Spatial Modelling of Soil-Transmitted Helminth Infections in Kenya: A Disease Control Planning Tool

Background: Implementation of control of parasitic diseases requires accurate, contemporary maps that provide intervention recommendations at policy-relevant spatial scales. To guide control of soil transmitted helminths (STHs), maps are required of the combined prevalence of infection, indicating where this prevalence exceeds an intervention threshold of 20%. Here we present a new approach for mapping the observed prevalence of STHs, using the example of Kenya in 2009. Methods and Findings: Observed prevalence data for hookworm, Ascaris lumbricoides and Trichuris trichiura were assembled for 106,370 individuals from 945 cross-sectional surveys undertaken between 1974 and 2009. Ecological and climatic covariates were extracted from high-resolution satellite data and matched to survey locations. Bayesian space-time geostatistical models were developed for each species, and were used to interpolate the probability that infection prevalence exceeded the 20% threshold across the country for both 1989 and 2009. Maps for each species were integrated to estimate combined STH prevalence using the law of total probability and incorporating a correction factor to adjust for associations between species. Population census data were combined with risk models and projected to estimate the population at risk and requiring treatment in 2009. In most areas for 2009, there was high certainty that endemicity was below the 20% threshold, with areas of endemicity ≥20% located around the shores of Lake Victoria and on the coast. Comparison of the predicted distributions for 1989 and 2009 show how observed STH prevalence has gradually decreased over time. The model estimated that a total of 2.8 million school-age children live in districts which warrant mass treatment. Conclusions: Bayesian space-time geostatistical models can be used to reliably estimate the combined observed prevalence of STH and suggest that a quarter of Kenya's school-aged children live in areas of high prevalence and warrant mass treatment. As control is successful in reducing infection levels, updated models can be used to refine decision making in helminth control. Author Summary: Effective targeting of mass drug administration for the treatment of soil-transmitted helminths (STH) requires reliable, up-to-date maps that indicate where prevalence exceeds the 20% intervention threshold recommended by the World Health Organization. We present a new approach for mapping the prevalence of STH in Kenya, incorporating observed prevalence data from 945 cross-sectional surveys undertaken between 1974 and 2009. The distribution of each species was modelled using model-based geostatistics; models included information on environmental factors, the spatial distribution of existing surveys and when these surveys were conducted. Resulting risk maps were combined and linked with population data enabling estimation of the population at risk of any STH infection and requiring treatment in 2009. In most areas, there was high certainty that combined STH prevalence was below the 20% intervention threshold, with areas of high prevalence located around the shores of Lake Victoria and on the coast. Results also suggest that observed prevalence decreased over time and emphasise the importance of continued surveillance in areas where observed prevalence was historically high. We show how spatial modelling can be used to develop up-to-date maps of STH risk to help improve the precision of decision making in disease control.