Epidemiology of soil-transmitted helminth infections in Timor-Leste

Abstract

Soil-transmitted helminths (STH) are significant human parasites, causing long-term morbidity. They are prevalent in impoverished regions lacking adequate water, sanitation and hygiene (WASH).

This PhD has been undertaken during a period of debate around the health benefits of deworming, and global prioritisation of neglected tropical disease (NTD) control and elimination activities. Therefore, this thesis commences by presenting two comprehensive reviews. In one, recent evidence of STH morbidity was analysed, and systematic reviews appraised, to highlight evidence shortfalls for direct morbidity measures indicating possible benefits from chemotherapy. In the second, evidence for chemotherapy, WASH, and current NTD integration were analysed, and the need for “multi-component” integration highlighted, being more holistic integration to achieve more sustainable STH control.

Quantitative epidemiological analyses presented in this thesis used baseline data from a randomised controlled trial (RCT) to analyse STH epidemiology in Manufahi District, Timor-Leste; an impoverished, post-conflict country. This thesis explores: what is the prevalence of STH, and what WASH risk factors contribute towards infections? What are the associations between STH infections and plausible STH-related health outcomes, and finally, what are the associations between aspects of WASH and the village environment with intensity of STH infection, in these communities?

WASH risk factors for STH infections were analysed, stratified by age, using principal component analysis and mixed-effects logistic regression. The main findings were a high prevalence of parasitic infections, however few WASH risk factors significantly associated with STH infection. The impact of STH intensity on community anaemia, and also stunting, wasting and being underweight in children aged one to 18 years, was investigated. An algorithm correlating DNA intensity to eggs per gram of faeces equivalents was used to assign cut-points for PCR-derived Ascaris spp. and Necator americanus infection intensity. STH were found not to be strong predictors of anaemia, stunting or wasting in the study communities. Finally, given exposure-related risks, and associations between heavy-intensity infection and morbidity, the hypothesis that WASH and environmental risk factors may vary according to infection intensity was tested. Environmental variables, but again few WASH variables, were associated with intensity of STH infection in this analysis. Despite this, WASH is the only identified mechanism that could reduce or prevent transmission in this high-transmission environment. It should be included in integrated control strategies.

All analyses are the first reported examples for Timor-Leste. The thesis findings provide an informed position for establishing national STH control strategies, and a useful baseline for monitoring and evaluating control programmes once implemented. Analyses additionally provide essential baseline information for the RCT in which this research is embedded. Research findings also contribute to international knowledge: few analyses have investigated WASH risk factors stratified by age and STH species separately. These analyses additionally provide the first epidemiological investigation of STH infection intensity from PCR-diagnosed infection. This requires verification in different epidemiological settings. Finally, the thesis provides the first investigation of adjusted environmental and WASH risk factors in any community setting. Show more