Cholera in coastal Africa: a systematic review of its heterogeneous environmental determinants

According to the "cholera paradigm," epidemiology of this prototypical waterborne disease is considered to be driven directly by climate-induced variations in coastal aquatic reservoirs of Vibrio cholerae. This systematic review on environmental determinants of cholera in coastal Africa shows that instead coastal epidemics constitute a minor part of the continental cholera burden. Most of coastal cholera foci are located near estuaries, lagoons, mangrove forests, and on islands. Yet outbreaks often originate in coastal cities, where cholera is more likely to be imported from distant areas. Cholera outbreaks also may intensify in densely populated slum quarters before spreading to adjacent regions. Frequent seasonality of cholera incidence appears driven by the rainfall-induced contamination of unprotected water sources through latrine overflow and sewage, as well as by the periodicity of human activities like fishing or traveling. Lulls in transmission periods of several years are repeatedly recorded even in high-risk coastal areas. To date, environmental studies have failed to demonstrate a perennial aquatic reservoir of toxigenic V. cholerae around the continent. Finally, applicability of the cholera paradigm therefore appears questionable in Africa, although available data remain limited. Thorough surveys with microbiological analyses of water samples and prospective genotyping of environmental and clinical strains of V. cholerae are needed to understand determinants of cholera in coastal Africa and better target prevention and control measures.