Risk mapping and socio-ecological drivers of soil-transmitted helminth infections in the Philippines: a spatial modelling study

BACKGROUND: The Philippines reports a high prevalence of soil-transmitted helminth (STH) infections despite the implementation of nationwide mass drug administration since 2006. The spatial variation of STH infections in the Philippines was last described using the 2005-2007 national STH and schistosomiasis survey. This study aimed to identify sociodemographic and environmental factors that drive STH transmission and predict high-risk areas in the Philippines.

METHODS: Epidemiological data on STH for students aged 5-16 years were obtained from the 2015 Philippines National Prevalence survey, while environmental data were extracted from satellite images and publicly available sources. Model-based geostatistics, implemented in a Bayesian framework, was used to identify sociodemographic and environmental correlates and predict high-risk areas for STH across the Philippines. The best-fitting model with the lowest deviance information criterion (DIC) was used to interpret the findings of the model and predict STH infection risk for the entire country. Risk maps were developed for each STH infection using the posterior means derived from the model.

FINDINGS: The prevalence of Ascaris lumbricoides (20.0%) and Trichuris trichiura (29.3%) was higher in the Visayas Island than in the Luzon and Mindanao Islands. Hookworm prevalence was highest in Mindanao Island (1.3%). Risk of A. lumbricoides was positively associated with males (odds ratio [OR]: 1.197; 97.5% Credible Interval [CrI]: 1.114, 1.286) and temperature (OR: 1.148; 97.5% CrI: 1.033, 1.291), while normalized difference vegetation index (OR: 0.354; 97.5% CrI: 0.138, 0.930) and soil pH (OR: 0.606; 97.5% CrI: 0.338, 0.949) were negatively associated with the transmission. T. trichiura risk was positively associated with males (OR: 1.261; 97.5% CrI: 1.173, 1.341), temperature (OR: 1.153; 97.5% CrI: 1.001, 1.301), and rainfall (OR: 1.004; 97.5% CrI: 1.011, 1.069). Hookworm risk was positively associated with males (OR: 2.142; 97.5% CrI: 1.537, 2.998), while children aged ≤12 years (OR: 0.435; 97.5% CrI: 0.252, 0.753) had a negative association with risk compared to those over 12 years. Focal areas of high risk were identified for A. lumbricoides and T. trichiura in the Visayas Island, and hookworm in the Mindanao Island.

INTERPRETATION: The spatial distribution of all three STH infections has considerably decreased since a previous national risk-mapping exercise. The high-risk areas identified in the study can be used to strategically target deworming and health education activities to further reduce the burden of STH and support progress toward elimination.

FUNDING: The Australian Centre for the Control and Elimination of Neglected Tropical Diseases and the Australian National Health and Medical Research Council.