Biologically meaningful coverage indicators for eliminating malaria transmission

Abstract

Mosquitoes, which evade contact with long-lasting insecticidal nets and indoor residual sprays, by feeding outdoors or upon animals, are primary malaria vectors in many tropical countries. They can also dominate residual transmission where high coverage of these front-line vector control measures is achieved. Complementary strategies, which extend insecticide coverage beyond houses and humans, are required to eliminate malaria transmission in most settings. The overwhelming diversity of the world's malaria transmission systems and optimal strategies for controlling them can be simply conceptualized and mapped across two-dimensional scenario space defined by the proportion of blood meals that vectors obtain from humans and the proportion of human exposure to them which occurs indoors.

Figure 1.

Conceptual schematic of the difference between current demographic indicators of coverage of all humans (N_h) and true biological coverage of all available mosquito blood resources (A). In all panels, the proportion considered covered by the stated indicator is represented by the shaded fraction. (a) Conventional view of current LLIN/IRS target of 80% crude demographic coverage of all humans while indoor (C_h = 0.8). (b) Protective coverage of humans at all times when either indoors or outdoors (C_h,p; equation (1.1)) where half of human exposure to vectors occurs outdoors (π_i,0 = 0.5). (c) Biological coverage of all blood resources (C_A,p), equivalent to the covered proportion of all available human and animal blood (C_h,pA_h/A; equation (1.2)) in a scenario where half of human exposure to vectors occurs outdoors (π_i,0 = 0.5) and animals previously accounted for half of all bloodmeals (Q_h,0 = 0.5).

Figure 2.

A conceptual summary of the conclusions of recent deterministic modelling analyses [2,4,12,20,21] comparing vector control product profiles with a variety of repellent and/or toxic properties in a diversity of vector scenarios, mapped across the full range of preferences for feeding upon humans indoor versus outdoor (π_i,0) and upon humans versus animals (Q_h,0).