Mosquito Repellents: What Works?
Abstract & Commentary
Synopsis: The efficacy of several commercially available insect repellents was compared. The topical application of compounds containing DEET was far more protective against mosquito bites than was application of products containing soybean oil or citronella. With the resurgence of West Nile virus in the southern United States this summer, and the potential for spread of infections, mosquito protection assumes even greater relevance at this time.
Source: Fradin MS, Day JF. Comparative efficacy of insect repellents against mosquito bites. N Engl J Med. 2002;347: 13-18.
In a laboratory setting, adult laboratory workers exposed an arm to caged, hungry female Aedes mosquitoes while using a variety of commercially available insect repellents. Sixteen separate repellent formulations were evaluated in a total of 270 test episodes on the 15 volunteer subjects. The time elapsed until the first bite was noted as the end point of "complete protection time."
The Table highlights a few of the relevant results. Topically applied products were generally more effective than were impregnated wristbands. DEET-containing products varied in efficacy with longer protection noted with higher concentration formulations. Citronella-containing products provided a maximum of 30 minutes of protection.
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Comment by Philip R. Fischer, MD, DTM&H
Travel medicine practitioners are well aware of the importance of insect repellents in preventing mosquito-transmitted infections such as malaria, yellow fever, and Japanese encephalitis. In addition, the use of insect repellents is also protective against many tick-borne diseases. With the westward and southward expansion of the endemic region of West Nile virus in North America, even nontravelers are increasingly interested in avoiding insect bites.
Advertisements, anecdotes, and the lay literature include many claims about the effectiveness and safety of various insect repellents. Fradin and Day, in a recent issue of the New England Journal of Medicine, have provided practically useful information about the relative efficacy of various readily available repellents.
For approximately 4 decades, DEET (also known as N, N-diethyl-meta-toluamide or N, N-diethyl-3-methylbenzamide) has been the most widely used insect repellent.1 With questions about the safety of DEET for human usage, alternative products have been developed. Fradin and Day have now clearly shown that DEET-containing repellents are markedly more effective than other common repellent preparations. Under standardized conditions, DEET protected against mosquito bites for much longer than any of the other preparations. As suggested in previous studies, the duration of DEET’s protection depends on the DEET concentration; peak protection is conferred by products containing about 30% DEET. Though not specifically studied in this investigation, it is not clear that high concentrations of DEET (such as the 90+% products available in some camping stores) are more effective than products containing "only" 20-30% DEET. Interestingly, a "long-acting" formulation of DEET did not seem to protect for any longer than did a similarly concentrated standard formulation. Botanical preparations containing citronella protected only for a few brief minutes. Of note, repellents were not significantly protective when applied only to a nearby wristband. The message should be clear to travelers and to domestic residents at risk of exposure to mosquitoes carrying disease: DEET is the most effective mosquito repellent.
But, is laboratory testing using caged Aedes mosquitoes relevant to the conditions faced by travelers? It is impossible to perfectly replicate any individual traveler’s situation in an artificial environment. Different individuals are differentially attractive to mosquitoes [with pregnant women being particularly appealing2], and insect repellents are not always applied in a completely careful manner. Repellent efficacy varies somewhat between different genera and species of mosquitoes as well. These factors should make us cautious about guaranteeing any specific repellent’s duration of protection for a particular traveler, but the relative efficacy for the different products tested would not be expected to change between individuals and settings.
What about safety? When ingested, DEET can have neurotoxic effects. It can also cause irritation when rubbed into eyes or, in some individuals, when rubbed vigorously (such as in elbow creases) on the skin in high concentrations. Clearly, oral and ocular applications of DEET (such as can occur inadvertently in children who rub their faces with DEET-laden hands or lick their DEET-treated forearms) should be avoided.
However, there have been a few, albeit rare, reports of serious and even fatal episodes in children who used DEET. In those cases, inappropriately frequent application (up to 10 times in a day) and oral ingestion (licking of arms) were sometimes reported, and there is no good evidence linking DEET concentration to the risk of toxicity. Of course, no product is perfectly safe. Citronella toxicity has also been reported.3
Since 1998, the American Environmental Protection Agency has removed restrictions on the concentration of DEET used in children. The American Academy of Pediatrics has acknowledged that lower concentrations of DEET are not safer than higher concentrations and that higher concentrations of DEET are more effective in repelling insects than are formulations with lower concentrations.4 Each organization does stress the importance of appropriate use of DEET with care: 1) not to apply the repellent on or near the eyes or mouth; 2) to apply the repellent only on exposed skin (not skin covered by clothes); and 3) to wash remaining DEET off the skin when leaving an area of risk for insect bites. In Canada, where recent safety concerns have limited DEET concentrations, the use of products containing up to 30% DEET is still accepted and advocated.
The editorial accompanying the report by Fradin and Day5 wisely summarizes DEET safety when writing that 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.
This summer, Fradin and Day have provided a great service though their New England Journal of Medicine paper. Their solidly scientific study reminds us that DEET-containing insect repellents are much more effective than other products and that DEET efficacy increases with increasing DEET concentration. Appropriate use of DEET, both at home and in travelers, should result in improved health for many people.
Dr. Fischer, Professor of Pediatrics, Department of Pediatric & Adolescent Medicine, Mayo Clinic, Rochester, MN, is Associate Editor of Travel Medicine Alert, a sister publication to Infectious Disease Alert.
References
1. Fradin MS. Ann Intern Med. 1998;128:931-940.
2. Ansell JJ, et al. Trans R Soc Trop Med Hyg. 2002;96:113-116.
3. Temple WA, et al. J Toxicol Clin Toxicol. 1991;29:257-262.
4. AAP News pages 52-53, August 2001.
5. Pollack RJ, et al. N Engl J Med. 2002;347:2-3.
This study compared the efficacy of several commercially available insect repellents. The topical application of compounds containing DEET was far more protective against mosquito bites than was application of products containing soybean oil or citronella.
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