Distribution of freshwater snails in irrigation schemes in the Sudan.
The distribution of aquatic macrophytes and of 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) was investigated. The largest number of snail species was recorded from GMAS, where Biomphalaria pfeifferi and Bulinus truncatus were very abundant and equally common. In RAS, B. pfeifferi was less common than B. truncatus; the opposite was found in NHAS. 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 between 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. Contingency tests revealed no significant negative correlations between pairs of snails species. On the contrary, a number of positive correlations were found. In GMAS, B. pfeifferi was positively correlated with B. truncatus, Lymnaea natalensis, Cleopatra bulimoides and Lanistes carinatus. In RAS, Melanoides tuberculata was positively correlated with B. pfeifferi.