Effect of a sanitation intervention on soil-transmitted helminth prevalence and concentration in household soil: A cluster-randomized controlled trial and risk factor analysis.

Lauren Steinbaum,John Mboya,Sammy M Njenga,Clair Null,Amy J Pickering,Ryan Mahoney,María-Gloria Basáñez

doi:10.1371/journal.pntd.0007180

Abstract

Improved sanitation has been associated with a reduced prevalence of soil-transmitted helminth (STH) infection and has been hypothesized to prevent fecal contamination from spreading throughout the household environment. We evaluated the effect of providing households with a pit latrine with a plastic slab and drophole cover, child feces management tools, and associated behavioral messaging on reducing STH eggs in household soil. We collected soil samples from 2107 households (898 control and 1209 improved sanitation intervention households) that were enrolled in the WASH Benefits cluster randomized controlled trial in rural Kenya and performed a post-intervention analysis after two years of intervention exposure. Following a pre-specified analysis plan, we combined all households that received the sanitation intervention into one group for comparison to control households. The prevalence of STH eggs in soil was 18.9% in control households and 17.0% in intervention households. The unadjusted prevalence ratio of total STH eggs in the intervention groups compared to the control group was 0.94 (95% CI: 0.78–1.13). The geometric mean concentration was 0.05 eggs/g dry soil in control households and intervention households. Unadjusted and adjusted models gave similar results. We found use of a shared latrine, presence of a roof over the sampling area, and the number of dogs owned at baseline was associated with an increased prevalence of STH eggs in soil; the presence of a latrine that was at least 2 years old and a latrine with a covered drophole was associated with a reduction in the prevalence of STH eggs in soil. Soil moisture content was also associated with an increased prevalence of STH eggs in soil. Our results indicate that an intervention designed to increase access to improved latrines and child feces management tools may not be enough to impact environmental occurrence of STH in endemic areas where latrine coverage is already high.

Highlights

Soil plays a crucial role in the lifecycle of soil transmitted helminths (STH)
Soil is crucial for the lifecycle and transmission of soil-transmitted helminths (STH), a class of parasitic intestinal worms that infect over a billion people globally
The sanitation intervention did not reduce STH contamination in household soil, suggesting households with access to improved sanitation may still be exposed to STH in the household environment

Summary

Introduction

Eggs need to incubate in soil in a warm, moist environment to become infective. Two species of STH, Ascaris lumbricoides and Trichuris trichiura, are transmitted through the ingestion of infective eggs. Both hookworm species, Ancylostoma duodenale and Necator americanus, infect people through larvae penetrating the skin, but Ancylostoma duodenale is transmitted by ingesting infective larvae[2]. Soil-transmitted helminth (STH) eggs are excreted in the feces of an infected person and are present in the environment in endemic areas. STH eggs have been found in soil in rural households in Poland, southern Thailand, the Philippines, Tanzania, Kenya, and South Africa; urban households in Ethiopia and Jamaica; informal settlements in Brazil and Turkey; urban and rural towns in Nepal; and primary schools in northern Vietnam and South Africa[3,4,5,6,7,8,9,10,11,12,13,14]

Results

Discussion

Conclusion