Target product profile choices for intra-domiciliary malaria vector control pesticide products: repel or kill?

Abstract

Background: The most common pesticide products for controlling malaria-transmitting mosquitoes combine two distinct modes of action: 1) conventional insecticidal activity which kills mosquitoes exposed to the pesticide and 2) deterrence of mosquitoes away from protected humans. While deterrence enhances personal or household protection of long-lasting insecticidal nets and indoor residual sprays, it may also attenuate or even reverse communal protection if it diverts mosquitoes to non-users rather than killing them outright.

Methods: A process-explicit model of malaria transmission is described which captures the sequential interaction between deterrent and toxic actions of vector control pesticides and accounts for the distinctive impacts of toxic activities which kill mosquitoes before or after they have fed upon the occupant of a covered house or sleeping space.

Results: Increasing deterrency increases personal protection but consistently reduces communal protection because deterrent sub-lethal exposure inevitably reduces the proportion subsequently exposed to higher lethal doses. If the high coverage targets of the World Health Organization are achieved, purely toxic products with no deterrence are predicted to generally provide superior protection to non-users and even users, especially where vectors feed exclusively on humans and a substantial amount of transmission occurs outdoors. Remarkably, this is even the case if that product confers no personal protection and only kills mosquitoes after they have fed.

Conclusions: Products with purely mosquito-toxic profiles may, therefore, be preferable for programmes with universal coverage targets, rather than those with equivalent toxicity but which also have higher deterrence. However, if purely mosquito-toxic products confer little personal protection because they do not deter mosquitoes and only kill them after they have fed, then they will require aggressive "catch up" campaigns, with behaviour change communication strategies that emphasize the communal nature of protection, to achieve high coverage rapidly.

Figure 1

A schematic outline how the model captures the sequential nature of deterrent (θ_Δ) and toxic actions (θ_Δ) of vector control pesticides and account for the distinctive impacts of toxic activities which kill mosquitoes before (θ_μ,pre) or after (θ_μ,post) they have fed upon the occupant of a covered house (IRS) or sleeping space (LLIN).

Figure 2

The sensitivity of mosquito survival per feeding cycle (P_f) and sporozoite infection prevalence (S) upon assumed values for daily survival while foraging for vertebrate blood or oviposition resources (P_ov) in the absence of any LLIN or IRS intervention (C_h = 0).

Figure 3

Predicted impact of increasing levels of deterrence (θ_Δ) upon exposure to malaria transmission for LLIN or IRS products with (θ_μ,pre = 0.5, θ_μ,post = 0 and without (θ_μ,pre = 0, θ_μ,post = 0) toxic properties, assuming either fixed or survival-dependent emergence rates (E) at 80% crude coverage (C_h = 0.8).

Figure 4

Predicted impact of increasing levels of deterrence (θ_Δ) upon underlying biodemographic mosquito and sprogonic-stage parasite population parameters that determine malaria transmission for LLIN or IRS products with (θ_μ,pre = 0.5, θ_μ,post = 0) and without (θ_μ,pre = 0, θ_μ,post = 0) toxic properties at 80% crude coverage (C_h = 0.8). Only the model with survival-dependent emergence rates (E) is presented.

Figure 5

Predicted impact of increasing levels of deterrence upon the share of total blood availability (Z) that human users and non-users of LLINs (Z_h) constitute as the deterrence of an LLIN or IRS product at 80% crude coverage (C_h = 0.8).

Figure 6

Predicted impact of LLIN or IRS products with purely pre-feeding toxic (θ_μ,pre = 0.8 θ_μ,post = 0, θ_Δ = 0), post-feeding toxic (θ_μ,pre = 0, θ_μ,post = 0.8, θ_Δ = 0), deterrent (θ_μ,pre = 0, θ_μ,post = 0, θ_Δ = 0.8) and pre-feeding toxic-deterrent hybrid (θ_μ,pre = 0.8 θ_μ,post = 0, θ_Δ = 0.8) properties upon malaria transmission exposure for users and non-users where either most (π_i = 0.9) or half (π_i = 0.5) of baseline transmission occurs indoors.

Figure 7

Relative residual malaria transmission achieved with varying levels of crude coverage of a purely post-feeding toxic LLIN or IRS product (θ_μ,pre = 0 θ_μ,post = 0.8, θ_Δ = 0), compared with products with both purely deterrent (θ_μ,pre = 0, θ_μ,post = 0, θ_Δ = 0.8) and pre-feeding toxic-deterrent hybrid (θ_μ,pre = 0.8 θ_μ,post = 0, θ_Δ = 0.8) properties for users and non-users where either most (π_i = 0.9) or half (π_i = 0.5) of baseline transmission occurs indoors.