Assessing relationship between human settlement patterns a malaria risk in a residual transmission setting in south-eastern Tanzania

Kaindoa, Emmanuel W
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Background: Spatial targeting of interventions is increasingly recognized as essential for malaria control, particularly in areas aiming for elimination. The associations between house characteristics and malaria transmission is known, but gaps remain on whether transmission is also influenced by factors such as distances between households or the degree to which houses are clustered. It is also important to identify new interventions that can be implemented by targeting critical points in the biology of major malaria vectors and common characteristics of human settlements. Such efforts will be essential to complement current major tools. Aims: This study had the following aims; (i) understand how households densities and their distances influence malaria transmission the study area , (ii) identify housing-related factors affecting residual transmission risks in the area (iii) assess opinions of residents on how house characteristics, settlement patterns and other environmental factors influencing malaria transmission, and (iv) identify effective complementary approaches that could be used to improve malaria control in these residual transmission area with high coverage of basic interventions such as long lasting treated nets. Methodology: A mixed methods approach was used, which included: (i) indoor and outdoor mosquito collections from fixed as well as randomly selected households over 12 months, (ii) characterization of physical and microclimatic conditions of the houses and surrounding environmental variables, (iii) assessing effects of spatial clustering of households on malaria transmission risks, and (iv) assessing community concerns regarding household and environmental factors influencing transmission. During these studies, swarms of Anopheles mosquitoes were discovered for the first time in this area. Therefore, additional quantitative and qualitative observations were made to characterize the swarms and assess how they could be targeted to complement malaria control efforts. Key findings: Despite the expansive use of LLINs in the area, factors associated with housing characteristics and settlement patterns as well as people’s perceptions contribute to persistent malaria transmission, and will need to be addressed for eventual elimination to be reached. The main findings of this study were as follows: (i) household occupancy influenced indoor mosquito density (ii) high house densities increased Anopheles biting risk but mosquito density declined as distances between houses increased beyond 50m; (iii) abundance of the vector Anopheles arabiensis peaked during high rainfall months (February-May), but An. funestus densities remained stable into the dry season (May-August); (iv) there were higher indoor densities of An. arabiensis and An. funestus in houses with mud walls compared to plastered or brick walls, open eaves compared to closed eaves and unscreened windows compared to screened windows; (v) most respondents were aware of associations between their house characteristics and malaria risks but carrying out improvements was constrained by financial costs and other household priorities; (vi) in all our surveys, An. funestus mosquitoes contributed approximately 85% of ongoing malaria transmission in the areas, even when occurring in far smaller densities than An. arabiensis; and (vii) with regards to complementary interventions, small scale studies demonstrated that mating swarms of both An. arabiensis and An. funestus could be readily identified and characterized (in this case by volunteer community members), and that targeting the swarms using aerosol spraying could possibly reduce overall biting risk in the communities. Conclusions: This study yielded evidence that malaria transmission risks are significantly lower when the distances between houses are high. Variation in malaria transmission risk within a village might be important for planning and implementing spatially targeted interventions. With limited resources, malaria control efforts must be prioritized in locations with significantly higher risk in order to obtain maximum benefit. Thus, the distances between houses should be considered when planning malaria vector control strategies. Furthermore, the overall clustering of indoor densities of major disease-transmitting mosquitoes is significantly associated with household occupancy and its spatial distribution within the villages. Further research is needed to assess the potential of using this information for predicting, identifying and targeting the most intense foci of mosquito-borne disease transmission based on household occupancy. Understanding the factors influencing malaria transmission at a small scale is crucial for planning malaria control interventions. Malaria transmission heterogeneity does not only exist on a large scale, but also at small scales, which may be influenced by small variations of environmental features, such as seasonality. These variations should be considered when improving malaria control interventions. While An. funestus showed variation in density during the year, numbers of mosquitoes remained high throughout the dry season, whereas An. arabiensis showed a definite peak in the rainy season with a subsequent drop during the dry season. Further studies are required to investigate the survival strategies of major malaria vectors during the dry season. Understanding this particular feature of An. funestus may lead to the design of new control strategies or improve existing interventions so as to reduce the malaria burden in such rural areas. This study also documented high mosquito densities in mud-walled houses compared to houses with brick or plastered walls. These findings further indicate the necessity of considering house improvement as a malaria control strategy. While the community members had fairly high awareness that improved houses were associated with reduced mosquito nuisance, many families in rural areas are living in poorly constructed houses. It is therefore necessary to consider inter-sectoral collaborations to integrate housing into health policies in Tanzania. Further studies are also needed to provide epidemiological evidence on how housing design affects malaria transmission. Furthermore, community-based house improvement programs should be promoted, including simple, scalable and affordable house improvements for the populations living in high malaria endemic areas in rural communities. Generally, this study has generated information which sets the basis for further studies on the relationship between the exact house location and malaria transmission risk in rural malaria endemic countries.
Thesis submitted to the Faculty of Science, University of the Witwatersrand, in fulfilment of the requirement for the degree of Doctor of Philosophy 2019 School of Public Health University of the Witwatersrand, South Africa
Kaindoa, Emmanuel Willson (2019) Assessing relationship between human settlement patterns and malaria risk in a residual transmission setting in South-Eastern Tanzania, University of the Witwatersrand, Johannesburg, <>