Synergy between repellents and organophosphates on bed nets: efficacy and behavioural response of natural free-flying An. gambiae mosquitoes

Abstract

Background: Chemicals are used on bed nets in order to prevent infected bites and to kill aggressive malaria vectors. Because pyrethroid resistance has become widespread in the main malaria vectors, research for alternative active ingredients becomes urgent. Mixing a repellent and a non-pyrethroid insecticide seemed to be a promising tool as mixtures in the laboratory showed the same features as pyrethroids.

Methodology/principal findings: We present here the results of two trials run against free-flying Anopheles gambiae populations comparing the effects of two insect repellents (either DEET or KBR 3023, also known as icaridin) and an organophosphate insecticide at low-doses (pirimiphos-methyl, PM) used alone and in combination on bed nets. We showed that mixtures of PM and the repellents induced higher exophily, blood feeding inhibition and mortality among wild susceptible and resistant malaria vectors than compounds used alone. Nevertheless the synergistic interactions are only involved in the high mortality induced by the two mixtures.

Conclusion: These field trials argue in favour of the strategy of mixing repellent and organophosphate on bed nets to better control resistant malaria vectors.

Figure 1. Log of the difference between numbers of mosquitoes entering treated hut and the control one, with 95% confidence intervals.

Axis of abscissa represents the week numbers after the beginning of the trial. For each week are written the number of An. gambiae mosquitoes collected in the treated hut (at the top) and the number of An. gambiae mosquitoes collected in the control hut.

Figure 2. Exophily rates over time in the minimal adequate logistic regression model with standard error bounds.

a and c illustrate the trial during dry season 2006; b and d illustrate the trial during rainy season 2006.

Figure 3. Blood feeding rates over time in the minimal adequate logistic regression model standard error bounds.

a, c and e illustrate the trial during dry season 2006; b, d and f illustrate the trial during rainy season 2006.

Figure 4. Mortality rates over time in the minimal adequate logistic regression model with standard error bounds.

a, c, e, g and i illustrate the trial during dry season 2006; b, d, f, h and j illustrate the trial during rainy season 2006.