to a record-breaking wet spring, mosquitoes are hatching at an unprecedented rate. And although scratching those bites around your ankles may seem like a minor annoyance, there are potential consequences to being snacked on. Scientists are predicting that the large pest population could soon result in higher rates of the West Nile virus, which is most commonly transmitted to humans by mosquitoes that have bitten infected birds.
Most cases of West Nile disease are mild, but the elderly and those with compromised immune systems are at greatest risk for serious infections. The virus first appeared in the United States in New York state in 1999 and has been working its way across the country ever since.
Still, rates of West Nile are low, especially when compared to rates of insect-borne disease in other parts of the world. Dr. Mark Fradin, a clinical associate professor of dermatology at the University of North Carolina at Chapel Hill, who did a study comparing commercially available insect repellents that was published in The New England Journal of Medicine last summer, says that there are a number of ways that people can safely protect themselves from mosquitoes. Below, Dr. Fradin discusses the most effective repellents on the market, as well as other approaches for avoiding bites.
How can people reduce their risk of being bitten?
I always talk to people about a three-pronged approach. The first step is avoidance, which means that if you find that the mosquitoes are most likely to be around your house and biting at dusk or dawn, avoid being outside at those times of day. If you're camping, keep your tent away from wet areas that tend to be havens for mosquitoes.
The second step is protection that can be achieved through covering the skin by wearing long pants, a long-sleeved shirt and a hat. The third step is protecting the skin by applying insect repellent.
What types of repellents are commercially available?
Repellents essentially fall into two categories. The first category is "chemical," which means that they're synthetic or man-made. The chemical repellents sold in the United States are DEET (N,N-diethyl-3-methylbenzamide) and IR3535. In a laboratory study that we did with a particular species of mosquito, DEET was found to be the most effective commercially available repellent.
The second category is the so-called "plant-based" insect repellents. The most common repellents that are botanical in origin are based on citronella. Oil of citronella, which has a somewhat lemony smell, was originally isolated from a couple of cultivated grasses. It is used in skin repellents as well as candles. It's been in used in this country as a repellent since about 1948, but very variable efficacy has been reported with its use.
Citronella, for the most part, has a very limited duration of action; in our study, we were not able to get more than about 20 minutes of protection out of any citronella product we purchased, regardless of the concentration.
There are, however, a few other plant-based repellents that are now available. There's a single soybean-oil–based insect repellent on the market. And just this year, a couple of repellents that are based on oil of eucalyptus became available.
Is there a concern about the safety of DEET?
There are fewer than 50 reported cases of significant toxicity directly related to DEET use published in the English language medical literature. Those relatively small numbers are against a background of an estimated 8 billion applications of DEET since it first came to market in 1957. Even if there is a 10-fold concern of under-reporting, it's still a relatively small risk. And many of the toxicity cases involved very heavy and frequent application of repellent. People would apply the repellent to the whole body for days on end, and not wash it off when they went back inside. This was not common sense application.
In 1998, the Environmental Protection Agency (EPA) issued a re-registration standard for DEET, to ensure that it met newer, more stringent, safety requirements. After looking at the results of many studies that it required, the EPA did not feel that they had to change the labeling requirements based on the new data. The bottom line is that if you're careful about where and how DEET is applied, it should be a safe product.
It is possible to be allergic to it, but it's a very rare scenario. There are some people that get hives or a rash from it.
How do you know what concentration of DEET you need?
The vast majority of the DEET-based repellents that you see have a DEET concentration of between 5 percent and 15 or 20 percent. There are some products that go up to 35 percent concentration. And then, there are a few 100 percent DEET repellents still sold.
When people go to buy DEET insect repellents, the natural tendency is to buy the highest concentration. But any DEET-based repellent should give you near 100 percent protection. It's the length of time that it will provide that protection that varies with the concentration.
If you only need two hours of protection from a DEET-based repellent, you can probably use a repellent with 10 percent DEET. But if your child is going to be outside at camp for eight hours, and you only have one chance to put a repellent on, then the repellent that's only 10 percent DEET is not going to give eight hours of complete protection.
What are some effective non-DEET repellents?
