Insecticidal efficacy of residual spraying with deltamethrin–clothianidin (Fludora® Fusion) in Papua New Guinea

Abstract Background In Papua New Guinea (PNG), insecticide-treated nets leave considerable gaps in protection from Anopheles mosquito bites and, as a result, malaria cases are increasing. Residual spraying (RS) may be an effective complementary malaria control strategy. To understand the potential impact of RS and guide RS implementation, it is important to quantify the duration of insecticidal efficacy of RS formulations. The materials used to construct houses in rural PNG differ significantly from the African study sites where most such data have been generated. This study investigated determinants of duration of insecticidal efficacy of Fludora® Fusion, a co-formulation of clothianidin and deltamethrin in two villages in PNG, following a RS intervention study. Methods Eight sprayed houses and four control houses with different wall materials (six sago palm, six bamboo) were selected from each village. Test surfaces were either inside the houses or in protected outdoor locations such as on verandas, which had also been sprayed. Mortality at 24 h post exposure (M24h) was measured over 11 months post RS with wall cone bioassays and susceptible Anopheles farauti colony mosquitoes. The duration that M24h remained above 80% was estimated using a statistical model. Results M24h exceeded 95% on all sprayed surfaces within 1 month after spraying but decreased by 51 percentage points during the study period. Overall, M24h fell below 80% at 25 weeks post RS but variability was high [95% confidence interval (CI) 9–41 weeks]. The duration of insecticidal efficacy was significantly associated with the application rate, surface material, indoor versus outdoor location and height of the tested surface area. Conclusions The duration of insecticidal efficacy of Fludora® Fusion (M24h > 80%) was an average of 6 months in these field conditions but was highly variable and depended significantly on surface parameters. RS with Fludora® Fusion could be a valuable addition to insecticide-treated nets (ITNs) to control malaria in PNG. However, frequent reapplication may be required depending on surface properties and location. Achieving consistent target application rates is critical to achieve maximum duration of insecticidal efficacy. Graphical abstract