the Annals of Internal Medicine,
1 June 1998. By Mark S. Fradin, MD 128:931-940.
This paper is intended to provide the clinician with the
detailed and scientific information needed to advise patients who seek safe and
effective ways of preventing mosquito bites.
N,N-diethyl-3-methylbenzamide (DEET) is the most
effective, and best studied, insect repellent currently on the market. This
substance has a remarkable safety profile after 40 years of worldwide use, but
toxic reactions can occur (usually when the product is misused). When DEET-based
repellents are applied in combination with permethrin-treated clothing,
protection against bites of nearly 100% can be achieved. Plant-based
repellents are generally less effective than DEET-based products. Ultrasonic
devices, outdoor bug "zappers," and bat houses are not effective
against mosquitoes. Highly sensitive persons may want to take oral
antihistamines to minimize cutaneous reactions to mosquito bites.
Only female mosquitoes bite. Male mosquitoes feed primarily
on flower nectar, whereas female mosquitoes require a blood meal to produce
eggs. They usually feed every 3 to 4 days; in a single feeding, a
female mosquito typically consumes more than its own weight in blood. Certain
species of mosquitoes prefer to feed at twilight or nighttime; others bite
mostly during the day.
Despite the obvious desirability of finding an effective oral
mosquito repellent, no such agent has been identified.
Thus, the search for the perfect topical insect repellent continues. This ideal
agent would repel multiple species of biting arthropods, remain effective for at
least 8 hours, cause no irritation to the skin or mucous membranes, cause no
systemic toxicity, be resistant to abrasion and rub-off, and be greaseless and
odorless. No available insect repellent meets all of these criteria.
How To Choose and Apply DEET
For casual use, a high concentration of DEET is not needed.
Products with 10% to 35% DEET will provide adequate protection under most
conditions. The American Academy of Pediatrics recommends that repellents
used on children contain no more than 10% DEET. Products with a DEET
concentration of more than 50% are probably best reserved for circumstances in
which insect biting pressures are intense and in which other factors, such as
high temperature and humidity, may promote rapid loss of repellent from the skin
Repellents may be applied directly to the skin or to
clothing, window screens, mesh insect nets, tents, or sleeping bags.
Persons who are particularly concerned about potential toxicity from DEET may
limit application of the repellent to their clothes. If DEET-treated
garments are stored in a plastic bag between wearings, the repellent effect can
last for many weeks.
Repellents containing DEET must be carefully applied because
they can damage plastics (such as watch crystals and eyeglasses frames), rayon,
spandex, other synthetic fabrics, leather, and painted or varnished surfaces.
DEET does not damage natural fibers, such as cotton or wool, and has no
effect on nylon.
Until 1989, the standard-issue insect repellent of the U.S.
military consisted of 75% DEET in an alcohol base. Complaints about the
aesthetic feel of this product and concerns about potential toxicity under
long-term daily use led to U.S. Army-sponsored studies to produce new
formulations. The 3M Company (St. Paul, Minnesota) developed a
slow-release, polymer-based product containing 35% DEET; this has become
the repellent provided to all U.S. military personnel. This product is
available to the general public exclusively through the Amway
Corporation (New York, New York) under the brand name HourGuard. If
lower-strength formulations of extended-release DEET are desired,
Minnetonka Brands (Eden Prairie, Minnesota) offers products containing
6.5% and 10% DEET. As a general rule, higher concentrations of DEET provide
longer-lasting protection. Unfortunately, no guidelines are available to help
consumers decide what concentration of DEET is appropriate for their specific
needs. The number of variables that affect a repellent's effectiveness precludes
assigning an "insect repellent factor" to individual products.
Avon (New York, New York) Skin-So-Soft bath oil
received considerable media attention several years ago when some consumers
reported it to be effective as a mosquito repellent. When tested under
laboratory conditions against Aedes
aegypti mosquitoes, this product's effective half-life was 30 minutes.
