Mapping the Risk of Soil-Transmitted Helminthic Infections in the Philippines

Abstract

Background: In order to increase the efficient allocation of soil-transmitted helminth (STH) disease control resources in the Philippines, we aimed to describe for the first time the spatial variation in the prevalence of A. lumbricoides, T. trichiura and hookworm across the country, quantify the association between the physical environment and spatial variation of STH infection and develop predictive risk maps for each infection.

Methodology/principal findings: Data on STH infection from 35,573 individuals across the country were geolocated at the barangay level and included in the analysis. The analysis was stratified geographically in two major regions: 1) Luzon and the Visayas and 2) Mindanao. Bayesian geostatistical models of STH prevalence were developed, including age and sex of individuals and environmental variables (rainfall, land surface temperature and distance to inland water bodies) as predictors, and diagnostic uncertainty was incorporated. The role of environmental variables was different between regions of the Philippines. This analysis revealed that while A. lumbricoides and T. trichiura infections were widespread and highly endemic, hookworm infections were more circumscribed to smaller foci in the Visayas and Mindanao.

Conclusions/significance: This analysis revealed significant spatial variation in STH infection prevalence within provinces of the Philippines. This suggests that a spatially targeted approach to STH interventions, including mass drug administration, is warranted. When financially possible, additional STH surveys should be prioritized to high-risk areas identified by our study in Luzon.

Fig 1. Geographical distribution of survey locations and observed prevalence for Ascaris lumbricoides (A), Trichuris trichiura (B) and hookworm (C).

Note: Luzon–left, Visayas–top right and Mindanao–bottom right.

Fig 2. Residual semivariograms for Ascaris lumbricoides, Trichuris trichiura and hookworm in Luzon, the Visayas and Mindanao.

Note: distance is recorded in decimal degrees (1 decimal degree is 111km at the equator).

Fig 3. Predicted spatial distribution of Ascaris lumbricoides (A), Trichuris trichiura (B) and hookworm (C) for boys aged ≥20 years for the Philippines.