Characterizing, controlling and eliminating residual malaria transmission

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.

Developing an expanded vector control toolbox for malaria elimination

“…LLINs and IRS have been highly effective interventions in LMICs, 1 but they have fundamental limitations, including (1) their vulnerability to selection for insecticide resistance, 12 13 (2) their reliance on population-wide human compliance for operational effectiveness, 14 15 (3) considerable cost, 2 16-18 and (4) important biological constraints to their efficacy caused by mosquitoes that feed on humans and/or animals outdoors, rest outdoors, or enter houses but then rapidly exit from them without being exposed to insecticides. 5 6 19 Resistance to all four classes of insecticide available for public health, especially the pyrethroids we rely on for LLINs, is now prevalent across Africa. 12 13 New chemical insecticides are expected to enter the market over the next few years, but these may be similarly vulnerable to selection for physiological resistance if used as single active ingredients.…”

“…5 6 19 26 Reducing malaria transmission to levels where the rate of reinfection is low enough to eliminate parasite reservoirs from humans will require improved protection against human-biting mosquitoes, as well as more broadly effective population control of all major vector species, regardless of their diverse behavioural traits. 5 6 19 In this analysis, we outline immediate opportunities for developing and implementing more aggressive malaria vector control strategies in LMICs, by leveraging transferable programmatic experiences from HICs with existing technologies, as well as exploiting repurposed and emerging new technologies. Additional new technologies include autodissemination of larvicides, 27 genetic control, 28 biological control 29 and endectocides (systemic insecticides that are delivered to the tissues of target animals through oral, injectable or implant formulations) for humans, 30 31 but these are unlikely to be ready for programmatic assessment 19 in <10 years.…”

Measuring, manipulating and exploiting behaviours of adult mosquitoes to optimise malaria vector control impact

“…While the theoretical evidence base emphasising the importance of such behavioural measurements has become stronger in recent years, 3 Restoring the problem-solving traditions of malaria vector surveillance Developing and evaluating a simple set of affordable, practical, scalable entomological indicators of vector control opportunities will require considerable consensus and funding investment; it will also need a new generation of entomologists to embrace the quantitative ethos of what was once known as epidemiological entomology 76 and update the underlying science. After decades The estimated fraction of A. arabiensis which rest indoors after feeding (reported originally as the estimated usage rate for indoor resting sites per feeding cycle 52 ) varies across a range of more than 300-fold in 21 distinct villages surveyed all across Africa.…”

Surveillance and response for high-risk populations: what can malaria elimination programmes learn from the experience of HIV?

“…In many elimination settings, residual transmission exists due to anopheline species that exhibit outdoor feeding and resting behaviour (exophagy and exophily), which often coincides with outdoor human behaviours during biting hours [23, 24]. While long-lasting insecticide-treated nets (LLINs) and indoor residual spraying (IRS) are effective for more endophagic and endophilic anopheline species, where high coverage of LLINs and IRS has been achieved, programmes often observe behavioural shifts towards increased outdoor feeding and resting [25–27], as well as increased proportional abundance of more exophilic species such as Anopheles arabiensis [28].…”