Diarrhoeal diseases kill an estimated 2.5 million people each year, the majority being children under 5 years (Kosek et al. 2003). An estimated 4 billion cases annually account for 5.7% of the global burden of disease and place diarrhoeal disease as the third highest cause of morbidity and sixth highest cause of mortality (Pruess et al. 2002). Among children under 5 years in developing countries, diarrhoeal disease accounts for 21% of all deaths (Parashar et al. 2003). By inhibiting normal consumption of foods and adsorption of nutrients, diarrhoeal diseases are also an important cause of malnutrition, leading to impaired physical growth and cognitive development (Guerrant et al. 1999), reduced resistance to infection (Baqui et al. 1993) and potentially long-term gastrointestinal disorders (Schneider et al. 1978).
Infectious agents associated with diarrhoeal disease are transmitted chiefly through the faecal-oral route (Byers et al. 2001). A wide variety of bacterial, viral and protozoan pathogens excreted in the faeces of humans and animals are known to cause diarrhoea. Many of these are potentially waterborne – transmitted through the ingestion of contaminated water (Leclerc et al. 2002). Accordingly, a number of interventions have been developed to treat water. These include (i) physical removal of pathogens (e.g. filtration, adsorption and sedimentation); (ii) chemical treatment (e.g. assisted sedimentation, chemical disinfection and ion exchange); or (iii) heat and ultra violet (UV) radiation. Because of the risk of recontamination (Clasen & Bastable 2003), interventions to improve water quality also include steps to maintain the microbiological quality of safe drinking water, such as piped distribution, residual disinfection and improved storage. These efforts are expected to receive additional priority as a result of the United Nation's commitment to reduce by one-half of the 1.5 billion people without sustainable access to improved water, one of the United Nation's Millennium Development Goals (United Nations 2000), and by the World Health Organization's steps to accelerate the health gains of safe water to the remaining population by improved treatment and storage of water at the household level (Sobsey 2002).
Health authorities generally accept that safe water plays an important role in preventing outbreaks of diarrhoeal disease (Hunter 1997). Accordingly, the most widely accepted standard for water quality allows no detectable level of harmful pathogens at the point of distribution (WHO 1993). However, in those settings in which diarrhoeal disease is endemic, much of the epidemiological evidence for increased health benefits following improvements in the quality of drinking water has been equivocal (Esrey & Habicht 1986; Lindskog et al. 1987; Cairncross 1989). As many of these same waterborne pathogens are also transmitted via ingestion of contaminated food and other beverages, by person-to-person contact, and by direct or indirect contact with infected faeces, improvements in water quality alone may not necessarily interrupt transmission (Briscoe 1984).
As a result of this variety of risk factors, interventions for the prevention of diarrhoeal disease not only include enhanced water quality but also steps to (i) improve the proper disposal of human faeces (sanitation), (ii) increase the quantity and improve access to water (water supply), and (iii) promote hand washing and other hygiene practices within domestic and community settings (hygiene). As in the case of studies of water quality, there is a wide range in the reported measure of effect on diarrhoea morbidity of each of these other environmental interventions (Esrey et al. 1985). Even more fundamentally, there are also questions about the methods and validity of studies designed to assess the health impact of such interventions (Briscoe et al. 1986; Imo State Evaluation Team 1989).
As part of a larger evaluation of interventions for the control of diarrhoeal disease (Feachem et al. 1983), Esrey et al. (1985) reviewed 67 studies to determine the health impact from improvements in water supplies and excreta disposal facilities (Esrey et al. 1985). The median reduction in diarrhoeal morbidity from improved water quality was 16% (range 0–90%). This compared with 22% for improvements in excreta disposal, 25% for improvements in water availability and 37% for combined improvements in water quality and availability. In 1991, the review was updated and expanded to cover 144 studies addressing a variety of specific pathogens associated with poor water and sanitation (Esrey et al. 1991). The median reduction in diarrhoeal disease from improvements in water quality from which calculations could be made was 17% (15% from studies the authors deemed rigorous), compared with 22% (36%) for sanitation, 27% (20%) for water quantity, 20% (30%) for combined water and sanitation, 33% (33%) for hygiene and only 16% (17%) for combined water quality and quantity.
These reviews led to league tables that established a simple and understandable priority to environmental health interventions for preventing diarrhoeal disease. Ubiquitously cited in both professional journals and practical guides, the reviews have led to the dominant paradigm respecting water supply and sanitation interventions: that to achieve broad health impact, greater attention should be given to safe excreta disposal and proper use of water for personal and domestic hygiene rather than to drinking-water quality. The corollary has become equally established: that interventions aimed solely at improving drinking water quality would have relatively little impact in reducing diarrhoeal disease.
There is, however, increasing evidence that has begun to call into question, the validity of the dominant paradigm. The first body of evidence, although admittedly indirect, should nevertheless give pause to those working on environmental interventions to reduce diarrhoeal disease. While substantial progress has been made over the last decade in reducing the mortality associated with diarrhoeal disease, morbidity remains essentially unchanged (Kosek et al. 2003). Although there has been substantial success from interventions to improve case management, such as oral rehydration therapy, the dominant paradigm and the priority it establishes with respect to water and sanitation initiatives has not led to corresponding success in reducing transmission of the pathogenic agents (Huttly et al. 1997).
