Attribution of reductions in malaria prevalence in Dar es Salaam, Tanzania – Authors' reply
Authors: Gerry F Killeen, Nicodem J Govella, Yeromin P Mlacha, Prosper P Chaki
Journal: The Lancet Planetary Health (2019)

We thank Mathieu Maheu-Giroux and Marcia Castro for their Correspondence about our Article.1Killeen GF Govella NJ Mlacha YP Chaki PP Suppression of malaria vector densities and human infection prevalence associated with scale-up of mosquito-proofed housing in Dar es Salaam, Tanzania: re-analysis of an observational series of parasitological and entomological surveys.Lancet Planet Health. 2019; 3: e132-e143Summary Full Text Full Text PDF PubMed Scopus (22) Google Scholar Regarding concerns about the stepped-wedge design of the larviciding scale-up in our study, we agree and also note that scale-up was not randomised, but rather introduced earliest to the best-prepared wards. However, such compromises are normal and healthy in pragmatic assessments of effectiveness under realistic programmatic conditions, rather than efficacy under artificially controlled experimental conditions. As acknowledged in the discussion and emphasised in the title, we also agree that evidence for window screening impact is purely observational, and that long-overdue, cluster-randomised, experimentally controlled analyses of well developed delivery practices for both these supplementary vector control measures are urgently needed.1Killeen GF Govella NJ Mlacha YP Chaki PP Suppression of malaria vector densities and human infection prevalence associated with scale-up of mosquito-proofed housing in Dar es Salaam, Tanzania: re-analysis of an observational series of parasitological and entomological surveys.Lancet Planet Health. 2019; 3: e132-e143Summary Full Text Full Text PDF PubMed Scopus (22) Google Scholar However, we disagree with the view that our attributions of intervention effects are haphazard and that potential confounders were “arbitrarily disregarded”. In our Article,1Killeen GF Govella NJ Mlacha YP Chaki PP Suppression of malaria vector densities and human infection prevalence associated with scale-up of mosquito-proofed housing in Dar es Salaam, Tanzania: re-analysis of an observational series of parasitological and entomological surveys.Lancet Planet Health. 2019; 3: e132-e143Summary Full Text Full Text PDF PubMed Scopus (22) Google Scholar factors such as bed nets, antimalarial drugs, and household-level screening were all considered in the analysis, with relevant effect sizes reported and discussed in detail. Although major shifts in vector population composition were observed in rural Tanzania around the time of the first mass distributions for insecticide-treated nets in 2008,2Russell TL Govella NJ Azizi S Drakeley CJ Kachur SP Killeen GF Increased proportions of outdoor feeding among residual malaria vector populations following increased use of insecticide-treated nets in rural Tanzania.Malar J. 2011; 10: 80Crossref PubMed Scopus (421) Google Scholar, 3Derua YA Alifrangis M Hosea KM et al.Change in composition of the Anopheles gambiae complex and its possible implications for the transmission of malaria and lymphatic filariasis in north-eastern Tanzania.Malar J. 2012; 11: 188Crossref PubMed Scopus (88) Google Scholar no equivalent changes were observed in Dar es Salaam over the same period, because the city was given lowest priority in both national campaigns so coverage remained low until 2010.4Msellemu D Namango HI Mwakalinga VM et al.The epidemiology of residual Plasmodium falciparum malaria transmission and infection burden in an African city with high coverage of multiple vector control measures.Malar J. 2016; 15: 288Crossref PubMed Scopus (22) Google Scholar Although a slight upward trend in net usage coincided with the downward trend in human malaria prevalence in Dar es Salaam, and artemisinin-based therapies also came into use, uptake of both measures was far too low to plausibly account for such massive reductions of malaria burden.1Killeen GF Govella NJ Mlacha YP Chaki PP Suppression of malaria vector densities and human infection prevalence associated with scale-up of mosquito-proofed housing in Dar es Salaam, Tanzania: re-analysis of an observational series of parasitological and entomological surveys.Lancet Planet Health. 2019; 3: e132-e143Summary Full Text Full Text PDF PubMed Scopus (22) Google Scholar The only interventions for which substantive upward coverage trends were observed over the study period were larviciding and window screening. Together, these two intervention scale-ups were associated with prevalence reductions equivalent to those accounted for with a simple, unattributed empirical time trend.1Killeen GF Govella NJ Mlacha YP Chaki PP Suppression of malaria vector densities and human infection prevalence associated with scale-up of mosquito-proofed housing in Dar es Salaam, Tanzania: re-analysis of an observational series of parasitological and entomological surveys.Lancet Planet Health. 