Target product profiles for protecting against outdoor malaria transmission

Abstract

Background: Long-lasting insecticidal nets (LLINs) and indoor residual sprays (IRS) have decimated malaria transmission by killing indoor-feeding mosquitoes. However, complete elimination of malaria transmission with these proven methods is confounded by vectors that evade pesticide contact by feeding outdoors.

Methods: For any assumed level of indoor coverage and personal protective efficacy with insecticidal products, process-explicit malaria transmission models suggest that insecticides that repel mosquitoes will achieve less impact upon transmission than those that kill them outright. Here such models are extended to explore how outdoor use of products containing either contact toxins or spatial repellents might augment or attenuate impact of high indoor coverage of LLINs relying primarily upon contact toxicity.

Results: LLIN impact could be dramatically enhanced by high coverage with spatial repellents conferring near-complete personal protection, but only if combined indoor use of both measures can be avoided where vectors persist that prefer feeding indoors upon humans. While very high levels of coverage and efficacy will be required for spatial repellents to substantially augment the impact of LLINs or IRS, these ambitious targets may well be at least as practically achievable as the lower requirements for equivalent impact using contact insecticides.

Conclusions: Vapour-phase repellents may be more acceptable, practical and effective than contact insecticides for preventing outdoor malaria transmission because they need not be applied to skin or clothing and may protect multiple occupants of spaces outside of treatable structures such as nets or houses.

Figure 1

Impact of products for outdoor malaria prevention expressed in terms of the mean relative risk of exposure experienced by the average community member (ψ_h,Ω). Scenarios are considered in which LLIN products that provide 50% personal protection (ρ_i = 0·5) by killing half of all mosquitoes that attack them (θ_μ,pre,i = 0·5) are complemented by use of additional products conferring equivalent personal protection (ρ_o or ρ_i+o = 0·5) with one of the three following profiles: Products for exclusively outdoor use that kill attacking mosquitoes before they feed (θ_μ,pre,o = 0·5) or products that deter mosquitoes from attacking that are used either outdoors only (θ_Δ,o = 0·5) or are used both indoors and outdoors (θ_Δ,i+o = 0·5). Further details of the model and symbol definitions are available online (See Additional file 4)

Figure 2

Additional incremental impact of outdoor contact toxins (θ_μ,pre,o = 0·5) or repellents that are exclusively used outdoors (θ_Δ,o = 0·5) or used both indoors and outdoors (θ_Δ,i+o = 0·5) when combined with indoor LLINs with contact toxins (θ_μ,pre,i = 0·5), compared with their direct impact as stand-alone intervention strategies. Impact is expressed in terms of the mean relative risk of exposure to residual transmission for the average community member where LLINs are combined with additional products with the above profiles (ψ_{h,combination}) compared with when they are applied as a stand-alone measure (ψ_{h,LLINs alone}). All products are assumed to confer 50% personal protection (ρ_o or ρ_i+o = 0·5) by either repelling or killing half of all mosquitoes that attack them (θ = 0·5). Further details of the model and symbol definitions are available online (See Additional file 4)

Figure 3

Progressive impact upon a completely outdoor transmission system (π_i = 0) of products with increasing efficacy of personal protection (ρ_o) achieved by either repelling (θ_Δ,o) or killing (θ_μ,pre,o) attacking mosquitoes before they feed upon human users. The grey arrows represent interpolation of the efficacy thresholds at which the toxic mode of action achieves equivalent transmission control to the theoretical limit at complete protective efficacy (θ_Δ,o = 1·0 so ρ_o = 1·0) for spatial repellents at high coverage (C_h = 0·8). Impact is expressed in terms of the mean relative risk of exposure experienced by the average community member (ψ_h,Ω). Further details of the model and symbol definitions are available online (See Additional file 4)