Abstract: 　AIM: To observe longitudinally the changes in survival and infection rate of Oncomelania snails in a positive snail spot in Yunnan.METHODS: A positive snail spot in a ditch in Shalimuzhuang Village,Dali City,was continually observed on the changes in the density of snail population and natural infection rate of the snails by random environmental sampling for 8 times in 4 successive years. RESULTS: During the 8 times of snail survey,living snails were found relatively stable in the ditch,in which positive snails were obtained in 6 occasions,the average infection rate being 3.99%. CONCLUSION: The density and the positive rate of snails in the mountain region remains relatively stable as a source of human infection. Elimination of the positive snail spots is necessary for controlling schistosomiasis.

Key words: Schistosomiasis, positive snail spot, source of infection