Abstract
Dams have long been associated with elevated burdens of human schistosomiasis, but how dams increase disease is not always clear, in part because dams have many ecological and socio-economic effects. A recent hypothesis argues that dams block reproduction of the migratory river prawns that eat the snail hosts of schistosomiasis. In the Senegal River Basin, there is evidence that prawn populations declined and schistosomiasis increased after completion of the Diama Dam. Restoring prawns to a water-access site upstream of the dam reduced snail density and reinfection rates in people. However, whether a similar cascade of effects (from dams to prawns to snails to human schistosomiasis) occurs elsewhere is unknown. Here, we examine large dams worldwide and identify where their catchments intersect with endemic schistosomiasis and the historical habitat ranges of large, migratory Macrobrachium spp. prawns. River prawn habitats are widespread, and we estimate that 277–385 million people live within schistosomiasis-endemic regions where river prawns are or were present (out of the 800 million people who are at risk of schistosomiasis). Using a published repository of schistosomiasis studies in sub-Saharan Africa, we compared infection before and after the construction of 14 large dams for people living in: (i) upstream catchments within historical habitats of native prawns, (ii) comparable undammed watersheds, and (iii) dammed catchments beyond the historical reach of migratory prawns. Damming was followed by greater increases in schistosomiasis within prawn habitats than outside prawn habitats. We estimate that one third to one half of the global population-at-risk of schistosomiasis could benefit from restoration of native prawns. Because dams block prawn migrations, our results suggest that prawn extirpation contributes to the sharp increase of schistosomiasis after damming, and points to prawn restoration as an ecological solution for reducing human disease.This article is part of the themed issue ‘Conservation, biodiversity and infectious disease: scientific evidence and policy implications’.
Highlights
Dams stabilize water availability, generate power and facilitate navigation, and provide substantial benefits to human society, but they sometimes lead to disease epidemics [1]
We examined countries where prawns, dams and schistosomiasis co-occur, estimating the number of people living: (i) in areas subject to endemic schistosomiasis and (ii) within watersheds that are part of the native, historical ranges of 24 Macrobrachium spp. prawns identified by the Food and Agriculture Organization of the United Nations (FAO) as large-bodied and migratory [21]
Schistosomiasis data pertinent to our three scenarios and available from the Global Neglected Tropical Diseases (GNTD) Database included 679 054 individual human subjects tested for S. haematobium and 462 859 for S. mansoni in 4976 and 3725 unique locations, respectively, within the African continent
Summary
Generate power and facilitate navigation, and provide substantial benefits to human society, but they sometimes lead to disease epidemics [1]. One hypothesis that has received less attention argues that the Diama Dam excludes native, migratory snail predators, notably Macrobrachium spp. river prawns, by blocking the predators’ migrations. This causes snail populations to be released from a primary regulating force: predation pressure [5,6,7]. In a test of this hypothesis, a demonstration intervention in Senegal showed that restoring native M. vollenhovenii prawns to one village water-access point led to significantly lower reinfection rates for human schistosomiasis, compared to a nearby control village [5]
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