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Mosquito repellent evidence

The test
Results
Comment

Avoiding being bitten by mosquitoes or other little nasties is usually about preventing discomfort in the short or medium term. Because those little nasties can sometimes be vectors of several unpleasant diseases in different parts of the world, though, it can be more important than that. So, as one Bandolier reader asked, what's the evidence for which repellent works best?

This is not quite so easy to answer as one might think, because the number of variables is legion. The mosquito, for a start. It's sex, the number of them, how hungry they are, the temperature, humidity, and wind speed can all affect the likelihood of being bitten, with or without any repellent present. So controlled laboratory tests are the order of the day, and the best is a study looking at 16 different repellents tested in 15 intrepid volunteers in controlled circumstances [1].


The test

Basically, the test comprised one arm being placed in a cage containing a fixed number of unfed mosquitoes and measuring how long it took until the first bite. Cages contained 10 disease-free female Aedes aegypti mosquitoes between seven and 24 days old, reared in a laboratory, and not previously exposed to a repellent. Temperature, humidity and light-dark cycles were controlled.

Before each test, the readiness of mosquitoes to bite was measured by inserting an untreated arm, and observing five mosquito landings on the arm. Repellent was then applied (or worn in the case of wristbands), and the arm reinserted, according to a set schedule.

Simply, this started with insertion of the treated arm for one minute in every five for the first 20 minutes, then one minute every 15 minutes until the first bite, the time of which was recorded. Each volunteer made three measures for each repellent. This was done for each of 16 commercially obtained products. The order of testing was determined by a random number generator for each subject.


Results

The results are shown in Table 1. Certain results are obvious.



Table 1: Complete protection time against mosquito bite



Active ingredient
Mean complete protection time
(minutes)
Synthetics
DEET, 24%
302
DEET, 20%
234
DEET, 6.7%
112
DEET, 4.8%
88
IR3535, 7.5%
23
Natural products
Soybean oil, 2%
95
Citronella, 10%
20
Citronella, 12% plus other oils
19
Citronella, 10% plus peppermint
14
Citronella, 5%
14
Citronella, 1% bath oil
10
Uncertain mix bath oil
10
Citronella, 0.05%
2.8
Wristbands
DEET, 9.5% wristband
0.3
DEET, 9.5% wristband
0.2
Citronella, 25% wristband
0.2
Products were applied to the skin from elbow to fingertips, except wristbands.



Comment

This paper tested 16 products available commercially in the USA. It was one of those studies, beautifully done, that did not need statistics. The answer was obvious.

But all may not be up for natural products, or different products available in different countries, though we could not find any that had been tested in the same stringent way. For instance, testing of 38 essential oils in laboratory conditions found that only four (citronella, patchuli, clove, and makaen) gave two hours protection, and only at 100% concentration [2]. Diluted to 50% or 10%, they were largely ineffective, echoing the US findings [1]. However, three commercially available products in South Africa (one with 15% DEET, one with another synthetic agent, and one containing a mixture of oils) all produced 3-4 hours of almost complete protection, though the natural oil product declined rapidly after that time [3].

Undoubtedly there is more literature than this, but right now, unless there is good data to show that another product works, choosing DEET repellent products is the sensible and safe thing to do. For travellers, the Centres for Disease Control has much useful advice about general preventative measures and repellents, at www2.ncid.cdc.gov/travel (look for the yellow book section). The main points are:

References:

  1. MS Fradin, JF Day. Comparative efficacy of insect repellents against mosquito bites. NEJM 2002 347: 13-18.
  2. Y Trongtokit et al. Comparative repellency of 38 essential oils against mosquito bites. Phytotherapy Research 2005 19: 303-309.
  3. J Govere et al. Efficacy of three insect repellents against the malaria vector Anopheles arabiensis. Medical and Veterinary Entomology 2000 14: 441-444.

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