With the Zika fever outbreak spreading across Latin America, many travelers and people in endemic areas are left wondering how best to protect themselves from mosquitoes. The tropical yellow fever mosquito, Aedes aegypti, is the primary vector for transmitting the Zika virus, as well as the Dengue and Chikungunya viruses, although other mosquitoes such as Aedes albopictus, also called the Asian tiger mosquito, can also act as carriers. Those in the U.S. are more likely to come in contact with Ae. albopictus, which can survive below-freezing temperatures and inhabits temperate climates in the South and Northeast. Both species bite during daylight hours when people are most active, making insect repellent an essential tool for anyone spending time outdoors in endemic zones.
Many products claim to deter mosquitoes, but which have been scientifically proven as effective? Most commercially-available insect repellents contain one or several active ingredients including N,N-Diethyl-meta-toluamide, commonly known as DEET, IR3535,or picaridin.
A recent study by Rodriguez and colleagues in the Journal of Insect Science tested commercially available mosquito repellents against both Ae. aegypti and Ae. albopictus. Those sprays containing DEET were most effective, repelling both species with >70% efficacy for at least four hours.
While many repellents use DEET as an active ingredient, several reports linking use of DEET with negative health effects have led to public distrust and chemophobia. However, research shows with an estimated 200 million applications of DEET occurring every year, there have been only 14 reported incidents of adverse effects, and most were the result of overuse.
It was once thought that DEET worked by masking the presence of human scents that mosquitoes find attractive, but research shows mosquitoes indeed sense DEET and then avoid the scent. In a conversation with one of the study authors, University of Notre Dame professor Zain Syed said, “This matters because this depends on the unique composition of scents on each person’s skin.” DEET’s repellency may be less effective if the combination of compounds from a particular person’s scent is attractive enough to the mosquito’s olfactory system.
The general consensus within the scientific and medical communities is that DEET is safe to use at low doses, and the formula is still considered the gold standard of insect repellents by many vector biologists. Conventional commercial products are typically formulated with 20-25% DEET.
Concerns about negative effects of DEET has contributed to the popularity of repellents composed of plant-based compounds. Formulations containing essential oils are frequently found as active ingredients in mosquito repellents marketed as “organic” or “natural”. Despite their newfound popularity, these alternatives are largely ineffective when compared to DEET. However, research by Rodriguez et al found that while DEET-free formulations did not perform as well as those containing DEET, products containing only essential oils from lemongrass, citronella, soybean, rosemary, and cinnamon provided brief repellency immediately upon application. But within four hours after application, sprays were no longer repellent or had substantially reduced repellancy.
“These plant compounds are highly volatile,” says Syed. While plant-based products can provide effective protection, their effect is usually temporary compared to solutions containing synthetic chemicals, which tend to break down less easily.
The exception is essential oil from the lemon eucalyptus tree, which contains p-menthane-3-8-diol, a chemical previously associated with mosquito repellency. Rodriguez et al found that products with p-menthane-3-8-diol provided repellency effects equivalent to products containing DEET. Unlike related compounds, p-methane-3-diol is one of the few botanicals with low volatility, and therefore dissipates off the skin less quickly.
For those still concerned by DEET, a recent PLOS Neglected Tropical Diseases (NTDs) study found that formulations with 10-20% picaridin performed with similar efficacy to DEET, protecting against 97% of bites in indoor tests. Formulations with 20% picaridin proved more effective than those with only 10%, and Ae. aegypti were more repelled by DEET than Ae. albopictus. Picaridin is also a synthetic compound, but is based on a compound found in plants used to produce black pepper. It has the benefit of being more biodegradable and is less prone to absorption by the skin.
As the weather heats up for summer and mosquito populations ramp up, officials are concerned by the possibility of a Zika outbreak in the U.S. With no specific treatment and vaccine trials at least 18 months away, officials agree that prevention is key, which makes finding an effective insect repellent compound critical to curbing transmission. Most repellents use DEET as an active ingredient, but the CDC also recommends any repellent with picaridin, IR3535, or p-methane-3-8-diol. For those traveling to Zika-endemic areas, the CDC suggests wearing long-sleeved shirts and pants, sleeping in screened or air-conditioned rooms, and using a bed-net when sleeping in exposed outdoor areas, to avoid being bitten.