Evaluating survey designs for targeting preventive chemotherapy against Schistosoma haematobium and Schistosoma mansoni across sub-Saharan Africa: a geostatistical analysis and modelling study

Abstract

Background Schistosomiasis control programmes primarily use school-based surveys to identify areas for mass drug administration of preventive chemotherapy. However, as the spatial distribution of schistosomiasis can be highly focal, transmission may not be detected by surveys implemented at districts or larger spatial units. Improved mapping strategies are required to accurately and cost-effectively target preventive chemotherapy to remaining foci across all possible spatial distributions of schistosomiasis. Methods Here, we use geostatistical models to quantify the spatial heterogeneity of Schistosoma haematobium and S. mansoni across sub-Saharan Africa using the most comprehensive dataset available on school-based surveys. Applying this information to parameterise simulations, we assess the accuracy and cost of targeting alternative implementation unit sizes across the range of plausible schistosomiasis distributions. We evaluate the consequences of decisions based on survey designs implemented at district and subdistrict levels sampling different numbers of schools. Cost data were obtained from field surveys conducted across multiple countries and years, with cost effectiveness evaluated as the cost per correctly identified school. Results Models identified marked differences in prevalence and spatial distributions between countries and species; however, results suggest implementing surveys at subdistrict level increase the accuracy of treatment classifications across most scenarios. While sampling intensively at the subdistrict level resulted in the highest classification accuracy, this sampling strategy resulted in the highest costs. Alternatively, sampling the same numbers of schools currently recommended at the district level but stratifying by subdistrict increased cost effectiveness. Conclusions This study provides a new tool to evaluate schistosomiasis survey designs across a range of transmission settings. Results highlight the importance of considering spatial structure when designing sampling strategies, illustrating that a substantial proportion of children may be undertreated even when an implementation unit is correctly classified. Control programmes need to weigh the increased accuracy of more detailed mapping strategies against the survey costs and treatment priorities.

GPS-based fine-scale mapping surveys for schistosomiasis assessment: a practical introduction and documentation of field implementation

“…In areas where schistosomiasis control progresses towards elimination, heterogeneity of transmission becomes more pronounced and fine-scale mapping of infection patterns is needed for effective micro-targeting of interventions at sub-district level [ 14 , 23 , 24 , 28 ]. Novel tablet-based tools containing offline high-resolution maps offer new opportunities for disease mapping and household identification, particularly in remote areas where the distribution of housing structures do not follow an order such as street names or house numbers and where internet connectivity is poor [ 26 ].…”

“…The sustained implementation of control interventions, including mass drug administration (MDA) as the cornerstone, often supplemented with snail control, educational measures, or improvements in the socio-economic standard and access to clean water and sanitation, has resulted in decreasing schistosomiasis prevalences in many countries [ 8 – 11 ]. In areas where prevalences decline, the focality and heterogeneity of schistosomiasis becomes more pronounced [ 8 , 12 – 14 ]. Typically, within a few years of annual or biannual MDA across districts and countries, many communities reach low schistosomiasis prevalence and infection intensity [ 10 , 15 , 16 ].…”

“…Typically, within a few years of annual or biannual MDA across districts and countries, many communities reach low schistosomiasis prevalence and infection intensity [ 10 , 15 , 16 ]. Some areas, however, remain as pockets of high transmission with persistent or reoccurring high prevalence levels despite intense interventions [ 8 , 12 – 14 , 17 – 20 ].…”

“…To achieve the WHO elimination goals, the pronounced spatial and temporal heterogeneity in schistosomiasis elimination settings will need to be considered in future intervention planning [ 8 , 14 , 20 – 23 ].…”

“…Fine-scale mapping of schistosomiasis is considered an essential requirement to move from morbidity control towards interruption of transmission in endemic areas [ 23 ]. Detailed fine-scale mapping of infection and disease patterns can help to identify low-risk and high-risk areas of transmission at sub-district level and to micro-target interventions in line with infection levels for optimal treatment and resource allocation [ 10 , 14 , 24 , 25 ]. Android tablet-based applications containing or being able to connect to high-resolution maps to geolocate individuals’ residences in remote resource poor areas have been suggested as attractive approaches for fine-scale disease mapping [ 26 , 27 ].…”