Water-related Disease and Climate

Human-induced climate change is altering precipitation patterns in most parts of the world (Stocker et al., 2013). In the future, climate change will likely exacerbate droughts (Trenberth et al., 2014; Dai, 2012) and drastically increase the likelihood of floods throughout many parts of South America, Africa and southern Asia (Hirabayashi  et al., 2013). These changes are likely to negatively affect the roughly 800 million people around the world who lack basic improved water supplies and the 2.5 billion who lack a basic toilet, further increasing the global burden of water-related diseases. This perspective reviews the connections between climate and diarrheal diseases with the aim of understanding how much climate change might exacerbate the deleterious affects of poor quality water which is already responsible for approximately 1.6 million deaths each year (WHO, 2006).

The relationship between climate and diarrheal diseases is complex because of the large number of confounding variables (Mellor et al., 2012a, 2014) and transmission routes (Mellor et al., 2013) that can affect disease rates and the fact that diarrhea is caused by a number of different pathogens (Walker et al., 2010; Guerrant et al., 1990). Despite this complexity, a number of researchers have carried out correlational studies to see how diarrhea rates change with precipitation, temperature and flooding.

Researchers in Dhaka, Bangladesh used hospital records to show that diarrhea incidences increased by 5.1% and 3.9% for every 10 mm rainfall increase and decrease respectively around a threshold of 52 mm in the proceeding 0 - 8 weeks. They also found that a 1oC rise in temperature increases rates by 5.6%. The effect of temperature was higher for people with lower levels of education, poorly constructed houses and unsanitary toilets. However, the effects of rainfall were not associated with these modifiers (Hashizume et al., 2007). This basic tenet was confirmed by researchers who studied hospital reports in the Pacific Islands and found a positive association with diarrhea reports and temperature as well as high and low rainfall events (Singh et al., 2001). Likewise researchers found that diarrhea increased by 8% per degree Celsius rise in ambient temperature during an El Nin˜o event in Lima, Peru (Checkley et al., 2000). A study in China compared diarrhea rates for temperate and subtropical climates and found that a 1oC increase in maximum or minimum temperature was associated with a 12% increase in bacillary dysentery in the temperate city and a 16% increase in the subtropical city. Interestingly, they found that the northern city had a threshold for temperature effects while the southern city had no such threshold (Zhang et al., 2007). Findings in Dhaka indicate that floods can double the cases of diarrhea and that lower socio-economic groups suffer disproportionately (Hashizume et al., 2008a). However, a more recent study in Ecuador found that heavy rainfall events following wet periods resulted in fewer reported diarrhea cases while similar events following dry periods increased diarrhea reports. The presence of improved sanitation and hygiene infrastructure did not modify these findings (Carlton et al., 2013). A final study conducted in Botswana found a distinct cyclical pattern with peaks in the wet and dry seasons with the dry season peak being 20% higher than the yearly mean (Alexander et al., 2013).

A number of other researchers have investigated the risk from specific pathogens including rotavirus and norovirus. One such study found that the risk of getting diarrhea caused by rotavirus peaked with both low and high temperatures (Hashizume et al., 2008b) and that river level was positively associated with such diarrhea. However, a meta-analysis that looked solely at rotavirus found that temperature and precipitation increases reduced rotavirus incidence such that it thrives in cooler and drier seasons (Levy et al., 2009b). Research conducted in England and Wales found similar results for norovirus, which is one of the most commonly, reported cause of gastroenteritis in industrialized countries (Lopman et al., 2009). A more global meta-analysis found that norovirus is wintertime disease, however, they found that average rainfall in the wettest month was positively associated with norovirus incidence. Unexpectedly, the same authors did not find a similar seasonal correlation in the southern hemisphere (Ahmed et al., 2013).

There have been only a couple of studies that used computer models or statistical analyses informed by empirical studies to predict the global effects of climate change on diarrhea diseases. One such study used climate model ensembles with results of field studies to predict the global increase of the relative risk associated with diarrhea. They found that the risk of diarrhea will increase by 8-11% by 2010-2039 and 22-29% by 2070-2099. Despite these results, the authors caution that the lack of empirical climate-health data leads to large uncertainties in these future projections. The authors also note that their model predicts substantial diarrhea increase for even moderate climate change scenarios (Kolstad & Johansson, 2011). A second global analysis using a conservative approach by taking only temperature effects into account, concluded that global warming was already causing an additional 1.5 million cases of diarrhea worldwide in 2000 which led to an additional 47,000 deaths annually (WHO, 2002).

Many diarrhea-causing pathogens are sensitive to ambient temperature which  might explain why non-specific diarrhea peaks during warm weather in many areas. For instance, a study of salmonella in a rural watershed of Georgia indicated that concentrations were higher during summer months and were positively associated with water temperature and average daily rainfall (D’Souza et al., 2004). A similar finding in Australia found that foodborne salmonella notifications were positively correlated with temperature (Haley et al., 2009). Another study of campylobacter in developed countries found a distinct seasonality with peak infection rates in the spring, but only a weak correlation with temperature and no effects of rainfall (Kovats et al., 2005).

Most of the aforementioned research supports the reasoning that pathogens can be concentrated during dry periods and flushed into drinking water sources during heavy rainfall events (Levy et al., 2009a). This makes it difficult to directly correlate mean rainfall amounts with diarrhea incidences since heavy rainfall events following particularly wet periods might not lead to elevated diarrhea rates. A further open question is exactly how increased rainfall correlates to the availability of water for resource-limited communities. Although increased rainfall does not always lead to increased water availability (Schweitzer et al., 2013), the availability of water resources is positively correlated with lower diarrheal disease rates (Fry

et al., 2010). Furthermore, longer distance to a water source has been shown to be positively associated with decreases in diarrheal disease rates across Africa (Pickering & Davis, 2012) despite the possibility that collection distances and water usage might be poorly correlated (Mellor et al., 2012b). The reasons for this are unclear, but it could be due to longer storage times and recontamination, which have been shown to be important in such settings (Mellor, 2013). Or, it could be that time fetching water takes mothers away from important child-care priorities (Miller, 2010). Therefore the occurrence of droughts, which might limit the reliability of nearby water sources, could be detrimental.

This analysis of the literature has highlighted a number of important areas of research that should be undertaken to further elucidate how climate change is related to diarrheal diseases. Most notably, little empirical research has been conducted to understand how developing world communities cope with weather variability. There should also be a push to take a more mechanistic approach to try to understand not just if weather and diarrhea are correlated, but also to explain why and how that correlation occurs. This will require the collaboration of environmental engineers, hydrologists, microbiologists and epidemiologists along with international development professionals. By taking a more mechanistic approach, researchers might be able to elucidate intra-regional variability.

Based on the available literature, it is likely that climate change will exacerbate diarrhea rates in much of the developing world although the magnitude of this change will likely vary by region and local climate. It will also depend on the relative importance of different pathogens in that region and by the adaptive capacity of local communities. It is therefore imperative that governments, NGOs and other stakeholders improve the resilience of safe water systems in resource limited settings while also spreading novel technologies to purify water at the household level (Mellor et al., 2014).

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