This Disease Burden Is Largely Preventable with Proven, Cost-Effective Interventions

Figure 2 shows the average reductions in diarrhoea incidence found to be associated with HSW interventions in several literature reviews. The impact of “real world” interventions varies widely in response to local factors such as which pathogens are contributing to disease and the relative contribution of different transmission routes.

Figure 2. Results of reviews of the effect on diarrhoea of HSW interventions.

Results of the previous reviews are for the better quality studies. The reduction for household drinking water connections is in addition to reductions for water quality and availability of public sources. Previous reviews: (a), (c)–(f) [8]; (b) [11]). Fewtrell et al. [57].

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A balanced interpretation of the available evidence suggests that a reasonably well-implemented intervention in one or more of hygiene, sanitation, water supply or water quality, where preexisting conditions are poor, is likely to reduce diarrhoeal disease prevalence by up to a third. Still greater reductions (up to 63%) are associated with water piped to one or more taps on a property [8]. Such a major impact merits far more attention from health professionals and health systems than has been common in recent decades.

We are still learning about the role of HSW in disease control. For example, Ascaris and other intestinal worms are known to be associated with poor sanitation, but a recent review [9] found evidence that hand washing with soap can also help to prevent transmission of ascariasis. We know that trachoma is prevented by facial hygiene and hand washing, but recent research has also highlighted the role of latrines in controlling the Musca sorbens flies that carry the Chlamydia pathogen between children's faces [10]. Even regarding the effect of hygiene on diarrhoea among young children in poor communities, we still have much to learn. There is good evidence to justify promotion of hand washing with soap [11], but for other aspects of hygiene behaviour, such as proper disposal of children's stools[12], the epidemiological evidence is from observational studies, which are subject to confounding.

The most effective means of promoting behaviour change is also a fruitful research field. It has only recently become clear to health professionals that emotional levers (“Clean hands feel good”) change people's health behaviours more effectively than cognitive statements (“Dirty hands cause disease”). Advertising agencies have known this for years. They also know the importance of investing in formative research, testing, and evaluation, to tailor the messages to local people's beliefs and aspirations [13]. If health workers can divest themselves of the unsubstantiated belief that health considerations motivate behaviour, they can become a more effective force for hygiene behaviour change.

There are alternative ways to tackle some of the HSW-associated disease burden. The widespread introduction of oral rehydration therapy (ORT) in the 1980s, for example, contributed much to reducing mortality from diarrhoeal disease [14]. However, such interventions focus on mortality rather than morbidity and on secondary rather than primary prevention. Moreover, ORT does not address the problems of persistent diarrhoea and dysentery.

It is sometimes claimed that the lack of an overall decline in diarrhoea morbidity rates despite increasing coverage with water and sanitation shows that the health benefits of HSW are illusory. However, there are other possible explanations for the apparent contradiction. First, coverage has not advanced as rapidly as one would wish, or as some official figures suggest. Second, the diarrhoea morbidity data are subject to a variety of interpretations; for example, reviews have found that apparent geographical variations could be explained by differences in study design [15]. Third, if challenge by diarrhoea pathogens can cause tropical enteropathy [16] without diarrhoea, a reduction in that challenge could reduce mortality risk without necessarily reducing diarrhoea morbidity.

In fact, the benefits to health of improving HSW are far greater than implied by disease-specific statistics. In the early 1900s, sanitary engineers in the US and Germany identified the “Mills-Reincke phenomenon.” Their studies showed that for every death from typhoid fever averted by water supply improvements, two to three deaths from other causes, including tuberculosis, pneumonia, and other causes of child mortality, were also avoided [17].

We now know that frequent bouts of diarrhoea and intestinal parasitosis are important causes of malnutrition, which renders children more susceptible to other diseases. For example, when malnourished children are recovering from an episode of diarrhoea, they are unusually susceptible to pneumonia; this diarrhoea-induced susceptibility may be associated with as much as 26% of all childhood pneumonia episodes [18]. Similarly, while 7% of the HSW-associated disease burden is directly associated with malnutrition, reductions in diarrhoea also reduce the incidence of diseases that are the consequence of malnutrition and that account for 29% of the disease burden (Figure 1).

The disease burden weighs heavily on both households and health systems. It has been estimated that the health costs alone amount to some US$340 million for households lacking water supply and sanitation and US$7 billion for national health systems [19]. The household burden weighs most heavily upon the poor, but well-conceived sanitation and water programmes can weaken the link between poverty and disease [20] (Figure 3) and so contribute to health equity.

Figure 3. Determinants of diarrhoea in Salvador, Brazil, 1997–2004: Results of a hierarchical effect decomposition analysis.

The width of each vertical bar shows the proportion of diarrhoea risk attributable to socioeconomic status and mediated by the intermediate variables shown. The two figures show conditions respectively (A) before and (B) after implementation of a major sanitation project. The project was associated with a 21% reduction in diarrhoea citywide, and 42% in the high incidence areas. Socioeconomic status accounted for 23% of the variance in diarrhoea rates before the project, but afterwards the strength of that link had been halved, to 11%. The proportion of that association mediated by intermediate variables, particularly sanitation, was also greatly diminished. Source: [20].

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The World Bank/WHO Disease Control Priorities Project judged most interventions in HSW in developing countries to be highly cost-effective health interventions (Table 2). Indeed, hygiene promotion was the most cost-effective of all major disease control interventions at US$5 per DALY averted, with sanitation promotion also in the top ten at just over US$10 per DALY [21]. Although these figures do not consider the construction costs of water and sanitation facilities (which would lower cost-effectiveness if included) or the indirect costs of malnutrition (which would increase cost-effectiveness if included), Table 2 clearly shows that the HSW interventions most appropriate for the health sector are among the most cost-effective interventions it can make. Furthermore, most investments in water and sanitation infrastructure are made from other sources and for reasons other than health.

Table 2. Cost-effectiveness of HSW compared with other public health interventions.

doi:10.1371/journal.pmed.1000367.t002