Water, sanitation, hygiene and soil-transmitted helminth infection: a systematic review and meta-analysis

Methodological quality of the review: High confidence

Author: Strunz EC, Addiss DG, Stocks ME, Ogden S, Utzinger J, Freeman MC

Geographical coverage: Asia, Africa and the Americas

Sector: WASH

Sub-sector: Soil-transmitted helminth infection

Equity focus: None specified

Review type: Effectiveness review

Quantitative synthesis method: Narrative synthesis

Qualitative synthesis method: Not applicable

Background:

Preventive chemotherapy represents a powerful but short-term control strategy for soil-transmitted helminthiasis. Since humans are often re-infected rapidly, long-term solutions require improvements in water, sanitation and hygiene (WASH).

Objectives:

The purpose of this study was to quantitatively summarise the relationship between WASH access or practices and soil-transmitted helminth (STH) infection.

Main findings:

A total of 94 studies were included in the review. Studies researched populations in Asia (42), Africa (29) and the Americas (23). Studies investigated access and practices relating to water (56), sanitation (79) and hygiene (53). The most commonly explored were access to sanitation (63), access to water (45), handwashing (30) and wearing shoes (27).

Studies reported investigating infection with A. lumbricoides (n = 69), T. trichiura (n = 60), hookworm (n = 63), S. stercoralis (n =12), and any STH collectively (n = 52). Of 94 studies, 89 were observational: 75 used a cross-sectional epidemiologic design, 13 were prospective, and the remaining was a case-control study. Authors noted that the overall quality of studies were low due to the preponderance of observational studies.

Findings from meta-analyses suggest that the use of treated water was associated with lower odds of STH infection (odds ratio [OR] 0.46, 95% CI 0.36–0.60). Results showed that piped water access was associated with lower odds of A. lumbricoides (OR 0.40, 95% CI 0.39–0.41) and T. trichiura infection (OR 0.57, 95% CI 0.45–0.72), but not any STH infection (OR 0.93, 95% CI 0.28–3.11).

Access to sanitation was associated with the decreased likelihood of infection with any STH (OR 0.66, 95% CI 0.57–0.76), T. trichiura (OR 0.61, 95% CI 0.50–0.74), and A. lumbricoides (OR 0.62, 95% CI 0.44–0.88), but not with hookworm infection (OR 0.80, 95% CI 0.61–1.06). Wearing shoes was associated with reduced odds of hookworm infection (OR 0.29, 95% CI 0.18–0.47) and infection with any STH (OR 0.30, 95% CI 0.11–0.83).

Authors noted that handwashing, both before eating (OR 0.38, 95% CI 0.26–0.55) and after defecating (OR 0.45, 95% CI 0.35–0.58), was associated with lower odds of A. lumbricoides infection. Soap use or availability was significantly associated with lower infection with any STH (OR 0.53, 95% CI 0.29–0.98), as was handwashing after defecation (OR 0.47, 95% CI 0.24–0.90).

Authors stated that observational evidence constituted the majority of included literature, which limits any attempt to make causal inferences. In addition, reviewers acknowledged that due to underlying heterogeneity across observational studies, the meta-analysis results reflect an average of many potentially distinct effects, not an average of one specific exposure-outcome relationship.

Overall, based on findings, authors concluded that except for two meta-analyses, the pooled data indicated at least 33% reduction in odds of infection associated with individual WASH practices or access.

Methodology:

An article was eligible for inclusion if it presented a measure of effect between WASH and STH. Authors did not exclude studies based on methodology, population characteristics or the language in which the report was written. Studies that evaluated multiple WASH components were included as long as the components could be assessed separately.

Authors searched PUBMED, EMBASE, Web of Science, and LILACS for relevant articles from inception to October 2013. In addition, authors also indexed relevant studies from the bibliography of reviews, and abstracts without published articles were considered eligible for inclusion. Authors also contacted researchers for relevant unpublished studies. Screenings of studies were conducted by two investigators independently. Relevant data was extracted by reviewers and independently by assistants. Extracted data included study design, setting years, populations, characteristics, EAHS components measured, diagnostic approach, STH species and relevant effect measures.

Odds ratio served as the primary effect measure in the reviewed literature. Authors collected both crude and adjusted estimates if available. To assess potential sources of bias within included studies and determine overall strength of evidence for each meta-analysis, authors used the GRADE framework.

Authors conducted meta-analyses for groups of effect estimates that related similar WASH access or practices to a common outcome. A random-effects model was employed to account for the expected heterogeneity between studies. When necessary, authors noted inverting estimates to reflect the effect of WASH, rather than the absence of WASH. Estimates of effect not included in meta-analyses were summarised in the text. Inconsistency was assessed with I² and Cochrane’s Q-test. Publication bias was assessed by analysing funnel plots, though Egger’s test also informed the authors’ interpretation.

Applicability/external validity:

Authors noted in the review that by limiting their focus to adjusted measures of effect, this reduced the number of eligible studies, which may impact on the generalizability of the results.

Geographic focus:

Authors included studies from low- and middle-income countries. However, authors stated that results may be applicable depending on the context of each country as interventions are tailored to specific areas.

Summary of quality assessment:

Overall, there is high confidence in the conclusions about the effects of this study. Authors used appropriate methods to screen, extract data and critically appraise included studies and pool data from individual studies. Limitations were appropriately acknowledged and no strong policy conclusions were drawn from the review.