Priorities for Broadening the Malaria Vector Control Tool Kit

Abstract

Long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) have contributed substantially to reductions in the burden of malaria in the past 15 years. Building on this foundation, the goal is now to drive malaria towards elimination. Vector control remains central to this goal, but there are limitations to what is achievable with the current tools. Here we highlight how a broader appreciation of adult mosquito behavior is yielding a number of supplementary approaches to bolster the vector-control tool kit. We emphasize tools that offer new modes of control and could realistically contribute to operational control in the next 5 years. Promoting complementary tools that are close to field-ready is a priority for achieving the global malaria-control targets.

Keywords: Anopheles; behavior; insecticide resistance; integrated vector management; malaria; vector control.

Figure 1. Estimate of historic and projected global deaths due to malaria based on different control scenarios

The figure (modified from [9]) shows estimates of global malaria deaths from 2000–2045. The 50% decline in malaria related mortality recorded from 2000–2015 is largely attributable to the wide scale implementation of vector control tools (Long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS)) [1,2]. The future projections are based on a model analysis that considers different scenarios of access to vector control, together with malaria drug treatments [9]. The graph is modified from Figure 1B of Griffin et al. [9] by using data from the 2016 World Malaria Report [89] to convert the original y-axis of ‘deaths per 1000 people per year’ into estimates of overall malaria mortality per year, and adding the target line for future decline in malaria deaths from the WHO Global Technical Strategy. The back line indicates resurgence in malaria deaths if control efforts remain at current levels. The blue line is the predicted decline in deaths assuming coverage of current control tools can be increased to reach 80% of the population at risk. The red line represents the target set out in the WHO Global Technical Strategy [1], which aims for a 90% decline in malaria deaths by 2030 and then ultimate elimination thereafter. The arrows A and B illustrate the differences between the WHO target and the two control scenarios. Business as usual clearly represents a massive failure (A). Perhaps more notably, even substantial intensification of existing tools still yields a substantial shortfall (B). These gaps in control demonstrate the need for new interventions. The numbered horizontal lines refer to the estimated timelines for implementation of a range of prospective control tools where: (1) refers to tools that are close to field ready (e.g. attractive toxic sugar baits, housing improvement, livestock targets, next generation LLINs and IRS); (2) represents tools that require a few more years for product development (e.g. improved topical repellents, long lasting endectocides for human use); and (3) tools that either for technical and/or regulatory reasons are still far from operational use (e.g. transinfection with Wolbachia, population replacement strategies using genetically modified mosquitoes and gene drive). The fact that the WHO target shows an immediate deviation from the two control scenarios highlights a critical role for tools that can be implemented in the short- and medium-term (1 and 2).

Figure 2. Diverse behaviors and activities of adult malaria mosquitoes as they progress from emergence through to egg laying over one or more gonotrophic cycles

Adult mosquitoes emerge from aquatic habitats (1) and mate within a few days (2), potentially taking a sugar meal for energy (*). Male mosquitoes then tend to die quite quickly, while females go in search of a blood meal (3). Blood feeding could be on a diversity of hosts, either indoors or outdoors. After blood feeding the mosquitoes will tend to rest for 2–4 days while they digest the blood to produce eggs (4). Resting can occur in a range of indoor or outdoor environments. Once the eggs are fully developed the mosquitoes then search for a suitable oviposition site (5), potentially taking another sugar meal (*) to boost energy reserves for flight. Once a suitable aquatic habitat is located and the eggs are laid, female mosquitoes can repeat the blood feeding and egg production process over subsequent days to complete multiple gonotrophic cycles. Current core vector control tools (Long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS)) target female mosquitoes at just two points in the adult life cycle within domestic dwellings only.