Repellents are volatile chemicals that repel insects (1, 2, 3). An ideal one would protect against a wide variety of insects for several hours at a stretch without causing any adverse reactions. However, currently available repellents are far from this ideal. Answer to which repellent is best isn’t straightforward for several reasons.
- In the case of mosquitoes, each repellent compound has a different efficacy against different mosquito species so efficacy data against one can’t be extrapolated to another. Usually, a repellent will be able to keep any/all mosquito species at bay at first, with none managing to land and bite, but often the effect wanes differentially against different mosquito species and soon they start biting (4).
- Formulations, which range from aerosols, creams, grease sticks, lotions, oils to sprays, greatly change the efficacy of the repellent substance (4). The formulation’s boiling point determines how much evaporates from the skin which in turn determines the concentration of volatile repellent emanating from it which in turn determines the distance mosquitoes are repelled. Two things about the repellent formulation come into play, how fast it evaporates and how deep it penetrates into the skin. Faster the former and deeper the latter, faster the repellent loses its efficacy. For e.g., formulations containing alcohol can penetrate deeper into the skin leading to faster efficacy loss.
- Intrinsic factors influence repellent action. For e.g., sweating both attracts mosquitoes and dilutes the repellent.
- External factors such as air temperature, humidity and wind speed also influence repellent efficacy. Warm, humid climates or high wind speed shorten the effective duration of repellent so frequent reapplications are necessary (4).
The CDC () lists the following as effective mosquito repellents: , (1-piperidinecarboxylic acid, 2-(2-hydroxyethyl)-, 1-methylpropylester; Picaridin; KBR 3023; Bayrepel), (Insect Repellent ethyl-butylacetyl-amino-propionate, EBAAP), oil of lemon eucalyptus ( , PMD, Citriodora). For clothing, it recommends -treated clothing.
In the case of Zika, protection is required against not just Aedes aegypti but also A. albopictus, aka the tiger mosquito, since both mosquito species have been implicated in human Zika outbreaks. A. aegypti prefers clean indoor and outdoor water reservoirs while A. albopictus prefers natural reservoirs in gardens and forests, and usually bites outdoors. Both are day biters, preferring to bite at dawn and dusk (crepuscular periods) (6) so control is more difficult compared to night biting malaria-spreading Anopheles, where long-lasting insecticidal nets (LLIN) have proven quite effective in malaria control programs.
Most studied repellents effective at repelling A. aegypti and A. albopictus are synthetic DEET, Icaridin , IR3535 , and plant-derived PMD. Of these DEET and Icaridin have the best track record of effectiveness against Aedes.
DEET: used since the 1950s, it’s very effective against mosquitoes, ticks, biting flies, chiggers and fleas (7). Commercial repellents typically contain 20-25% DEET.
Is DEET safe, especially for pregnant women? Thus far only one randomized, double blind study tested 897 pregnant women in Thailand for regular DEET usage during their 2nd or 3rd trimester. Study showed no adverse effect on fetuses, during birth or at one year follow-up (). However, DEET crossed the placenta since it was found in 8% of 50 randomly selected DEET users in the study. A pesticide survey among pregnant New Jersey women also found DEET crossed the placenta since again it was detected in blood and cord blood in all 150 subjects (9). However, again, with no adverse effects on new borns in terms of birth weight, head circumference or birth length. There was only a borderline association between higher DEET cord blood levels and higher abdominal circumference though its medical relevance is unclear.
DEET has some drawbacks. Effectiveness needs high concentrations of >10% and frequent re-applications. Not being very volatile means its spatial protection is short-range (). Some evidence suggests A. aegypti can become DEET-resistant ( ). Products with >25% DEET aren’t necessarily more effective against Aedes mosquitoes and they may not be as safe. Long term usage can cause skin irritation (12) and it can be absorbed through skin. If accidentally eaten, high blood concentrations are found in blood ( ) with anecdotal reports of hypotension, seizures, coma or encephalopathy (14).
Icaridin: Developed by Bayer in the 1980s, unlike DEET, this synthetic repellent is odorless, non-sticky, non-greasy, doesn’t dissolve plastic, coatings or sealants, doesn’t stain clothes and is biodegradable. As effective as DEET against Aedes, also feels and smells better compared to DEET (). While there are few clinical trials of Icaridin, it’s reported to be safe even in children ( ) and less of it gets into the blood ( ) compared to DEET.
- For A. aegypti and A. albopictus, a field study in Cambodia showed a commercial 20% Icaridin spray was as effective as ethanol solution of 20% DEET over a 5 hour period, confirming results of an earlier study that compared ethanol formulations of Icaridin and DEET ( ). These authors also note that adults prefer spray formulation containing 20% Icaridin, a concentration considered safe for their long-term use. OTOH, they say that 10% Icaridin lotion’s better for children since it’s adapted for their long-term use plus avoids risks associated with spraying on delicate parts of their body such as eyes, mouth, nose and skin abrasions. However, this study also found 10% Icaridin lotion was much less effective compared to 20% spray.
- A small field trial on 10 subjects in Brazil showed IR3535 as 10, 15 or 20% spray or 10 or 15% lotion, and Icaridin as 20% spray or 10% lotion were equally effective against A. aegypti for at least 6 hours ( ).
- Another study ( ) compared commercial formulations of DEET, PMD, citronella, geraniol, and vitamin B skin patch using cultured A. aegypti and A. albopictus. DEET and PMD were equally effective repellents though DEET’s effect lasted longer while citronella or geraniol containing formulations and the vitamin B skin patch were ineffective.
Permethrin-treated clothing can protect against A. aegypti bites (), usually for up to 5 to 10 washes since washing and ironing reduces the effectiveness. Interesting feature of this study was that hand- and factory-dipped clothing provided similar protection, though hand-dipped clothing is more inconsistent so requires more frequent re-application. Nevertheless, dipping clothes in permethrin could be a viable option for children’s school uniforms, for example. This concords with an older study that found permethrin-treated clothing to effectively repel A. albopictus ( ). In fact, one study on permethrin-treated army uniforms found effectiveness against A. aegypti up to even 55 washings (22).
These are a mere handful of numerous studies. Bottomline, DEET = Icaridin > PMD = IR3535 would be a good rule of thumb for repellents effective against Aedes. Formulations matter. Alcohol-free ones may be effective longer. All require some level of re-application, more mosquitoes, more frequent re-applications necessary. Since mosquito-intensive areas would require more re-applications, Icaridin, being less skin-irritating compared to DEET, may be a safer choice. Lotions may be safer for children though they may not be as effective as sprays.
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