2019; 3: e132-e143Summary Full Text Full Text PDF PubMed Scopus (22) Google Scholar As detailed in the methods, we did account for temporal autocorrelation when fitting these models.1Killeen GF Govella NJ Mlacha YP Chaki PP Suppression of malaria vector densities and human infection prevalence associated with scale-up of mosquito-proofed housing in Dar es Salaam, Tanzania: re-analysis of an observational series of parasitological and entomological surveys.Lancet Planet Health. 2019; 3: e132-e143Summary Full Text Full Text PDF PubMed Scopus (22) Google Scholar Indeed, our models that included a simple empirical time trend as per previous studies5Lawson AB Carroll R Castro M Joint spatial Bayesian modeling for studies combining longitudinal and cross-sectional data.Stat Methods Med Res. 2014; 23: 611-624Crossref PubMed Scopus (9) Google Scholar, 6Maheu-Giroux M Castro MC Impact of community-based larviciding on the prevalence of malaria infection in Dar es Salaam, Tanzania.PLoS One. 2013; 8: e71638Crossref PubMed Scopus (58) Google Scholar essentially reproduced the previous results, notably the modest effect size estimates for larviciding. The major issue therefore seems to be that fitting an unattributed empirical time trend may have spuriously assimilated the effects of both observed intervention scale-up processes.1Killeen GF Govella NJ Mlacha YP Chaki PP Suppression of malaria vector densities and human infection prevalence associated with scale-up of mosquito-proofed housing in Dar es Salaam, Tanzania: re-analysis of an observational series of parasitological and entomological surveys.Lancet Planet Health. 2019; 3: e132-e143Summary Full Text Full Text PDF PubMed Scopus (22) Google Scholar Underestimating effect sizes may be as dangerous as overestimating them, and regression dilution bias inherently exacerbates risk of underestimation. Notably, the entomological secondary outcomes and explanatory variables clearly and independently implicated window screening as the most important, obvious, and parsimonious explanation for the remarkable decline in malaria prevalence.1Killeen GF Govella NJ Mlacha YP Chaki PP Suppression of malaria vector densities and human infection prevalence associated with scale-up of mosquito-proofed housing in Dar es Salaam, Tanzania: re-analysis of an observational series of parasitological and entomological surveys.Lancet Planet Health. 2019; 3: e132-e143Summary Full Text Full Text PDF PubMed Scopus (22) Google Scholar Substantial reductions of mosquito population density were associated with scale-up of both larviciding and window screens. Effect sizes for different taxonomic groups of mosquitoes were fully consistent with those attributed to the same two interventions for the epidemiological data. Crucially, the effect sizes for window screening were greatest for the most efficient malaria vectors with the greatest preference for human hosts, and therefore the greatest dependence upon accessing human blood. It is difficult to envision how changes in demography, socioeconomic status, or any intervention other than a personal protection measure like window screens could cause such behaviour-dependent effects on mosquito populations. GFK, NJG, YPM, and PPC report grants from the United States Agency for International Development, the Bill & Melinda Gates Foundation, the Wellcome Trust (fellowship award to GFK), and Valent BioSciences LLC. Suppression of malaria vector densities and human infection prevalence associated with scale-up of mosquito-proofed housing in Dar es Salaam, Tanzania: re-analysis of an observational series of parasitological and entomological surveysCommunity-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. Full-Text PDF Open AccessAttribution of reductions in malaria prevalence in Dar es Salaam, TanzaniaDar es Salaam's Urban Malaria Control Project (UMCP) was launched in 2004 with the objective of implementing and assessing a microbial larviciding intervention. From 2004 to 2008, malaria prevalence decreased from 28% to less than 2%.1 This collapse of malaria vector populations was also observed in neighbouring regions of Tanzania.2 Part of the reduction preceded the initiation of larviciding activities and the introduction of artemisinin-based combination therapies. Yet, previous analyses suggested that larviciding was effective in reducing malaria prevalence. Full-Text PDF Open Access