Impact of changing water levels and weather on Oncomelania hupensis hupensis populations, the snail host of Schistosoma japonicum, downstream of the Three Gorges Dam.

Increasing evidence indicates that dams impact riverine ecosystems and human diseases. Poyang Lake, one of the largest schistosomiasis endemic environments in China, will change due to the construction of the Yangtze River Three Gorges Dam. We assess changes in Oncomelania hupensis hupensis, the snail host for Schistosoma japonicum, in response to changing water levels and weather from 1998 to 2002. In the 5 years following the major flooding of Poyang Lake in 1998, seasonal water levels have gradually decreased, concomitant with decreases in mean and variance of fall snail densities. Nonlinear relationships suggest that the highest spring density is associated with current, 2-, and 3-month prior temperatures of 18 degrees, 9.1 degrees, and 5.8 degrees C, while the highest fall density is associated with 2- and 3-month prior water levels of 17 and 18 m, respectively. This suggests that lower, more stable water levels downstream of the dam may result in a reduction in mean fall densities and their variance. However, additional data are needed to determine whether snail populations that are typically destroyed by seasonal floods may live longer in more stable environments created by the dam.