There are few studies, if any, that have looked at the seasonality of diarrhoea incidence in Zambia. This is a global gap in the research because of the complexity of the causal patterns of disease and the regional nature of climate change. Plus, the extent of climatic influence is highly dependent on the pathogens, as well as the water and sanitation context.
Needless to say, evidence of direct links between disease and climatic patterns is scarce and varied. Some studies have found that rainfall doesn’t affect transmission of specific diarrhoea pathogens (Zhang et al, 2007), while others found that low levels of rainfall are associated with higher incidence of diarrhoea (Singh et al, 2001).
Our lack of understanding in this area is a concern given that climate change is an important health determinant for people, particularly in vulnerable areas (Confalonieri et al, 2007; Patz et al, 2005). For example there are growing concerns that climate change could lead to increased mortality from malnutrition because of drought and crop failure (Schmidhuber and Tubiello, 2007), diarrhoea [McMichael et al, 2004; World Health Organization, 2004], respiratory diseases (Beggs and Bambrick, 2005), and vector-borne diseases like malaria (Tanser et al, 2003).
Further to our previous blog post, Rohit has superimposed the graph that was included on diarrhoea cases reported at Monze health centres with a climate graph for Zambia. Take a look at the trend illustrated in the graphs below. It shows a very clear inverse relationship between precipitation and diarrhoea incidence.
The diarrhoea incidence pattern was very similar for the other three COTZ districts (Katete, Kalomo and Petauke).
Rohit looked into the research in this area a bit further and found some potential explanations. One recent study showed that low rainfall in the dry season increases the prevalence of diarrhoea across Sub-Saharan Africa (Bandyopadhyay et al, 2011). When/where water is scarce the prevalence of diarrhoea increases due to consumption of unsafe water and poorer hygiene practices (Fewtrell et al. 2005). As touched on above, we also know that rainfall and variations in temperature have an impact on the types of pathogens – bacteria, protozoa, viruses and helminthes – that cause diarrhoea.
For example, cholera spreads when there is higher rainfall in coastal regions at particular times of the year (Colwell, 1996), while rotavirus infections are most commonly associated with winter diarrhoea (increasing in frequency during the drier, cool months), and bacterial diarrhoea tends to peak during the warmer rainy season (Kale, 2004). Drought and low water levels can also have indirect impacts on diarrhoea incidence. The most obvious example of this is lower crop yields due to crops that are dependent on rainfall. We saw this first hand during the ColaLife trial. The crop yields were poor in the year of the trial. Because families in our project districts largely depend on agriculture for their livelihoods, there was an impact on food security. This in turn has an effect on the family’s ability to practice good prevention – accessing safe food and water. It is clear then how the nutritional status of children can be compromised under this scenario. We already know well the cycle of how stunted children are more susceptible to diarrhoea (Bern et al, 1992), and how cumulative diarrhoea burden (number of episodes and duration) can lead to stunting (Checkley et al, 2003; Moore et al, 2001).
We know globally that rotavirus is the most common cause of severe diarrhoea, responsible for about 28% of cases (Fischer-Walker, 2013). Based on the epidemiological evidence above, and the pattern seen in the graph above, we would expect rotavirus to be one of the main culprits in Zambia as well.
A note on the data in this post
The data contained in this blog post are unpublished and based on preliminary analysis of data from the ColaLife Operational Trial in Zambia (COTZ). Final calculations may vary and will be published in peer reviewed literature in due course. In the interim, the following citation may be used: Ramchandani, R. et al. (forthcoming). ColaLife Operational Trial Zambia (COTZ) Evaluation. Johns Hopkins Bloomberg School of Public Health, Baltimore. Related correspondence should be sent to Rohit Ramchandani (firstname.lastname@example.org) and copied to Simon Berry (email@example.com).