Through the study on epidemic of schistosomiasis and distribution of oncomelania snail in Yangtze River valley, it is claimed that due to the deposit of sand and mud and reclaiming land from marshes, the capacity of preventing flood was decreased and there was a tendency of increassing flooding disaster during the last centuries. Thus, the valley became an origin and reservoir of the snails, resulting in the difficulty for the eradication of the snail in the middle and lower valley. Moreover, the complicated condition of mountainous area made the snails very difficult to be eradicated, resulti...
