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In this issue of the Journal, Fradin and Day (see pages 13–18) clearly demonstrate that not all repellents are created equal by showing that some protect only transiently or not at all. Under ideal laboratory conditions, products containing N,N-diethyl-m-toluamide (now called N,N-diethyl-3-methylbenzamide and known as DEET) protect considerably longer than other synthetic and botanical repellents. Although concentrated DEET formulations protect longer than those that are more dilute, little improvement is offered by concentrations of the active ingredient higher than 50 percent. The transient protection offered by dilute preparations, however, can be extended by reapplication.
Few people willingly thrust their arms into small cages filled with hungry mosquitoes, as subjects in this study did. So how relevant to the real world are the observations made in such studies? Failure of a candidate repellent under laboratory conditions, of course, virtually ensures failure in the field, but success in the laboratory may not translate to success in the field. Although DEET-based preparations powerfully deter many biting arthropods — including the mosquito that carries yellow fever (Aëdes aegypti), the species tested by Fradin and Day — they are less effective against various other types of mosquitoes, including certain of the anopheline mosquitoes that transmit malaria.
DEET is far less toxic than many people believe. Adverse effects, though documented, are infrequent and are generally associated with gross overuse of the product. The risk of DEET-related adverse effects pales in comparison with the risk of acquiring vector-borne infection in places where such diseases are endemic. Users should avoid the temptation to apply the most concentrated formulation available and, when using a more dilute product, should reapply it as protection wanes. Alternative "natural" products generally fail to live up to their reputations for greater safety and effectiveness and offer their users a false sense of security.
Various mass-marketed devices are advertised as "most effective" in distracting mosquitoes from people. Such contraptions are said to protect against pests and vector-borne disease either by repelling blood-feeding insects or by attracting and trapping them. These claims rely heavily on subjective testimonials and have not been well documented in peer-reviewed studies. The attractants in such devices may even increase the local density of mosquitoes. A $10 bottle of a DEET-based repellent is more cost effective and eminently more portable than such elegantly constructed machinery.
In the United States, the sale of insect repellents is regulated by the Environmental Protection Agency (EPA). Registration requirements are more stringent for products for which health benefits are claimed than for those that are simply labeled as repellents. Results of assays such as those reported by Fradin and Day might motivate the EPA and perhaps the Federal Trade Commission to scrutinize these products and their manufacturers' claims more closely. What is needed is realistic labeling that offers at least a modicum of guidance as to how much repellent to apply and its expected duration of protection.
Concentrated DEET formulations (
35 percent) may be appropriate for those who are exposed for many hours to numerous black flies or mosquitoes or who work in tick-infested areas. Less concentrated products might be used where more transient protection is desired. Although the susceptibility of children to DEET-related toxic effects remains uncertain, the use of a relatively dilute product might be prudent. DEET does irritate mucous membranes, and concentrated formulations dissolve plastic. As Fradin and Day nicely demonstrate, we currently have access to effective insect repellents that are safe when used judiciously.
Richard J. Pollack, Ph.D.
Anthony E. Kiszewski, D.Sc.
Andrew Spielman, Sc.D.
Harvard School of Public Health
Boston, MA 02115
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Related Letters:
Insect Repellents and Mosquito Bites
Teal J. J., Gerberg E. J., Novak R. J., Gerrish S., Adams D. R., Anderson B. E., Ammirati C. T., Harvey S. C., Brownstone G., Fradin M. S., Day J. F.
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N Engl J Med 2002;
347:1719-1721, Nov 21, 2002.
Correspondence
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