Strongyloidiasis as a Focal Household Cluster of Soil-Transmitted Helminthiasis in Rural Tropical Rainforest Southern Thailand: An Observational Study.

Abstract

Background: In southern Thailand, characterized by high rainfall for 9 months each year (2340 mm annually), hookworm prevalence is consistently higher than that of Strongyloides stercoralis. Such wet climatic conditions may favor hookworm transmission while limiting S. stercoralis. Conversely, living in crowded, cluster settlements may enhance the transmission of S. stercoralis. We aimed to determine whether S. stercoralis exhibits a focal distribution pattern-often related to close contact within households or small communities-and whether this pattern distinctively differs from that of hookworm infection.

Methods: We conducted a comparative observational study involving 346 of 418 village residents across eight designated subareas. Subareas 2, 4, 6, and 7 featured cluster settlements; Subareas 3 and 5 were diffuse low-income settings; and Subareas 1 and 8 represented mixed-density low-income areas. Each subarea contained at least one household lacking latrines. Stool samples were collected and analyzed using agar plate culture and the modified formalin-ether concentration technique to detect helminths. The spatial distributions of both infections were subsequently mapped and analyzed using spatial dispersion indices.

Results: Of the eight subareas, six demonstrated a higher hookworm-to-S. stercoralis prevalence ratio, ranging from 1.59:1 to 10:1. Conversely, Subareas 2 and 8 exhibited near-equivalent ratios, ranging from 1.38:1 to 1.75:1 (exact binomial p = 0.453, OR = 2.50 [discordant pairs = 7] and exact binomial p = 0.109, OR = 4.00 [discordant pairs = 10], respectively). Subareas 3 and 5 showed the strongest dominance of hookworm over S. stercoralis, at 10:1 and 7.7:1, respectively. Geographical and inequality indices revealed that S. stercoralis clustered focally across all subareas except 3 and 5, whereas hookworm infections were more broadly and uniformly distributed.

Conclusion: In southern Thailand, strongyloidiasis appears to cluster within households, supporting the hypothesis of close-contact transmission, whereas hookworm infection is more widely distributed. These spatial differences suggest distinct, tailored targeting strategies are necessary for hookworm control and S. stercoralis interventions.