What Insect Repellent Should Your Patients Be Using?

Abstract & Commentary

Source: Fradin MS, Day JF. Comparative efficacy of insect repellents against mosquito bites. N Engl J Med 2002;347:13-18.

This study was undertaken to determine the efficacy of several insect repellent products available in the United States and compare their ability to provide reliable and prolonged protection from mosquito bites. A total of 16 products were chosen, including seven widely available botanical repellents, three DEET-based repellents (N,N-diethyl-3-methylbenzamide), a controlled-release 20% DEET formulation, a synthetic repellent containing IR3535, three wristbands impregnated with either DEET or citronella, and a moisturizer (Skin-So-Soft, Avon) that commonly is believed to have repellent effects.

Fifteen volunteers inserted their repellent-treated arms into a cage containing 10 laboratory-reared mosquitoes. The age and degree of hunger of the mosquitoes and the humidity, temperature, and light-dark cycle within the cage were kept constant. A total of 720 individual tests were conducted, with each repellent being randomly tested three times on each subject.

The DEET-based products provided complete protection for the longest time with a positive correlation between complete protection time and the concentration of DEET. The alcohol-based product containing 23.8% DEET protected for an average of 301.5 ± 37.6 minutes and protected significantly longer than the controlled-release formulation containing 20% DEET (234.4 ± 31.8 minutes; p < 0.001.) A 6.65% DEET product protected for a mean of 112.4 ± 20.3 minutes. A soybean-oil based repellent protected for an average of 94.6 ± 42.0 minutes and was not significantly different from the 4.75% DEET product with a complete protection time of 88.4 ± 21.4 minutes. The IR3535-based repellent protected for an average of 22.9 ± 11.2 minutes. All other botanical repellents provided protection for a mean of fewer than 20 minutes. The repellent-impregnated wristbands (including one with DEET) offered no protection. A eucalyptus oil repellent became available after the original study was completed and was evaluated with only five subjects and had a mean of 120.1 ± 44.8 minutes of complete protection time.

Commentary by Stephanie B. Abbuhl, MD, FACEP

Mosquitoes are found all over the world, except in Antarctica, and the diseases these arthropods transmit include malaria, yellow fever, dengue hemorrhagic fever, epidemic polyarthritis, Bancroftian filariasis, and several forms of encephalitis.1

Until a few years ago, in the United States mosquito bites were considered primarily a nuisance; however, the arrival of the West Nile Virus (WNV) in North America in 1999 raised the level of concern. In 2001, the Centers for Disease Control and Prevention reported 66 human cases of WNV in the United States, including seven deaths in New York City. It is too early in the current season to know the extent of this summer’s outbreak, but as of August 16, a total of 156 human cases of WNV had been reported in eight states and the District of Columbia, and WNV-positive mosquito pools had been reported in eight states and New York City.2 In addition to WNV, the three other mosquito-borne diseases that are seen in the United States include eastern equine encephalitis, LaCrosse encephalitis, and St. Louis encephalitis.

This study confirms that DEET-based products remain the gold standard of mosquito repellent protection. A formulation of 23.8% DEET (OFF! Deep Woods, SC Johnson) provided an average of five hours of protection after a single application. While higher concentrations of DEET provided longer lasting protection, the authors mention that the duration of action tends to plateau at a concentration of about 50%, and most commercially available formulations now contain 40% DEET or less.

One possible limitation to the study was that only Aedes aegypti mosquitoes were used in the study, and there are approximately 170 species of mosquito in North America alone. An accompanying editorial notes that other species, including certain types that transmit malaria, may be less susceptible to DEET protection.3

With DEET the winner of the repellents, it is reassuring to note that DEET has a remarkable safety profile after 40 years of use. Fewer than 50 cases of serious toxic effects have been documented since 1960, most resolving without sequelae and many due to long-term, frequent, or heavy use. The Travel Medicine Clinic at the University of Pennsylvania recommends the use of up to 35% DEET in adults, 10% or less in children, and no use in children younger than 1 year of age.

Dr. Abbuhl, Medical Director, Department of Emergency Medicine, The Hospital of the University of Pennsylvania; Associate Professor of Emergency Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA, is on the Editorial Board of Emergency Medicine Alert.

References

1. Fradin MS. Mosquitoes and mosquito repellent: A clinician’s guide. Ann Intern Med 1998;128:931-940.

2. Weekly Update: West Nile Virus Activity—United States, Aug. 12-16, 2002. MMWR Morb Mortal Wkly Rep 2002;51(32):708-709.

3. Pollack RJ, et al. Perspective: Repelling mosquitoes. N Engl J Med 2002;347:2-3.