In the study that we published last year, out of the chemical and botanical repellents available in this country, the repellent that seemed to work best after DEET was the oil of eucalyptus-based repellent. Bite Blocker, a soybean-oil–based repellent worked best after that.
Still, there's a huge falloff in effectiveness in terms of duration with those products when compared to DEET. You can get as much as 12 hours of continuous protection following a single application of DEET. The oil of eucalyptus gave, on average, about two hours of protection. So if you want 8 to 12 hours of continuous protection, you're not going to be able to get it from anything but DEET.
The other thing that I think is worth mentioning is Skin-So-Soft bath oil, which has mythical status as an effective repellent. In our study, we found that the bath oil does not do anything. And other attempts to repel insects by ingesting something like garlic or brewer's yeast have been pretty much proven not to work.
Why are certain people more likely to attract mosquitoes than others?
Some people just naturally draw insects to them more readily because of their body chemistry. It's not well understood. There may be three to four hundred compounds that are released from the body from the skin and/or breath as a byproduct of metabolism. There are a couple of things that we know attract mosquitoes to warm-blooded animals: carbon dioxide and lactic acid. Carbon dioxide, which comes from breath and also from the skin, can probably attract a mosquito from as along as 100 feet away. Lactic acid also seems to attract from very long distances.
What is the best way to apply insect repellent and how it should they be applied?
It should be applied sparingly, only to the exposed areas of the skin. There's no reason to put it, for example, under clothing. If it's a spray, it should not be sprayed so heavily that it drips off. Just a light spray is fine.
Repellent should not be applied to broken skin, or where there's eczema. It should be kept away from the eyes. If you want to apply repellent to your face, dispense it first into your hands and then wipe it carefully on your face. I usually tell parents with very young children not to let the children apply it themselves. They either over-apply it or they miss areas. Or they rub it into their eyes, where it can be quite irritating.
Typically, repellent should only be reapplied when it doesn't seem to be working any more. If you notice that mosquitoes are landing or beginning to bite, that's the time to reapply it. It's really not meant to be used like sunscreen, which you reapply every couple of hours.
You also want to wash the repellent off when you go inside, when you don't need it any longer.
Are there any groups of people who shouldn't wear insect repellent?
You're always going to have people who will be allergic to anything, so they're going to have to figure that out by trial and error. For all practical purposes, it shouldn't be used on very small children. It was originally recommended that DEET not be applied to children under two years old, but recent papers suggest that it's safe to apply sparingly to children over two months. If you apply an insect repellent—it doesn't matter if it's a botanical or it's a chemical—to a young baby's face or hands or arms, they're going to smear it all over the place. They're going to stick their hands in their mouth. They're going to rub it in their eyes. They're better protected with clothing, or you might throw one of those little insect meshes over the stroller.
Are other types of insect-repelling methods effective?
Sound-emitting devices have been proven not to work. They're banned in Europe and in Canada, though you can still see them for sale in this country.
Studies have been done on citronella candles and, essentially, they don't work. If you take a subject and put them next to a citronella candle and have another subject sitting next to nothing, the subject sitting next to nothing gets more bites. But the reason is just the candle; the burning flame is a source of warmth and of moisture and that probably serves as a decoy. There's no difference been a regular candle and a citronella candle in terms of helping with your biting rates.
Other things that don't work are these so-called citronella plants, which are supposed to repel insects from the environment. Not only do they not work, but you can actually see mosquitoes landing on the leaves.
The traditional bug zappers indiscriminately lure insects and electrocute them, and 99.9 percent of what they kill are benign bugs, not mosquitoes. You're beginning to see the next generation of these insect zappers in stores now. These zappers try to lure mosquitoes uniquely, with warmth, moisture and a chemical that was originally isolated from cow's breath. They can kill significant numbers of mosquitoes, but the issue is can you actually put enough of those machines into an environment to deplete the local population. That's not easy if you have a machine and your neighbors don't.
What is your mosquito protection advice to people who spend a lot of time outdoors?
I think protective clothing, when reasonable, is worth wearing. And then treat the unprotected parts of the skin. I would use a DEET-based repellent, because they have the longest duration of action. But if someone is averse to DEET, I would use the oil of eucalyptus or the soybean-oil based–repellent Bite Blocker.