A Clinician's Guide to Mosquitoes and Mosquito Repellents
Source: Fradin MS. Ann Intern Med 1998;128:931-940.
A recently published paper in Annals of Internal Medicine is intended to provide the clinician with the detailed scientific information needed to advise patients to seek safe and protective ways to prevent mosquito bites. Fradin took relevant material from MEDLINE database, World Wide Web, the Mosquito-L-electronic mailing list, Extension Toxicology Network, and publications from the U.S. Army, EPA, and Department of Agriculture. Salient points reviewed in his article will be discussed here.
The Mosquito Life Cycle
Fradin reviews the four stages of the life cycle of the mosquito, a ubiquitous creature found all over the world except in the Antarctic. He reminds clinicians that only the female mosquito requires a blood meal every 3-4 days, while the male mosquito feeds on nectar. He reviews the stimuli involved in attracting mosquitoes: visual, thermal, and olfactory, the latter being the most important. Of the 100 volatile compounds detected in human breath and the 300-400 compounds that are by-products of metabolism, carbon dioxide and lactic acid are the two best-studied mosquito attractants. CO2 serves as a long-range airborne attractant and can be detected by mosquitoes at up to 36 meters. Lactic acid is an attractant to which mosquitoes are found to have chemoreceptors on their antennae. These same lactic acid chemoreceptors are thought to be inhibited by DEET insect repellents. Fradin reviews how adults are more likely to be bitten than children, men more than women, and aging adults are less attractive to the feeding female.
Insect Repellents
Fradin reviews all the chemical insect repellents that contain DEET, including companies, percentages, and concentrations. He offers a telephone number to obtain the 35% slow-release polymer-based product provided to the U.S. military now marketed under the name HourGuard (Amway Corp., 1-800-544-7167). Higher concentrations of DEET relate to the length of protection, and the extended slow-release permits this low percentage formulation to provide 12 hours of 95% protection.
Fradin reviews how the choice and application of repellents may affect other products, describing interactions with sunscreen (citing studies revealing diminished sunscreen potency with repellent use).
He reviews the pharmacology and toxicity profile that led to the controversial recommendation by the American Academy of Pediatrics to limit the percentage of any DEET compound on a child to 10%. He reviews the 14 cases of encephalopathy in children and the details of the inappropriate use of DEET or enzyme deficiencies detected in many of these cases. He refers clinicians to an EPA-sponsored Nation Pesticide Network for safety questions at 1-800-858-7378 or on the World Wide Web at: http://www.ace.orst.edu/info/nptn.
Plant-Derived Repellents/Alternative Repellents
The last part of Fradin's paper reviews the lack of evidence for prolonged repellency of Skin-So-Soft (Avon). He details all the plant-derived insect repellents and how to obtain products. He describes Bite Blocker (Consep, Inc.; Bend, OR), a new plant-based repellent (soybean oil, geranium, coconut oil) released in the United States in 1997 and shown to have 97% protection in one Canadian study after 3.5 hours.
Fradin contrasts the insecticide potential or kill potential of permethrin, a synthetic derived from chrysanthemum plants, to the repellent efficacy of DEET compounds. The author even tackles the backyard bug zappers and deems them totally ineffective. Indeed, one study conducted in a homeowner's backyard revealed that only 0.13% of insects attracted were female mosquitoes. An estimated 71-350 billion beneficial insects may be killed unnecessarily by these devices annually.
Relief from Mosquito Bites
Fradin reviews the immunology behind the cutaneous responses to mosquito bites, which range from local wheal and flare to delayed bite papules; rare, systemic, Arthus-type reactions; and anaphylaxis. Pros and cons of the use of topical agents such as corticosteroids, diphenhydramine, and caine-containing derivatives are discussed, as well as newer studies with non-sedating antihistamines, which have been shown to reduce mosquito-induced wheal response and size of bite papule in placebo-controlled studies.
This paper is clearly a useful guide for the travel medicine clinician.
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