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

Abstract

BackgroundThe 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.MethodsA 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.ResultsIncreasing 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.ConclusionsProducts 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.

Characterizing, controlling and eliminating residual malaria transmission

“…The probability curves presented represent the outputs of simulations implemented exactly as previously described [14] at 0, 20, 40 and 60% biological coverage of all available blood resources [10, 13] with LLINs that kill 60% of all mosquitoes encountering them.…”

“…Such stimulant insecticides artificially induce or exacerbate early exit behaviours, ultimately attenuating mosquito exposure to lethal doses [14, 15, 42–44, 51]. Behaviour-modifying insecticides which require direct physical contact with a mosquito to induce an avoidance response are known as contact irritants, while those that the mosquito can sense in the air at a distance from the treated surface, and then choose to avoid contact with, are known as spatial repellents [51, 52].…”

Made-to-measure malaria vector control strategies: rational design based on insecticide properties and coverage of blood resources for mosquitoes

“…All simulations were implemented exactly as described previously [14], assuming that these mosquitoes differ only in their preferences for human and cattle hosts (parameterized as per [16]), and that high demographic coverage ( C h = 0.8) and protective efficacy ( ρ = 0.8) of the intervention measures are maintained at all times of the day ( π h = 1). All toxicity is assumed to act on contact before mosquitoes feed so that products with toxic ( θ μ,pre =0.8, θ μ,post =0) and repellent ( θ Δ =0.8) profiles confer equivalent personal protection ( ρ = 0.8) and differ only in the level of community-level protection achieved [14-16]. The proportional frequency of emerging mosquitoes which take a given number of human blood meals per lifetime ( F i ) is calculated as product of the mean probability of survival per feeding cycle ( p f ) and the human blood index ( Q h ) to the power of the number of blood meals ( i ) divided by the sum of the values for this term for all possible numbers of blood meals: ${\mathit{F}}_{\mathit{i}}={\left({\mathit{p}}_{\mathit{f}}{\mathit{Q}}_{\mathit{h}}\right)}^{\mathit{i}}/{\displaystyle {\sum }_{\mathit{i}\mathit{=}\mathit{0}}^{\mathit{\infty }}{\left({\mathit{p}}_{\mathit{f}}{\mathrm{italicQ}}_{\mathrm{italich}}\right)}^{\mathit{i}}}$.…”

The risk of incomplete personal protection coverage in vector-borne disease

“…Previous models have pointed out the potential negative effects that partial bednet coverage can have due to diversion of vectors from the TG to the UTG, when combined with low killing efficiency [11,21,34]. The effects estimated, however, were very low, particularly at the level of the entire population [11,31,32].…”