Against Aedes albopictus, Skin-So-Soft
oil provided 40 minutes of protection from bites, a duration 10 times less
than that of 12.5% DEET. It has been proposed that the limited mosquito
repellent effect of Skin-So-Soft oil could be caused by its fragrance or the
presence of diisopropyl adipate and benzophenone in the formulation, both of
which have some repellent activity. Avon now markets products under the
Skin-So-Soft label that contain an EPA-recognized repellent
Thousands of plants have been tested as potential sources of
insect repellents. None of the plant-derived chemicals tested to date
demonstrate the broad effectiveness and duration of DEET, but a few show
repellent activity. Plants whose essential oils have been reported to have
repellent activity include citronella, cedar, verbena, pennyroyal, geranium,
lavender, pine, cajeput, cinnamon, rosemary, basil, thyme, allspice, garlic, and
peppermint. Unlike synthetic insect repellents, plant-derived repellents have
been relatively poorly studied. When tested, most of these essential oils
tended to give short-lasting protection, usually less than 2 hours.
Citronella is the active ingredient most
commonly found in "natural" or "herbal" insect repellents
marketed in the United States. It is registered with the EPA as an insect
repellent. Citronella oil has a lemony scent and was originally extracted from
the grass plant Cymbopogon nardus. Limited
data are available from studies that directly compared the efficacy of
citronella-based products with that of DEET-based products. In one study, 0.01
µmol of DEET per L of air was sufficient to prevent 90% of mosquitoes from
landing on their targets; a 1000-fold higher concentration of citronellol
(one of the active chemicals in citronella oil) was required to achieve a
Studies show that citronella can be an effective repellent,
but it provides shorter complete protection time than most DEET-based products.
Frequent reapplication of the repellent can partially compensate for this. The
manufacturer of Natrapel (Tender Corp., Littleton, New Hampshire) has laboratory
data showing that their 10% lotion reduced mosquito bites by 84% during a
4-minute test period. In contrast, 14% DEET reduced biting by 96% in the same
test period. Buzz Away (Quantum, Inc., Eugene, Oregon) with 5% citronella oil
provided an average protection time of 1.9 hours against Aedes
aegypti. In field testing, Buzz Away Oil provided an average of 88%
repellency during a 2-hour exposure. In general, the repellency of Buzz Away was
greatest within the first 40 minutes after application and decreased over the
remainder of the test period.
Citronella candles have been promoted as an effective
way to repel mosquitoes in the backyard. One study compared the ability of
commercially available 3% citronella candles, 5% citronella incense, and plain
candles to prevent bites by Aedes
mosquitoes under field conditions. Persons near the citronella candles had
42% fewer bites than controls, who had no protection (a statistically
significant difference). However, burning ordinary candles reduced the number
of bites by 23%. The efficacy of citronella incense and plain candles did not
differ. The ability of plain candles to decrease biting may result from their
action as a decoy source of warmth, moisture, and carbon dioxide.
The citrosa plant (Pelargonium
citrosum 'van Leenii') has been marketed as being able to repel mosquitoes
through the continuous release of citronella oils. Unfortunately, when tested,
these plants offer no protection against bites.
Pyrethrum is a powerful, rapidly acting insecticide,
originally derived from the crushed and dried flowers of the daisy Chrysanthemum
cinerariifolium. Permethrin is a human-made synthetic pyrethroid. It does
not repel insects but works as a contact insecticide, causing nervous system
toxicity that leads to the death or "knockdown" (out of the air)
of the insect. The chemical is effective against mosquitoes, flies, ticks,
and chiggers. Permethrin has low toxicity in mammals, is poorly absorbed by the
skin, and is rapidly inactivated by ester hydrolysis.
Permethrin should be applied directly to clothing or other
fabrics (such as tent walls or mosquito nets), not to skin.
The spray form is nonstaining, nearly odorless, and resistant to degradation by
heat or sun and maintains its potency for at least 2 weeks, even through several
launderings. The combination of permethrin-treated clothing and skin application
of a DEET-based repellent creates a formidable barrier against mosquito bites.
In a field trial conducted in Alaska, persons wearing permethrin-treated
uniforms and a polymer-based 35% DEET product had more than 99.9% protection (1
bite/h) over 8 hours, even under conditions of intense biting pressures;
unprotected persons received an average of 1188 bites/h.
Permethrin-based insecticide sprays can be apply to clothing.
Spray each side of the fabric (outdoors) for 30 to 45 seconds, just enough to
moisten it. Allow the garment to dry for 2 to 4 hours before wearing it.
from Mosquito Bites
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