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Repellents: give those creepy-crawlies the heebie-jeebies

Carolyn Driver
RGN RM RHV FPCert
MSc(TravelMed)
Independent Travel Health Specialist Nurse
Cheshire
Chairperson
British Travel Health Association
E:thecompany@driverc.freeserve.co.uk

Insects are very significant vectors of disease in tropical countries. While malaria is probably the best- known disease to be transmitted to humans by the mosquito, there are very many others transmitted by mosquitoes, flies, ticks, fleas and other bugs (see Table 1). Vaccines are only available to protect individuals for an extremely limited number of these infections, and prevention relies on personal protection measures (PPMs) against insect bites.

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Although most insect-borne disease is associated with the more exotic parts of the world, outbreaks can occur almost anywhere that has spells of hot weather. In addition to disease risk, biting insects can cause a lot of discomfort to individuals through local reactions to bites, and even in many less tropical destinations good PPMs can help reduce this misery. The mainstay of PPMs is the use of insect repellents. PPMs can be applied to the skin, clothing and the environment.
 
Insect repellents
The quest for the perfect insect repellent is ongoing. Surprisingly little is known about how repellents act on their target insects, but different species appear to react differently to the same repellent.(3) To be effective a repellent needs to be volatile enough to give off a vapour but not so volatile that it evaporates completely. A number of factors will play a part in determining how effective a product will be, and these include uniformity of application as well as the user's underlying attractiveness to biting insects. Repellents must be applied to all exposed areas of skin, as unprotected skin just a few centimetres away from a treated area can by targeted by the insect.
Repellents containing DEET remain the most effective products.(4) DEET has been in use for more than 50 years and is effective against a wide variety of biting insects. Preparations containing less than 50% DEET have a proven safety record in adults,(5) and there seems to be little benefit in using formulations that contain more than this concentration. Children and pregnant women should use formulations at around 10-20% DEET because of theoretical concerns about toxicity, although these have failed to be demonstrated in animal studies, and instances of toxicity have followed ingestion of the product rather than dermal use.
Other repellents that are commonly available include "Autan", which is manufactured by Bayer. This contains a chemical repellent - 1-piperidinecarboxylic acid, 2-(2-hydroxyethyl)-, 1-methylpropylester - which needless to say required a more user-friendly name and has thus been patented as Bayrepel. This is a less toxic chemical and shows promising efficacy but has not been as extensively tested as DEET.
A number of natural products are also widely marketed. Of these, those that contain eucalyptus oil in combination with citronella appear to offer the best protection. There is some question over length of efficacy, although one study has estimated it to be almost as good as DEET.(6) It should be remembered that long-term safety has not been proven with these products and that they will also cause harm if ingested, so they should not be recommended as "safer" than DEET-based products.

Insecticides
Insecticides do not repel insects but are toxic to them on contact. Permethrin, a synthetic pyrethroid, is the most commonly used. Insecticides should not be applied to the skin but can be applied to natural fabrics, so they can be used on furnishings and clothing. Wrist and ankle bands treated with permethrin can be worn under loose clothing and add an extra level of protection, but should not be used on uncovered limbs without the addition of repellents as the insects will be attracted to skin only a few centimetres away. Treating young children's clothing with permethrin is a useful way of increasing protection when using repellents with a lower concentration of DEET. Clothing does not need to be treated every day, and the protection may even last through several washes - always check individual product packaging for specific instructions.
Insecticides can also be used to treat the environment (eg, plug-in vaporisers or burning coils). Mosquito nets should be treated with permethrin every six months as it improves their protective efficacy considerably.

Guidelines for using insect repellents

  • At high-risk times try to cover as much of the body as possible with loose-fitting clothing (mosquitoes can bite through skintight clothing).
  • Ensure that all exposed skin is treated with a repellent, including the face, but be careful not to allow the product to come into contact with mucous membranes.
  • Suncreams or other skincare products should be applied first - insect repellents should be the last thing to be applied.
  • Avoid heavy floral fragrances.
  • Check reapplication times with the product, but bear in mind that if swimming or sweating ­profusely these may need to be shortened.
  • Back up the insect repellent with environmental protection (screening on windows, knockdown sprays and plug-in vaporisers).
  • Sleep under a permethrin-treated mosquito net if not in an air-conditioned environment.
  • Keep all products out of reach of children. Wash hands after application of creams and use a spray or stick product to treat the back of the hands.

References

  1. Fradin M. Mosquitoes and mosquito repellents: a clinician's guide. Ann Int Med 1998;128:931-40.
  2. Lindsay S, et al. Effect of pregnancy on exposure to malaria mosquitoes. Lancet 2000;355:1972.
  3. Davis E. Insect repellents: concepts of their mode of action relative to potential sensory mechanisms in mosquitoes. J Med Entomol 1985;22:237-43.
  4. Sturchler M. The vector and ­measures against mosquito bites. In: Schlagenhauf P, editor. Traveller's malaria. Ontario: BC Decker; 2001.
  5. Are insect repellents safe? [editorial]. Lancet 1988;2:610-1.
  6. Trigg J, Hill N. Laboratory ­evaluation of a eucalyptus-based ­repellent against four biting ­arthropods. Phytother Res 1996;10:313-6.

Resources
Department of Health. Health information for overseas travel. London: HMSO; 2001
W:www.the-stationery-office.co.uk/doh/hinfo/index.htm
A copy was sent out to each GP and practice nurse when the book was published, but extra copies can be purchased through HMSO stores

WHO.
International travel and health. Geneva: WHO; 2001
W:www.who.int/ith Can be purchased from HMSO stores

Malaria Advisory Committee. Guidelines for malaria prevention in travellers from the United Kingdom for 2001. Commun Dis Rep 2001;4(2)
W:www.phls.co.uk

TRAVAX
W:www.travax.nhs.uk