Co-variation of Cholera with Climatic and Environmental Parameters in Coastal Regions of Tanzania

The bacterium causing cholera, Vibrio cholerae, is essentially a marine organism and its ecological dynamics have been linked to oceanographic conditions and climate. We used autoregressive models with external inputs to identify potential relationships between the number of cholera cases in the coastal regions of mainland Tanzania with climatic and environmental indices (maximum air temperature, sea surface temperature, wind speed and chlorophyll a). Results revealed that, between 2004 and 2010, coastal regions of mainland Tanzania inhabited by approximately 21% of the total population accounted for approximately 50% of the cholera cases and 40% of the total mortality. Significant co-variations were found between seasonally adjusted cholera cases and coastal ocean chlorophyll a and, to some degree, sea surface temperature, the outbreaks lagging behind by one to four months. Cholera cases in Dar es Salaam were also weakly related to the Indian Ocean Dipole Mode Index, lagging by five months, suggesting that it may be possible to predict cholera outbreaks for Dar es Salaam this period ahead. The results also suggest that the severity of cholera in coastal regions can be predicted by ocean conditions and that longer-term environmental and climate parameters may be used to predict cholera outbreaks along the coastal regions. Corresponding author: CL Email: [email protected] INTRODUCTION Vibrio cholerae, which causes the acute enteric infection of cholera, is essentially a marine bacterium, with coastal waters acting as an important reservoir (Colwell et al., 1981). The bacterium has been found in coastal environments around the world, both in areas where cholera is endemic and in cholera disease-free areas (Karunasagar et al., 2003). It is now well known that cholera occurs in regions with natural aquatic reservoirs where the bacteria can persist in a free-living state, or in association with phytoplankton, zooplankton and detritus (Nelson et al., 2009). These environmental reservoirs may play a significant role in cholera epidemiology by favouring persistence of the pathogen in periods between epidemics (Vezulli et al., 2010). Indeed, the dynamics of cholera outbreaks have been linked to climate and environmental variables, such as air and water temperature, rainfall, wind direction (e.g. Paz and Broza, 2007; Jutla et al., 2011). The dynamics of environmental V. cholerae have been linked, for example, to water salinity, nutrients and plankton biomass (Vezulli et al., 2010). Water temperature, in particular, has been shown to be an important factor governing the seasonal and geographical variation in V. cholerae (Igbinosa & Okoh, 2008). Evidently the ecology of V. cholerae is important, as are the socio-economic factors for endemic and epidemic cholera. Studies have shown that the cumulative incidence of cholera is strongly correlated with low scores on several socio-economic development indices (Ackers et al., 1998), including lack of adequate water and sanitation infrastructure in densely populated areas and ineffective cholera management (Mhalu et al., 1984; Sedas, 2007; Sow et al., 2011). Inadequate and leaking sanitation provides opportunities for transmission of the virulent forms of the pathogen into environmental reservoirs (Vezulli et al., 2010) where it may survive and propagate depending on environmental and climatic conditions. Studies on the relationship between cholera outbreaks and climatic or seasonal variables in Tanzania are few (Trærup et al., 2011) and mainly limited to the Lake Victoria Basin and the islands of Zanzibar (e.g. Nkoko et al., 2011; Reyburn et al., 2011). Furthermore, demonstrated relationships between cholera outbreaks and weather and climate in mainland Tanzania are scarce (Trærup et al., 2011). There has, for example, been no analysis of data for the mainland coastal regions connected to the marine environment. Given the potential importance of such relationships for the prediction of cholera outbreaks in a country like Tanzania, we sought to analyse the relationship between cholera cases and climate in the coastal regions of mainland Tanzania, focusing on the years 2004 to 2010. We also aimed to establish whether any identified co-variations of cholera with seasonal or climatic cues would be useful in predicting outbreaks in coastal Tanzania regions ahead of time.