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

Abstract

BackgroundIn 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.Methods 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.Findings 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.Interpretation Community-wide mosquito proofing of houses might deliver greater impacts on vector populations and malaria transmission than previously thought. The spontaneou...

Opinions of key stakeholders on alternative interventions for malaria control and elimination in Tanzania

“…In developing countries, simple modifications like screening windows and doors and closing eave spaces have resulted in some cases, in a 50% decline in entomological inoculation rates [38]. In Tanzania for example, housing improvement was linked to significant historical declines of malaria in Dar es Salaam [39], and was likely a major factor in more than 99% decline in malaria in Ifakara town, the main town in the area of our study [33] Larval source management Larval source management (LSM) refers to environmental manipulations to target mosquito larval habitats [13]. LSM can include the use of larvicides as well as environmental management methods [13,14,40].…”

Reduced human-biting preferences by the African malaria vectors Anopheles arabiensis and Anopheles gambiae in an urban context: Controlled, competitive host-preference experiments in Tanzania

“…gambiae complex and An. funestus [38]. This was achieved due to the scaling-up of larvicides [39] and LLINs [34,35].…”

“…This was achieved due to the scaling-up of larvicides [39] and LLINs [34,35]. The scaling-up of larvicides and LLINs coincided spontaneously with the wide use of window screening across the city of Dar es Salaam [38]. The average annual rainfall ranging from 800mm to 1300mm with a 25ºC annual temperature [40].…”

“…gambiae in Dar es Salaam may also contribute to the persistence of this species, even while it was virtually eliminated from other nearby ecological settings [11,16], following widespread scale-up of LLIN use [11][12][13]16]. The increasingly widespread use of LLINs [51] and high coverage of house window screening in urban Dar es salaam [38], which limit safe access of mosquitoes to human blood, may have forced this species to develop a strategy which enables them to evade personal target protective interventions for humans by exploiting animal blood whenever they can nd it.…”

Testing a pyriproxyfen auto-dissemination station attractive to gravid Anopheles gambiae sensu stricto for the development of novel attract-and-kill strategies for malaria vector control

“…Larval source management (LSM) targeting immature mosquito vectors in their aquatic habitats, such as larviciding and environmental management, can effectively serve as supplementary vector control measures (9,10). Studies in East Africa highlighted the benefit of combining long-lasting insecticidal nets (LLINs) and larviciding with microbial larvicides for reducing transmission (11)(12)(13)(14). However, the challenge associated specifically with larviciding is the need to reach all available and potentially suitable aquatic habitats in an intervention area (15)(16)(17)(18), many of which might only be reached by aerial application.…”