CRD summary

The authors concluded that sewerage interventions seemed to reduce the incidence of diarrhoea and related outcomes. Although the applicability of the results to other countries was unclear, the quality of included studies was limited and there were differences between studies, this was a well-conducted review. The authors' cautious conclusions reflect the evidence presented and are likely to be reliable.

Authors' objectives

To investigate the effects of the presence of sewerage systems on diarrhoeal disease and related outcomes.

Searching

MEDLINE (from 1966 to February 2010), EMBASE (1980-2009), the Latin American and Caribbean Health Sciences Literature (LILACS), and the Conference Proceedings Citation Index databases were searched; search terms were reported. References of articles and reviews were searched manually. Unpublished studies were not included.

Study selection

Eligible studies were case-control or cohort studies, non-randomised trials, or cross-sectional studies that evaluated the effects of any piped sewerage intervention compared with the pre-sewerage sanitation situation on diarrhoea or specified enteric infections (including helminth infection) in children or adults in any region of the world.

Most studies included in the review compared sewerage with another type of sanitation, such as flush toilets discharging to septic tanks or open drains. The age of the participants varied, but most of the studies were conducted in children up to 14 years of age. The primary outcomes reported were diarrhoea incidence and Giardia species infection incidence (in stool samples). One study evaluated Cryptosporidium infection using antibody incidence in the blood serum (seroprevalence). Outcomes were assessed/reported in the past day, or in the past two or four weeks. The comparators were poorly described in the included studies; they were mostly described as 'no sewerage'.

Most included studies were conducted in Brazil; the other studies were conducted in middle income countries such as Mexico, Nicaragua, Honduras, Peru, the USA, Iran, Syria, Saudi Arabia, and Australia.

Two reviewers independently selected the studies for inclusion; no disagreements arose.

Assessment of study quality

Study quality was assessed using a modified five-point binary scale (based on a validated published scale) that was specifically developed for this systematic review. Criteria assessed included exposure (how clearly sanitation levels were defined), outcome (rating whether the measurement was acceptable), comparison group (rating whether the selection of a control group was acceptable), participation (rating whether participation or follow-up was acceptable), and confounding (rating whether adjustment, matching, and restriction were acceptable).

Two reviewers independently assessed quality.

Data extraction

Odds ratio (OR), relative risk (RR) or longitudinal prevalence ratio (LPR) and 95% confidence intervals (CIs) were extracted or calculated for each study. When further clarification of data was necessary (particularly of sanitation levels), the reviewers attempted to contact study authors. When a single study made two or more comparisons, a weighted relative risk value was calculated and used in the meta-analysis. Adjusted relative risk was used if possible; if no adjusted estimate was available, then a crude estimate was used.

It appeared that two reviewers independently extracted the studies.

Methods of synthesis

The studies were pooled using both fixed-effect and random-effects methods. Where studies included two or more comparisons, a weighted average risk ratio value was calculated.

Heterogeneity was assessed using a parametric bootstrap version of the DerSimonian and Laird Q test. Sources of heterogeneity were also explored by visual examination of the forest plots.

Subgroup analysis was conducted by study design, outcome variable, subject age, study location, and type of sanitation comparison. Sensitivity analysis was conducted by study quality.

Publication bias was assessed using the Egger test.

Results of the review

Twenty-five studies were included in the review (with at least 15,443 participants). There were six cohort studies, four case-control studies, one non-randomised trial, and fourteen cross-sectional studies. The quality scores ranged from 1.0 to 5.0. Follow-up ranged from three to 35 months (where reported).

Sewerage systems typically reduced diarrhoea incidence by about 30% (RR 0.70, 95% CI 0.61 to 0.79; 25 studies). However, there was significant heterogeneity between the studies (between study variance was 0.87).

Most subgroup analysis did not alter the statistical heterogeneity between the studies. Subgroup analysis suggested a greater benefit where sanitation levels were very poor.

There was no evidence of publication bias.

Authors' conclusions

Sewerage interventions seemed to reduce the incidence of diarrhoea and related outcomes. However, the interpretation of these findings was cautious as sewerage might be less cost-effective and sustainable than onsite alternatives.

CRD commentary

The review question and inclusion criteria were clear. Attempts to identify all the relevant studies were undertaken. Although the search was restricted to published studies, the authors found no evidence of publication bias using an Egger test. Steps were taken throughout the systematic review process to minimise reviewer bias and error.

Study quality was assessed and the results were considered in the analysis of the data. Details of the individual studies were provided. Appropriate methods were used to pool the results, although due to statistical and study heterogeneity, and the apparent lack of high quality studies, the overall summary score should be treated with some degree of caution.

Although the generalisability of the results to other countries was unclear, this was a well-conducted review. The authors' conclusions reflect the evidence presented and are likely to be reliable.

Implications of the review for practice and research

Practice: The authors did not state explicit implications for practice. They did state that sewerage interventions were appropriate if cost-effective, sustainable for system maintenance and environmental impact, and of genuine benefit to poor communities.

Research: The authors stated that studies should be more clearly reported (they did not make recommendations for research design). The authors also stated that ongoing comparative evaluations were needed that assessed health effects and other outcomes, including costs of different types of sanitation solutions.

Funding

None.

Bibliographic details

Norman G, Pedley S, Takkouche B. Effects of sewerage on diarrhoea and enteric infections: a systematic review and meta-analysis. Lancet Infectious Diseases 2010; 10(8): 536-544. [PubMed: 20620115]

Original Paper URL

http://www.thelancet.com/journals/laninf/article/PIIS1473-3099(10)70123-7/abstract

Indexing Status

Subject indexing assigned by NLM

MeSH

Diarrhea /prevention & control; Humans; Intestinal Diseases, Parasitic /prevention & control; Sanitation; Sewage

AccessionNumber

12010006479

Database entry date

29/06/2012

Record Status

This is a critical abstract of a systematic review that meets the criteria for inclusion on DARE. Each critical abstract contains a brief summary of the review methods, results and conclusions followed by a detailed critical assessment on the reliability of the review and the conclusions drawn.