8.2.5 Water and disease
Climate-change-related alterations in rainfall, surface water availability and water quality could affect the burden of water-related diseases (see Chapter 3). Water-related diseases can be classified by route of transmission, thus distinguishing between water-borne (ingested) and water-washed diseases (caused by lack of hygiene). There are four main considerations to take into account when evaluating the relationship between health outcomes and exposure to changes in rainfall, water availability and quality:
- linkages between water availability, household access to improved water, and the health burden due to diarrhoeal diseases;
- the role of extreme rainfall (intense rainfall or drought) in facilitating water-borne outbreaks of diseases through piped water supplies or surface water;
- effects of temperature and runoff on microbiological and chemical contamination of coastal, recreational and surface waters;
- direct effects of temperature on the incidence of diarrhoeal disease.
Access to safe water remains an extremely important global health issue. More than 2 billion people live in the dry regions of the world and suffer disproportionately from malnutrition, infant mortality and diseases related to contaminated or insufficient water (WHO, 2005). A small and unquantified proportion of this burden can be attributed to climate variability or climate extremes. The effect of water scarcity on food availability and malnutrition is discussed in Section 8.2.3, and the effect of rainfall on outbreaks of mosquito-borne and rodent-borne disease is discussed in Section 8.2.8.
Childhood mortality due to diarrhoea in low-income countries, especially in sub-Saharan Africa, remains high despite improvements in care and the use of oral rehydration therapy (Kosek et al., 2003). Children may survive the acute illness but may later die due to persistent diarrhoea or malnutrition. Children in poor rural and urban slum areas are at high risk of diarrhoeal disease mortality and morbidity. Several studies have shown that transmission of enteric pathogens is higher during the rainy season (Nchito et al., 1998; Kang et al., 2001). Drainage and storm water management is important in low-income urban communities, as blocked drains are one of the causes of increased disease transmission (Parkinson and Butler, 2005).
Climate extremes cause both physical and managerial stresses on water supply systems (see Chapters 3 and 7), although well-managed public water supply systems should be able to cope with climate extremes (Nicholls, 2003; Wilby et al., 2005). Reductions in rainfall lead to low river flows, reducing effluent dilution and leading to increased pathogen loading. This could represent an increased challenge to water-treatment plants. During the dry summer of 2003, low flows of rivers in the Netherlands resulted in apparent changes in water quality (Senhorst and Zwolsman, 2005).
Extreme rainfall and runoff events may increase the total microbial load in watercourses and drinking-water reservoirs (Kistemann et al., 2002), although the linkage to cases of human disease is less certain (Schwartz and Levin, 1999; Aramini et al., 2000; Schwartz et al., 2000; Lim et al., 2002). A study in the USA found an association between extreme rainfall events and monthly reports of outbreaks of water-borne disease (Curriero et al., 2001). The seasonal contamination of surface water in early spring in North America and Europe may explain some of the seasonality in sporadic cases of water-borne diseases such as cryptosporidiosis and campylobacteriosis (Clark et al., 2003; Lake et al., 2005). The marked seasonality of cholera outbreaks in the Amazon is associated with low river flow in the dry season (Gerolomo and Penna, 1999), probably due to pathogen concentrations in pools.
Higher temperature was found to be strongly associated with increased episodes of diarrhoeal disease in adults and children in Peru (Checkley et al., 2000; Speelmon et al., 2000; Checkley et al., 2004; Lama et al., 2004). Associations between monthly temperature and diarrhoeal episodes have also been reported in the Pacific islands, Australia and Israel (Singh et al., 2001; McMichael et al., 2003b; Vasilev, 2003).
Although there is evidence that the bimodal seasonal pattern of cholera in Bangladesh is correlated with sea-surface temperatures in the Bay of Bengal and with seasonal plankton abundance (a possible environmental reservoir of the cholera pathogen, Vibrio cholerae) (Colwell, 1996; Bouma and Pascual, 2001), winter peaks in disease further inland are not associated with sea-surface temperatures (Bouma and Pascual, 2001). In many countries cholera transmission is primarily associated with poor sanitation. The effect of sea-surface temperatures in cholera transmission has been most studied in the Bay of Bengal (Pascual et al., 2000; Lipp et al., 2002; Rodo et al., 2002; Koelle et al., 2005). In sub-Saharan Africa, cholera outbreaks are often associated with flood events and faecal contamination of the water supplies.