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. 2017 Jun 5;372(1722):20160127.
doi: 10.1098/rstb.2016.0127.

Nearly 400 Million People Are at Higher Risk of Schistosomiasis Because Dams Block the Migration of Snail-Eating River Prawns

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Free PMC article

Nearly 400 Million People Are at Higher Risk of Schistosomiasis Because Dams Block the Migration of Snail-Eating River Prawns

Susanne H Sokolow et al. Philos Trans R Soc Lond B Biol Sci. .
Free PMC article

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'.

Keywords: bilharzia; biological control; dam; disease control; planetary health; schistosome.

Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1.
Figure 1.
Odds ratios after:before dams within prawn ranges and comparison areas. Black circles show country-specific odds ratios and their confidence intervals among the respective dams and watersheds listed on the left. Red diamonds show the overall odds ratios and confidence intervals resulting from a generalized linear mixed model (with binomial errors) comparing after against before dam building for all dams/watersheds in each respective catchment/watershed category. N shows the total sample size (number of human subjects) for that category and the p value indicates the statistical significance of the test of the hypothesis that the overall odds ratios (diamonds) differ from unity. The vertical dashed line marks 1 : 1 odds ratios. Those points to the right of the dashed line indicate a significant increase in schistosomiasis after dam building; those to the left indicate a significant decline in schistosomiasis; and those overlapping the vertical dashed line show no change. De novo indicates a de novo introduction of the parasite in that catchment after the dam, whereby the presence of zero infections before the dam renders the odds ratio impossible to calculate. Note the log scale. (Online version in colour.)
Figure 2.
Figure 2.
Schistosomiasis mean prevalences before (squares) and after (circles) the dams at (a) Akosombo (Volta River Basin, Ghana) and (b) Diama (Senegal River Basin, Senegal). Schistosoma haematobium and S. mansoni prevalences are stratified along latitudinal or longitudinal gradients that span the downstream, near upstream, and far upstream areas of the watersheds. Bars represent standard errors of the mean. ND = no data. Dashed lines connect the mean prevalences before and after the dam in each latitudinal/longitudinal band, to assist visualization of the temporal trend.
Figure 3.
Figure 3.
(a) Schistosoma spp. parasite world distributions (adapted from [–24]) and Macrobrachium spp. prawn world distributions (data from this paper), and (b) dams and Macrobrachium spp. world distributions.
Figure 4.
Figure 4.
Map showing locations of dams (coloured circles), upstream dam catchments (coloured regions), and Macrobrachium spp. prawn distributions (dark grey regions) on the African continent. Dam data were sourced from the freely available GRaND database: http://www.gwsp.org/products/grand-database.html.

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