Review
Currently, control of malaria vectors relies almost entirely on indoor residual-spraying (IRS) and long-lasting insecticide-treated nets (LLINs) [1]. These vector control tools have successfully reduced mosquito population densities and malaria by targeting indoor-feeding (endophagic) and indoor-resting (endophilic) mosquitoes [2]. The most successful IRS chemical active used to date is DDT, which, in addition to killing mosquitoes, also reduces indoor mosquito densities consequently reducing malaria transmission [3-6].
Literature shows that much of the success of DDT is due to excito-repellency [4,5]. An excito-repellent is defined as a chemical that causes insects to make undirected movements that set them apart from insecticides [7]. Excito-repellency results from insect’s physical contact with chemicals on treated surfaces or with vapour particles at a distance [8,9]. It has been demonstrated that volatile DDT can induce neural excitement in insects [10] and importantly, it was observed that insects exposed to sub-lethal concentrations of DDT move towards the light explaining why mosquitoes are likely to quickly leave a sprayed dwelling [11]. Excito-repellency was also originally seen as a beneficial feature of pyrethroid treated bednets to reduce the probability of mosquitoes developing resistance to insecticides through lower contact with insecticides [12]. It is known that DDT and pyrethroids act on the voltage-gated sodium channel proteins found in insect nerve cell membranes, disrupting transmission of nerve impulses thereby causing mortality [13]. Cross resistance between DDT and pyrethroids is conferred by point mutations on the voltage gated sodium channel in mosquitoes indicating a common mode of toxic action for these insecticides on mosquitoes [14]. Mechanisms underlying host-seeking and feeding behaviours of mosquitoes are largely unknown and have been the topic of current investigations. It is known that sublethal exposure to both pyrethroids and DDT has a differing effect on insect feeding responses: pyrethroids inhibit responses to attractants while DDT increases neural sensitivity to attractive sources [15,16]. New advancements in the field of neurobiology have demonstrated that perception of chemicals in the environment by insects begins when compounds activate ionotropic receptors, gustatory receptors and olfactory receptors (ORs) located on the dendritic surface of chemosensory neurons of the olfactory receptor cells (ORCs) housed in a head appendage (e.g. antenna or palp) [17]. ORs recognize biologically meaningful chemical ligands, and shape responses of olfactory sensory neurons (OSNs), thus regulating many behaviors including repellency.
Repellents either activate or inhibit action of ORs interfering with the host-seeking behaviour of mosquitoes, resulting in repellency or anti-feeding [18]. A repellent pyrethroid has been shown to disrupt insect behaviour not through targeting the voltage gated sodium channel but instead inhibits the response of odorant receptors (ORs) to attractants in a similar way to para-menthane 3,8 diol and nepetalactone [18]. Repellency is a characteristic of personal protection tools such as mosquito coils, liquid vaporizers, vaporizer mats and ambient emanators [19]. These tools have been extensively studied yet they have not been promoted as formal methods for mosquito control. In 2006 the consumer market for pesticides was about $8.4 billion, with expected double-digit annual growth mainly due to rising income levels in several developing-world markets, notably China [20]. By far the most popular segment was aerosols, at $3.6 billion, followed by topical repellents, powders, and gels at $2 billion. The smaller segments of mats and vaporizers accounted for $1.6 billion and coils for $1 billion [20]. These products are already widely used and would therefore be expected to have community uptake if they were introduced as a formal means of disease control in an integrated vector management (IVM) strategy.
In addition, due to increased need for effective vector control tools, to combat residual outdoor-biting and resting mosquitoes [21], it is timely to review studies of mosquito coils and emanators. This will enable better understanding of their mode of action and hence gain useful knowledge for development of effective spatially acting chemical products that can be used outdoors hence complement LLINs and IRS for integration into a malaria elimination strategy [22].
The main active ingredients recommended by the World Health Organization (WHO) for use in the vapour phase all belong to the pyrethroid chemical class. The most commonly used format; mosquito coils are cheap and effective but produce smoke [23] which is undesirable. Vaporizer mats are an alternative to coils. The mats contain embedded repellent active ingredients that are volatilised using an electric heating element. This need for electricity can increase product costs making them inappropriate for some rural and urban settings in low or middle-income countries.
Recently, other delivery formats that do not require heating or combustion have been developed. These are commonly known as emanators and are composed of insecticides impregnated on substrates such as paper, plastic or agarose-based gels [24,25]. Unlike coils and mats, emanators function through passive evaporation of chemical actives. These chemicals are less polar and have lower vapour pressure than conventional pyrethroids hence evaporate at ambient temperature without the need for an external source of energy. Examples of these insecticides include metofluthrin and transfluthrin.
The aim of this review was to determine effects of mosquito coils and emanators on mosquito responses that reduce human-vector contact and to propose scientific consensus on terminologies and methodologies used for evaluation of product formats that could contain spatial repellents including IRS, LLINs and insecticide treated materials (ITMs).
This review was conducted in accordance with PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines [26]. PubMed, (National Center for Biotechnology Information (NCBI), National Library of Medicine, NIH), MEDLINE, LILACS, Cochrane library, IBECS and Armed Forces Pest Management Board Literature Retrieval System were searched systematically for both field and laboratory studies that included pyrethroid based coils and/or emanators using the English key-words “Mosquito coils”, “Mosquito emanators” and “Spatial repellents”, between January and November 2011. In addition to journal articles, we searched reference lists of identified papers. We also checked the System for Information on Grey Literature in Europe (SIGLE) for unpublished data from sources such as conference proceedings, abstracts and there with ensured that there was no publication bias. The last search was conducted on 21st September 2012. We were confident that the search engines we used provided almost all relevant studies of interest. Data were extracted from selected articles that met all study criteria using a standardized spreadsheet. The information collected included first author, year of publication, methods and design, active ingredient, dose, mosquito species, sample size, description of the control, testing conditions (experimental huts, rooms, chambers or cylinders) and the outcome measures reported with any available statistical information.
