Distribution of Freshwater Snails in Irrigation Schemes in the Sudan

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Abstract
(1) This study had two aims. Firstly, to investigate the distribution of aquatic macrophytes and molluscan intermediate hosts of schistosomes in the following irrigation systems in the Sudan: the Gezira-Managil Agricultural Scheme (GMAS), the Rahad Agricultural Scheme (RAS) and the New Halfa Agricultural Scheme (NHAS). Secondly, to formulate hypotheses to account for the distribution observed and to evaluate the possibility that snail hosts of schistosomiasis may be excluded by other competitor snail species. (2) The most snail species were recorded from GMAS, where Biomphalaria pfeifferi (Krauss) and Bulinus truncatus (Audouin) were vey abundant and equally frequent. In RAS, B. pfeifferi was less common than B. truncatus; the opposite was found in NHAS. (3) Density of the intermediate hosts and of submerged plants was high, particularly in the terminal section of minor canals. Chemical and physical characteristics of the water showed remarkable variation among sites, which was related to the composition and density of the aquatic vegetation. In GMAS, positive and negative associations between snail and plant species were found. (4) Contingency tests revealed no significant negative correlations between pairs of snail species. On the contrary, a number of positive correlations was found. In GMAS, B. pfeifferi was positively correlated with B. truncatus, Lymnaea natalensis Krauss, Cleopatra bulimoides (Olivier) and Lanistes carinatus (Olivier). In RAS, Melanoides tuberculata (Mueller) was positively correlated with B. pfeifferi.