Abstract
Insecticide-treated bed nets (ITNs) have become a central tool for malaria control because they provide personal and community-wide protection through their repellent and insecticidal properties. Here we propose a model that allows to assess the relative importance of those two effects in different epidemiological contexts and we show that these two levels of protection may oppose each other. On the one hand, repellency offers personal protection to the users of ITNs. The repellent action, however, is a two-edged sword, for it diverts infectious mosquitoes to non-users, thereby increasing their risk. Furthermore, with increasing ITN coverage, the personal protection effect of repellency decreases as mosquitoes are forced to perform multiple feeding attempts even on ITN users. On the other hand, the insecticidal property, which offers community-wide protection by killing mosquitoes, requires that mosquitoes contact the insecticide on the ITN and is thus counteracted by the repellency. Our model confirms that ITNs are an effective intervention method by reducing total malaria prevalence in the population, but that there is a conflict between personal protection, offered by repellency, and community-wide protection, which relies on the ITN’s insecticidal properties. Crucially, the model suggests that weak repellency allows disease elimination at lower ITN coverage levels.
Highlights
Insecticide-treated bed nets (ITNs) are among the most important and cost-effective intervention measures against malaria relying on three main mechanisms: 1) the nets create a physical barrier between the human and the mosquito vector, 2) the insecticide used to treat the bed net repels mosquitoes (“excito-repellency” or “deterrence”, referred to as repellency in this paper), increasing the personal protection offered by the net, and 3) if a mosquito fails to be repelled, it will often rest on the bed net after biting, and may be killed by contacting the insecticide
We find that increased coverage of ITNs decreases malaria prevalence through a combination of the personal protection given by the repellency of the insecticide and the community protection given by its insecticidal action (Fig 2)
Insecticide-treated bed nets protect individuals against malaria by blocking and repelling mosquitoes, and they protect the community by killing mosquitoes
Summary
Insecticide-treated bed nets (ITNs) are among the most important and cost-effective intervention measures against malaria relying on three main mechanisms: 1) the nets create a physical barrier between the human and the mosquito vector, 2) the insecticide used to treat the bed net repels mosquitoes (“excito-repellency” or “deterrence”, referred to as repellency in this paper), increasing the personal protection offered by the net, and 3) if a mosquito fails to be repelled, it will often rest on the bed net after biting, and may be killed by contacting the insecticide. How Bed Nets Influence Malaria Transmission animal feeding), ITNs provide protection by diverting mosquitoes to non-human hosts [1,2,3]. ITNs offer a mix of personal protection—blocking the bites of mosquitoes, thereby reducing the transmission from mosquitoes to humans—and community protection—reducing the longevity of mosquitoes and the prevalence of sporozoites, the infectious stage of malaria, in mosquitoes. The personal protection offered by insecticide-treated bed nets has been documented in many studies e.g [5,6,7,8]. Hawley et al [12], for example, found reduced disease incidence up to 300m around a house where ITNs are used, and [13] report a 4.2 fold reduction of the entomological inoculation rate (EIR) experienced by unprotected people with a coverage of 75% of untreated nets and an 18-fold reduction if those nets are treated with insecticides
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