Delayed response of snails' vertical distribution on the bottomland to the changing water level at the downstream area of the Three Gorges project

The schistosomiasis, severely harming to the anthropic health, is one of six key tropical diseases identified by the World Health Organization (WHO). The downstream area of the Three Gorges reservoir is the major epidemic area of schistosomiasis in China. As the only intermediate host of schistosome, snails generally live at the bottomland of rivers and lakes. The vertical distribution of snails on the bottomland is closely related to the water level. Due to the changes in soil moisture and the snails' crawl speed, the adjustment of snails' vertical distribution usually lags behind the water level alteration. This delayed response of the snails' vertical distribution to water level changes has yet been comprehensively studied. The snails' vertical distribution is always difficult to describe or predict accurately. Based on theoretical analysis, a generalized model for the changes in snails' vertical distribution was proposed in this study. A calculation method was developed for predicting the theoretical vertical distribution of snails under fluctuating water levels. The calculation method reveals that: (1) the elevation, at which snails are mostly distributed, increases with the rise of water level and decreases with the decline of water level; (2) the difference between this elevation is where intense snail distribution is located and the water level decreases with the rising of water level and increases with the descending of water level. By using the field data of water level at Chenglingji station and snails' distribution elevation at the Dongkou bottomland of Dongting Lake during 2008 to 2010, the calculation method based on the delay response for predicting the snails' distribution elevation was tested. The results showed that the computed results agreed with the measured data and the correlation coefficient between them equals to 0.7. The relationship between the snails' vertical distribution and the water level alteration established in this paper may be used to predict the snails' distribution elevation, thus this study contributes to the prevention and control of schistosomiasis.