An experimental hut study to quantify the effect of DDT and airborne pyrethroids on entomological parameters of malaria transmission

Abstract

BackgroundCurrent malaria vector control programmes rely on insecticides with rapid contact toxicity. However, spatial repellents can also be applied to reduce man-vector contact, which might ultimately impact malaria transmission. The aim of this study was to quantify effects of airborne pyrethroids from coils and DDT used an indoor residual spray (IRS) on entomological parameters that influence malaria transmission.MethodsThe effect of Transfluthrin and Metofluthrin coils compared to DDT on house entry, exit and indoor feeding behaviour of Anopheles gambiae sensu lato were measured in experimental huts in the field and in the semi-field. Outcomes were deterrence - reduction in house entry of mosquitoes; irritancy or excito-repellency – induced premature exit of mosquitoes; blood feeding inhibition and effect on mosquito fecundity.ResultsTransfluthrin coils, Metofluthrin coils and DDT reduced human vector contact through deterrence by 38%, 30% and 8%, respectively and induced half of the mosquitoes to leave huts before feeding (56%, 55% and 48%, respectively). Almost all mosquitoes inside huts with Metofluthrin and Transfluthrin coils and more than three quarters of mosquitoes in the DDT hut did not feed, almost none laid eggs and 67%, 72% and 70% of all mosquitoes collected from Transfluthrin, Metofluthrin and DDT huts, respectively had died after 24 hours.ConclusionThis study highlights that airborne pyrethroids and DDT affect a range of anopheline mosquito behaviours that are important parameters in malaria transmission, namely deterrence, irritancy/excito-repellency and blood-feeding inhibition. These effects are in addition to significant toxicity and reduced mosquito fecundity that affect mosquito densities and, therefore, provide community protection against diseases for both users and non-users. Airborne insecticides and freshly applied DDT had similar effects on deterrence, irritancy and feeding inhibition. Therefore, it is suggested that airborne pyrethroids, if delivered in suitable formats, may complement existing mainstream vector control tools.

A low technology emanator treated with the volatile pyrethroid transfluthrin confers long term protection against outdoor biting vectors of lymphatic filariasis, arboviruses and malaria

“…Furthermore, transfluthrin can also kill mosquitoes that closely approach protected humans [6], which would also preclude diversion to unprotected individuals nearby. Transfluthrin-treated hessian emanators may therefore be far safer for nearby non-users than conventional topical repellents like N,N-Diethyl-meta-toluamide (DEET) [11,34] and might even provide area-wide protection of users and non-users alike, similarly to LLINs and IRS [12,35].…”

“…Several species of Culex , Anopheles , and Mansonia mosquitoes including several known vectors of not only malaria [6,19,20,21], but also filariasis, and five different arboviruses [22] have been reported in this locality. Since scale up of LLINs in 2008, Anopheles gambiae sensu stricto population densities have declined, leaving exophagic An .…”

“…Low doses of airborne pyrethroids induce sub-lethal responses such as: repellency, deterrence, feeding inhibition and reduce fecundity as opposed to higher doses of contact insecticides that induce knock down and mortality of mosquitoes. Nevertheless, these sub-lethal responses are likely to decrease vectorial capacity of mosquitoes and consequently reduce disease transmission that could be equivalent to toxic contact insecticides [6]. Vapor-phase insecticides are typically formulated as coils, emanators or candles [7].…”

Current and Future Repellent Technologies: The Potential of Spatial Repellents and Their Place in Mosquito-Borne Disease Control

“…Because the ultimate goal of repellent technologies is to deter host-blood-feeding, these compounds may exert their effects through a variety of means. Transfluthrin and metofluthrin were capable of deterring overall mosquito entry into treated huts and inhibited feeding, decreased fecundity, and higher percentage mortalities were observed in mosquito groups that entered these treated huts [56]. Other studies have also demonstrated the sublethal effects of spatial repellents on mosquitoes that have been exposed to these repellent vapors.…”

Repellents and New “Spaces of Concern” in Global Health

“…Cumulative insights on the impact of this chemical have shown that DDT's toxicity to the mosquito is only one and, in terms of disease control, possibly the least effective of its chemical effects. 7 In addition to killing mosquito on contact, the pesticide also works as a powerful repellent (Mabaso et al 2004;Ogoma et al 2014;Roberts et al 2000;WHO 1971). 8 Repellency is a mode of action with manifold public health advantages.…”