The second body of evidence stems from a relatively new approach to enhancing water quality as part of a public health initiative: improved household water management. While the extent to which even safe water becomes faecally contaminated during collection, transport, storage and drawing in the home is well known (Wright et al. 2003), only recently have low-cost health interventions been promoted to improve and preserve water quality at the household level (Mintz et al. 2001). Based on a comprehensive review of these interventions, the WHO concluded that there was now ‘conclusive evidence that simple, acceptable, low-cost interventions at the household and community level are capable of dramatically improving the microbial quality of household stored water and reducing the attendant risks of diarrhoeal disease and death’ (Sobsey 2002). This has lead to the formation of the WHO-sponsored International Network for the Promotion of Safe Household Water Treatment and Storage, a global collaboration of UN and bilateral agencies, NGO's, research institutions and the private sector committed to improved household water management as a component in water, sanitation and hygiene programmes.
Interventions at the household level may, in fact, represent an exception to the dominant paradigm. None of the studies that Esrey et al. examined for their conclusions regarding the impact of water quality reflected interventions at the point of use. A brief analysis of 21 controlled field trials over the last 20 years dealing specifically with interventions designed to enhance the microbiological quality of drinking water at the household level showed a median reduction in endemic diarrhoeal disease of 42% compared with the control groups (Clasen 2003). The result was fairly consistent regardless of the nature of the intervention. Nine studies using free chlorine produced a median reduction of 46% (Kirchhoff et al. 1985; Deb et al. 1986; Mahfouz et al. 1995; Handzel 1998; Semenza et al. 1998; Quick et al. 1999, 2002; Sobsey et al. 2003); five studies examining filtration had a median reduction of 40% (Payment et al. 1991; Hellard et al. 2001; Clasen et al. 2003; Colwell et al. 2003); three studies employing flocculation or a combination of flocculation and disinfection showed a median reduction of 38% (Luby et al. 2003; Reller et al. 2003;Kahn et al. 1984), and four studies of heat or solar radiation produced a median reduction of 35% (Conroy et al. 1996, 1999; Conroy et al. 2001; Iijima et al. 2001). Only 2 of the 21 intervention studies showed no statistically significant reduction when compared with controls (Kirchhoff et al. 1985; Hellard et al. 2001). Nevertheless, it must be said that these studies have not yet been systematically scrutinized to arrive at a pooled measure of effect, to explore potential heterogeneity or to evaluate other possible explanations such as differences in ambient water quality or in the population's hygiene and sanitation practices.
Another possible reason for the difference between these reported results and the 15–17% median reduction predicted by Esrey et al. is the methodology employed. Esrey et al. based their conclusions chiefly on observational studies. In addition to the confounding and bias inherent in such studies, significant and widespread methodological problems with these studies have been pointed out (Blum & Feachem 1983; Esrey & Habicht 1986). Although these previous reviews were helpful in identifying the broad questions and suggested answers, they did not employ the more rigorous methodologies and statistical methods, including meta analysis, of a systematic review (Egger et al. 2001). In terms of coverage, for example, neither review involved a comprehensive search strategy (Clarke & Oxman 2003). Accordingly, the conclusions with respect to water quality are based on a limited number of studies, and omitted a number of studies that appear to have met the inclusion criteria. The reviews were also limited to studies in the English language. With respect to statistical methods, the simple use of the median fails to take into account the size of the study and the variance observed in the results, factors that are weighted in meta-analysis to arrive at a pooled measure of effect (Deeks et al. 2001). Moreover, they do not distinguish between the various case definitions (Moy et al. 1991) and measures of diarrhoea morbidity (Morris et al. 1996; Pickering et al. 1987). In addition, while Esrey attempted to incorporate quality criteria in the reviews, there was no independent assessment of study quality or, for that matter, whether identified studies met the inclusion criteria (Juni et al. 2001). Furthermore, these prior reviews did not explore publication bias and sensitivity.
Some of these areas are addressed by a more recent review (Gundry et al. 2003). Unfortunately, however, this review also relies heavily on observational studies, does not include a number of studies that would appear to meet the investigators’ inclusion criteria, and otherwise fails to follow many of the procedures for disciplined systematic reviews recommended by the Cochrane Collaboration and its Infectious Diseases Review Group. The authors, together with colleagues at the London School of Hygiene and Tropical Medicine, have filed a protocol with the Cochrane Collaboration to undertake such a review (T. Clasen et al. in preparation).
In the final analysis, it seems most likely that the dominant paradigm may not be so much wrong as incomplete. While Esrey's league tables comparing the relative impact of various types of environmental interventions are enticingly simple, they fail to explain the potentially more important reasons for the broad differences within each type. In the case of water quality improvements, for example, Esrey cited a median reduction in diarrhoea disease from 9 studies of 16%, with a range in effect from 0% to 90%. The real headline—and what should have captured the attention of subsequent researchers—was this range and its possible explanations. Studies have demonstrated significant differences in diarrhoea morbidity because of differences in case definitions, recall periods for reporting episodes, reported vs. clinically confirmed cases, age, seasonality, ambient level of contamination, and pathogenicity of the aetiological agents (Byers et al. 2001). Analysis of sub-groups, such as the specific type of intervention, the point at which it is applied (e.g. point of supply vs. point of use), and whether or not the intervention includes components in addition to improved water quality (e.g. sanitation, hygiene promotion, improved supply, safe storage) may also be important (Mintz et al. 2001).
The encouraging results from studies of improved household water management provide a sufficient impetus for reexamining the potential health impact of interventions to improve drinking water quality. They also provide an important opportunity to investigate more subtle but potentially important differences in the environmental health interventions and the manner in which their impact is assessed. Understanding the reasons for the heterogeneity in the observed effect – a primary objective in disciplined systematic reviews – and the differences in key sub-groups should lead to more accurate predictions of the true effect that can be expected under the vastly different contextual circumstances presented in a particular disease setting from the type of intervention employed (Petitti 2000). This type of analysis should ultimately help refine the dominant paradigm, and lead to more focused guidance on the potential health impact of water quality interventions.