Insect Repellents
Excerpts from The Medical Letter Vol 31 (issue 792) May
19,1989. Insect repellents have been used on the skin for many years, primarily
to prevent mosquito bites. With recent increased concern about Lyme disease,
skin and clothing repellents are now also recommended for protection against
ticks.
SKIN REPELLENTS: Currently
available insect repellents for application to the skin (Off!; and others) are
usually effective for one to several hours, but can be removed by absorption,
evaporation, rain, sweating, swimming or wiping, and must be reapplied to
maintain effectiveness. The most effective topical insect repellent known is N,
N-diethyl-m-toluamide, commonly called " DEET". DEET repels a variety
of mosquitoes, chiggers, ticks, fleas and biting flies; no topical repellent is
effective against stinging insects, such as bees and wasps. A repellent commonly
recommended is Cutters (Miles Inc.) which contains 21.85% DEET.
DEET Plus (Sawyer) which contains
17.5% DEET (mosquitoes), and 2.5% R- 326 (flies, fleas, etc.) is available.
An insect repellent is now
available: Skedaddle (Little Point) that is EPA approved for children. Skedaddle
contains 9.5% DEET with an added polymer. Skedaddle repels mosquitoes, flies,
ticks, etc. and provides 4 hours of protection.
Other repellents effective
against both mosquitoes and ticks, but less so than DEET, include
2-ethyl-.1,3-hexanediol (Rutgers 612) and dimethyl phthalate. Citronella-based
repellents (Natrapel; and others) may provide short-term protection against
mosquitoes, but are probably not effective against ticks.
A CLOTHING REPELLENT: Permethrin,
actually a pesticide rather than a repellent, is used for treatment of lice and
is also marketed as a clothing spray for protection against both mosquitoes and
ticks. The aerosol is available in many areas of the USA as Permanone Tick
Repellent, sold mostly in lawn and garden stores or sports stores. Manufactured
by Fairfield American in Newark, NJ and distributed by Coulston International,
Easton, PA, it is non-staining, nearly odorless and resistant to degradation by
light, heat or immersion in water. Also available from Coulston is Duranon Tick
Repellent. This product contains permethrin, and repels ticks, chiggers and
mosquitoes. Duranon provides up to 2 weeks of protection.
CLINICAL TRIALS: A field trial
conducted with US Air Force volunteers in an area of Alaska with a large
population of mosquitoes, but few mosquito-borne diseases, tested both the new
35% long-acting cream formulation of DEET applied to both exposed skin and
perrnethrin treatment of clothing. The DEET formulation provided greater than
99% protection for more than eight hours (a mean of four mosquito bites per
person per hour), while a permethrin-treated uniform (0.125 Mg/CM2) alone
provided 93% protection (78 mosquito bites/hour), compared to 1, 1 88 per hour
with no protection; using both DEET on skin and permethrin on clothing provided
99.9% protection. (TH Lillie et al, J Med. Entomol., 25:475, 1988). Another
trial conducted in Pakistan eight hours after application of the same
long-acting DEET formulation found that the combination of DEET and permethrin-treated
clothing provided 100% protection from mosquito bites; long-acting DEET
repellent alone gave 89% protection (a mean of 3.9 bites), compared to 57% (1
4.8 bites) with treated clothing alone and 34.4 bites with no repellent (LL
Sholdt et al, J Am Mosq. Control Assoc., 4,233. 1938).
An earlier field trial in
Australia had found two long-acting repellents (3M Insect Repellent lotion, 33%
DEET; Biotek Long-Acting Insect Repellent, 42% DEET) no more effective (56% and
61 % protection over 14 hours) than the standard military formulation of 75%
DEET treated (54% protection) in preventing bites. Any one of the three used
together with permethrin treated clothing provided the most protection (74%, 82%
and 80%) (RK Gupta et al, J Am Mosq. Control Assoc., 3,556, 1987).
SKIN SO SOFT: A commercial
concentrated bath oil, Avon, Skin So Soft, has come into wide use as a
"folk medicine" mosquito repellent. According to Medical Letter
consultants, Skin So Soft may protect against mosquitoes for as little as 30
minutes, and the safety of repeated applications of the concentrated bath oil to
the skin is unknown.
CONCLUSION: DEET-containing
insect repellents applied to the skin or clothing can prevent mosquito and tick
bites, but DEET may cause allergic and toxic effects in children and adults,
especially when used on the skin repeatedly in high concentrations. Wearing
protective clothing treated with permethrin in addition to using DEET on exposed
skin provides the greatest degree of protection against mosquito and tick bites.
Copyright University of
Washington Medical Center
Revised December 1996 Travel Medicine Service
University of Washington Medical Center
1959 NE Pacific St Box 356123
Seattle, WA 98195
(206) 598-4878