Abstract
Long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) interventions can reduce malaria transmission by targeting mosquitoes when they feed upon sleeping humans and/or rest inside houses, livestock shelters or other man-made structures. However, many malaria vector species can maintain robust transmission, despite high coverage of LLINs/IRS containing insecticides to which they are physiologically fully susceptible, because they exhibit one or more behaviours that define the biological limits of achievable impact with these interventions: (1) Natural or insecticide-induced avoidance of contact with treated surfaces within houses and early exit from them, thus minimizing exposure hazard of vectors which feed indoors upon humans; (2) Feeding upon humans when they are active and unprotected outdoors, thereby attenuating personal protection and any consequent community-wide suppression of transmission; (3) Feeding upon animals, thus minimizing contact with insecticides targeted at humans or houses; (4) Resting outdoors, away from insecticide-treated surfaces of nets, walls and roofs. Residual malaria transmission is, therefore, defined as all forms of transmission that can persist after achieving full universal coverage with effective LLINs and/or IRS containing active ingredients to which local vector populations are fully susceptible. Residual transmission is sufficiently intense across most of the tropics to render malaria elimination infeasible without new or improved vector control methods. Many novel or improved vector control strategies to address residual transmission are emerging that either: (1) Enhance control of adult vectors that enter houses to feed and/or rest by killing, repelling or excluding them; (2) Kill or repel adult mosquitoes when they attack people outdoors; (3) Kill adult mosquitoes when they attack livestock; (4) Kill adult mosquitoes when they feed upon sugar or; (5) Kill immature mosquitoes in aquatic habitats. To date, none of these options has sufficient supporting evidence to justify full-scale programmatic implementation. Concerted investment in their rigorous selection, development and evaluation is required over the coming decade to enable control and, ultimately, elimination of residual malaria transmission. In the meantime, national programmes may assess options for addressing residual transmission under programmatic conditions through pilot studies with strong monitoring, evaluation and operational research components, similar to the Onchocerciasis Control Programme.
Highlights
Hundreds of Anopheles species have been described worldwide, certain biological and environmental factors distinguish a small subset of a few dozen that mediate transmission of Plasmodium parasites to humans in the wild [1,2]
The specific behaviours exhibited by each distinct vector population in a given locality are important determinants of their vectorial capacity to mediate malaria transmission [1,2,7] and their vulnerability to control or even elimination [8,9,10,11,12,13]
Sustained local transmission requires that local Anopheles mosquitoes are physiologically competent and survive long enough for complete sporogonic development of malaria parasites within their bodies, and that they feed at least occasionally upon humans
Summary
Behaviours exhibited by each distinct vector population in a given locality are important determinants of their vectorial capacity to mediate malaria transmission [1,2,7] and their vulnerability to control or even elimination [8,9,10,11,12,13]. The most obvious options for preventing outdoor exposure of humans, especially when they are active and cannot be practically enclosed within a structure like a net, include insecticidetreated clothes [98,99] and repellents delivered as topical applications or vapour-phase emanators [51,100] While such approaches to personal protection of people may achieve valuable community-wide impact upon transmission by reducing human-vector contact, they are unlikely to reduce the survival, density or vectorial capacity of the vector population where they obtain most of their blood from animals [8,10,11] (Figure 13). Authors’ contributions GFK drafted, edited and finalized the manuscript based on detailed discussions with a number of helpful colleagues who are acknowledged below
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