Mosquito-borne pathogens kill more than half a million people a year and make hundreds of thousands of people sick. Malaria is the worst of these, with recent reports from the World Health Organization suggesting the steady improvements in the burden of disease are slowing, and the situation may even be getting worse. Dengue/Chikungunya continues to have wide ranging impacts. Bangladesh has also seen record-breaking epidemics of Chikungunya in the last year.

First one, then another. Bite! Slap! Bite! How can such a tiny, fragile insect cause so much pain, suffering and annoyance? The sight and smell of smouldering mosquito coils/aerosol insect spray is a mainstay of summer. But is all that smoke really keeping away the swarms of mosquitoes, and is breathing in the smoke worse than mosquito bites for our health? After burning a mosquito coil, and/or spraying insect spray several times, we are about to die from suffocation but mosquitoes fly above!

The burning of aromatic plant material to keep away swarms of mosquitoes is an integral part of many cultural traditions around the world. But it wasn’t until the early 1900s that the instinctively shaped mosquito coil was born, thanks to Japanese entrepreneurs Eiichiro and Yuki Ueyama and their katori senk (mosquito-killing incense). Notwithstanding the death and disease that mosquito-borne pathogens cause around the world, the annoyance caused by mosquito bites can be seriously disruptive. The reactions to bites can be severe, and when swarms of mosquitoes swell in the suburbs, the impacts can be substantial.

Regardless of formulation, be it cream, lotion, gel, roll-on, pump-spray or aerosol, there needs to be complete coverage of all exposed skin. Instead of a smouldering coil, there are some battery-operated and “plug-in” devices that release products that kill or repel mosquitoes. These devices typically release an insecticide from a heated pad or oil reservoir.

Mosquito coils work in one of two ways. Those that contain insecticides will kill (or at least “knock down”) mosquitoes, while those that contain aromatic substances (such as citronella) will repel mosquitoes or reduce the likelihood they’ll bite. Mosquito coils and their role in killing or repelling mosquitoes have been well studied. Despite differences between the chemical constituents of products and the ways in which they are tested, they will generally reduce the ability of mosquitoes to bite people. Mosquito coils contain a mix of substances. Along with the products that deter mosquito biting, there are also products that hold the coil together and enable it to smoulder slowly.

Mosquito coils and aerosol insect spray are widely used throughout Asia and in Africa and South America, but for some reason they never caught on much in North America and Europe. Mosquito-borne disease is an annual problem in Bangladesh, with endemic pathogens. The recent emergence of Zika virus in South America and the Pacific, together with ongoing outbreaks of dengue viruses in Southeast Asia, generated great community interest in the most effective strategies to avoid mosquito bites.  The use of topical insect repellents is a key recommendation by health authorities to prevent mosquito-borne disease. All products sold in Bangladesh purporting to repel mosquitoes must be registered with the Licensing Authority. Despite several commercial products registered as repelling mosquitoes, there are relatively few active ingredients used across these formulations. The most common are diethyltoluamide (DEET), picaridin, p-menthane-3,8-diol (PMD) and a range of plant-derived products (e.g. melaleuca, eucalyptus, citronella oils). Research has shown that each of these active ingredients varies in the duration of protection provided against biting mosquitoes.

To prevent public health risk associated with mosquitoes, most people have to rely on “covering up” with long-sleeved shirts and long pants, sleeping under bed nets, applying topical insect repellents or burning mosquito coils.

Surveillance of mosquito populations and activity of mosquito-borne pathogens throughout Bangladesh informs decisions by health authorities to issue public health warnings above and beyond the routine seasonal mosquito-borne disease awareness programs. Promoting personal protection measures, including avoiding mosquito habitats, limiting outdoor activities at times of the day when mosquitoes are most active and covering up exposed skin with clothing, remain at the core of public health messages. The use of topical insect repellents is one of the strongest recommendations by health authorities.

Although insect repellents have been shown to provide protection against biting mosquitoes, for this approach to be effective at preventing mosquito-borne disease, it is important that health authorities provide recommendations on safe and effective use of these products.

Spatial repellents such as mosquito coils are used commonly and, although there is evidence that they provide some protection against biting mosquitoes, their role in reducing disease is less well studied. The growing community interest in spatial repellents and new insecticide technologies warrants further investigation.

