Abstract
In the city of Dar es Salaam, Tanzania, rapid and spontaneous scale-up of window screening occurred through purely horizontal commercial distribution systems without any public subsidies or promotion. Scale-up of window screening coincided with a planned evaluation of programmatic, vertically managed scale-up of regular larvicide application as an intervention against malaria vectors and transmission. We aimed to establish whether scale-up of window screening was associated with suppression of mosquito populations, especially for malaria vectors that strongly prefer humans as their source of blood. This study was a re-analysis of a previous observational series of epidemiological data plus new analyses of previously partly reported complementary entomological data, from Dar es Salaam. Between 2004 and 2008, six rounds of cluster-sampled, rolling, cross-sectional parasitological and questionnaire surveys were done in urban Dar es Salaam to assess the effect of larviciding and other determinants of malaria risk, such as use of bed nets and antimalarial drugs, socioeconomic status, age, sex, travel history, mosquito-proofed housing, and spending time outdoors. The effects of scaled-up larvicide application and window screening were estimated by fitting generalised linear mixed models that allowed for both spatial variation between survey locations and temporal autocorrelation within locations. We also conducted continuous longitudinal entomological surveys of outdoor human biting rates by mosquitoes and experimental measurements of mosquito host preferences. Best-fit models of Plasmodium falciparum malaria infection prevalence among humans were largely consistent with the results of the previous analyses. Re-analysis of previously reported epidemiological data revealed that most of the empirically fitted downward time trend in P falciparum malaria prevalence over the course of the study (odds ratio [OR] 0·04; 95% CI 0·03-0·06; p<0·0001), which was not previously reported numerically or attributed to any explanatory factor, could be plausibly explained by association with an upward trend in city-wide window screening coverage (OR 0·07; 0·05-0·09; p<0·0001) and progressive rollout of larviciding (OR 0·50; 0·41-0·60; p<0·0001). Increasing coverage of complete window screening was also associated with reduced biting densities of all taxonomic groups of mosquitoes (all p<0·0001), especially the Anopheles gambiae complex (relative rate [RR] 0·23; 95% CI 0·16-0·33) and Anopheles funestus group (RR 0·08; 0·04-0·16), which were confirmed as the most efficient vectors of malaria with strong preferences for humans over cattle. Larviciding was also associated with reduced biting densities of all mosquito taxa (p<0·0001), to an extent that varied consistently with the larvicide targeting scheme and known larval ecology of each taxon. Community-wide mosquito proofing of houses might deliver greater impacts on vector populations and malaria transmission than previously thought. The spontaneous nature of the scale-up observed here is also encouraging with regards to practicality, acceptability, and affordability in low-income settings. United States Agency for International Development, Bill & Melinda Gates Foundation, Wellcome Trust, and Valent BioSciences LLC.
Highlights
Vector control with insecticide-treated nets (ITNs) and indoor residual spraying (IRS) accounted for most of the 1·3 billion fewer malaria cases and 6·8 million fewer malaria-related deaths that occurred globally between2000 and 2015.1,2 direct protection of individuals and households by these approaches is obvious, most of their impressive effects on malaria transmission are mediated by area-wide population suppression of mosquitoes that feed and rest indoors, www.thelancet.com/planetary-health Vol 3 March 2019 e132Research in contextEvidence before this study We have proactively and retroactively surveyed the malaria vector control literature for more than 20 years, with a particular focus on larval control and mosquito-proofed housing, through regular active searches on PubMed, weekly publication alerts, advice from colleagues, and following reference trails
Most of the enormous reductions in malaria transmission and burden achieved since the turn of the century arose from scale-up of vector control with insecticide-treated nets (ITNs) and indoor residual spraying (IRS)
We examine complementary entomological data to establish whether scale-up of window screening was associated with suppression of mosquito populations, especially the most important malaria vectors, which strongly prefer humans as their source of blood
Summary
Vector control with insecticide-treated nets (ITNs) and indoor residual spraying (IRS) accounted for most of the 1·3 billion fewer malaria cases and 6·8 million fewer malaria-related deaths that occurred globally between2000 and 2015.1,2 direct protection of individuals and households by these approaches is obvious, most of their impressive effects on malaria transmission are mediated by area-wide population suppression of mosquitoes that feed and rest indoors, www.thelancet.com/planetary-health Vol 3 March 2019 e132Research in contextEvidence before this study We have proactively and retroactively surveyed the malaria vector control literature for more than 20 years, with a particular focus on larval control and mosquito-proofed housing, through regular active searches on PubMed, weekly publication alerts, advice from colleagues, and following reference trails. Most of the enormous reductions in malaria transmission and burden achieved since the turn of the century arose from scale-up of vector control with insecticide-treated nets (ITNs) and indoor residual spraying (IRS). Both of these approaches achieve these reductions by protecting indoor sleeping and living spaces, and by killing mosquitoes that attempt to feed or rest inside houses. Reported analyses of data from six rounds of repeated cross-sectional parasite surveys yielded quite modest estimates for the area-wide effects of larvicide application on infection prevalence of Plasmodium falciparum malaria (odds ratio [OR] 0·79; 95% CI 0·66–0·93), as well as the household-level effects of complete netting screens (OR 0·79; 0·66–0·93) and closed ceilings (OR 0·93; 0·85–1·01)
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