Attribution of reductions in malaria prevalence in Dar es Salaam, Tanzania

Dar 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%.1Maheu-Giroux M Castro M 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 This collapse of malaria vector populations was also observed in neighbouring regions of Tanzania.2Meyrowitsch DW Pedersen EM Alifrangis M et al.Is the current decline in malaria burden in sub-Saharan Africa due to a decrease in vector population?.Malar J. 2011; 10: 188Crossref PubMed Scopus (80) Google Scholar 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.1Maheu-Giroux M Castro M 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, 3Lawson 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 In their Article, Gerry Killeen and colleagues4Killeen 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 reanalysed the UMCP data. Although it is reassuring that most of their results confirm those previously reported, their paper raises important concerns. First, the rollout of larviciding followed a stepped-wedge design. It involved sequential crossover of the city's wards from control to intervention such that, ultimately, all wards received the larviciding intervention. This implies that calendar time is a potentially crucial confounder and, following standard practice, must be included in the analyses.5Hemming K Haines TP Chilton PJ Girling AJ Lilford RJ The stepped wedge cluster randomised trial: rationale, design, analysis, and reporting.BMJ. 2015; 350: h391Crossref PubMed Scopus (661) Google Scholar Substituting another covariate for time in the model could grossly overestimate the intervention's impact. A second concern is that the authors mistakenly state that earlier analyses did not adjust for within-ward covariance. This is incorrect because previously reported odds ratios for larviciding included this adjustment in sensitivity analyses using ward-level fixed effects—confirming their robustness. The advantage of this specification is that it controls for “any time-invariant measured or unmeasured confounders of the larviciding-malaria relationship”,1Maheu-Giroux M Castro M 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 which is especially important for the UMCP whose larviciding allocation sequence was not randomised. Using fixed effects therefore provides a more robust estimate than using a random effects specification. Third, attributing changes in an observed health outcome to one specific intervention requires an appropriate control group. Although it is plausible that improved house proofing had a role in reducing malaria transmission, attributing the reduction to this factor alone, as implied by their analyses substituting calendar time, does not follow principles of causal attribution. The authors disregard household-level variations in house proofing to use city-wide coverage (with unrealistic discrepancies between the two estimates) and do not consider potential confounders. From 2002 to 2012, Dar es Salaam's population increased from 2·5 million to 4·3 million people and its density nearly doubled. There were noticeable reductions in urban agriculture, increases in bednet usage, introduction of artemisinin-based combination therapy, and clear improvements in literacy and socioeconomic status during the UMCP. The decision to attribute the bulk of the malaria decline to improved housing seems arbitrary and could misguide public health efforts by overstating its potential benefits. We declare no competing interests. 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, Tanzania – Authors' replyWe thank Mathieu Maheu-Giroux and Marcia Castro for their Correspondence about our Article.1 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. Full-Text PDF Open Access

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