One of the most widely recommended mosquito repellents internationally is N,N-diethyl-3-methylbenzamide (commonly known as diethyltoluamide, or DEET). There are many and varied registered DEET formulations available, ranging from less than 10% through to 80% concentration. There is a considerable body of scientific work supporting their safety and effectiveness, with laboratory and field investigations demonstrating that DEET effectively protects against a range of nuisance-biting and vector mosquito species in Bangladesh.

Picaridin has been shown in laboratory and field investigations to provide protection from biting mosquitoes. Registered formulations range in concentration from 9% to 20%. Alongside DEET, it has been widely recommended by health authorities internationally. Picaridin-based formulations are generally considered cosmetically more pleasant to use.

Repellent formulations containing p-menthane-3, 8-diol (PMD) are becoming widely available and, given their effectiveness are increasingly included in lists of recommended products. However, this product should not be confused with eucalyptus-based essential oil formulations, which generally do not provide long periods of protection against biting mosquitoes.

A number of registered topical mosquito repellents contain plant-derived active ingredients including one or more of citronella, melaleuca, eucalyptus, leptospermum and lavender extracts. These formulations typically range from <1% through to approximately 8.5%, representing relatively low active ingredient concentrations compared with other repellents. Although there is evidence that repellents containing plant-derived extracts will repel biting insects for short periods they would need to be reapplied far more frequently that DEET-, picaridin- or PMD-based repellents to provide comparable protection.

There are inconsistencies in the advice included on registered labels in Bangladesh. There are no age-related warnings on repellent use, others only contain general warnings about use but nothing on children, so it is difficult for users to be specific in warnings about particular active ingredients and their concentrations.

There is often a perception that DEET is unpleasant to use or may pose a health risk. However, a number of studies that investigated health risks associated with use of topical DEET-based repellents found that, if used as recommended, it poses minimal health risk. Although the safety of picaridin has not been investigated to the same extent as DEET, it is also considered a safe product to use. DEET and picaridin are considered safe to use for pregnant and breastfeeding women.

Botanical-based repellents are often perceived as being ‘natural’ and may be preferred by many in the community because of perceived risks associated with other topical repellents. However, essential oils have the potential to cause adverse skin reactions, an important consideration when discouraging use of unregistered repellents, or ‘homemade’ mixtures that may contain relatively high doses of these products.

Irrespective of the inherent effectiveness of active ingredients in mosquito repellents, the best bite protection is achieved by thorough application to all exposed skin. Recommendations about how to use topical repellents are typically absent from public health messages.

Regardless of future changes to mosquito-borne disease risks, topical mosquito repellents will remain an essential component of recommended personal protection measures. Although it is unlikely there will be substantial changes in the active ingredients used in registered mosquito repellents in Australia, the landscape of commercial formulations will continue to evolve. Together with the emergence of new technologies for mosquito bite prevention (i.e. clip-on insecticide devices or insecticide-treated clothing), it will be important that health authorities routinely review academic studies of mosquito repellent effectiveness and safety, particularly for picaridin- and PMD-based repellents, to provide the most informed public health messages to reduce the burden of mosquito-borne disease.

There’s growing concern about the adverse health impacts associated with the burning of mosquito coils and sticks indoors. The insecticide products used are generally considered safe, but it’s the particulate matter produced from a smouldering mosquito coil that poses the greatest risk. Is it correct to conclude “burning one mosquito coil in a closed room amounts of smoking roughly 100 cigarettes” as some have claimed?

The link between smoking cigarettes and poor health outcomes is clear. What about mosquito coil smoke, especially if there’s almost daily exposure, as there is in some countries?

One study estimated the particulate matter produced from burning one mosquito coil was equivalent to burning 75-137 cigarettes. This amount of exposure poses a health risk, but there is a lack of clear evidence that the long-term exposure to mosquito coil smoke increases the risk of more serious health impacts such as lung cancer. In the face of this uncertainty, the key message should be to avoid prolonged exposure, especially in enclosed spaces.

In Bangladesh, all products that purport to kill or repel mosquitoes must be registered by the competent authority. Every user should check the packaging for a registration number. There are dozens of variations on “mosquito coils”, including sticks, coils, candles and a variety of “smokeless”, plug-in devices.

There’s enough evidence to show that when used outdoors, burning a mosquito coil will assist in reducing mosquito bites, but should be used judiciously. Using them in combination with topical insect repellents probably provides the best protection. Their use in closed rooms is best avoided – “smokeless” devices are worth considering as an alternative.

The writer is former Head, Department of Medical Sociology,

Institute of Epidemiology, Disease Control & Research (IEDCR)

Mohakhali, Dhaka – 1212

E-mail: [email protected]