Research Article: Three Gorges Dam: Impact of Water Level Changes on the Density of Schistosome-Transmitting Snail Oncomelania hupensis in Dongting Lake Area, China

Date Published: June 26, 2015

Publisher: Public Library of Science

Author(s): Jin-Yi Wu, Yi-Biao Zhou, Yue Chen, Song Liang, Lin-Han Li, Sheng-Bang Zheng, Shao-ping Zhu, Guang-Hui Ren, Xiu-Xia Song, Qing-Wu Jiang, Matty Knight.

Abstract: BackgroundSchistosomiasis remains an important public health issue in China and worldwide. Oncomelania hupensis is the unique intermediate host of schistosoma japonicum, and its change influences the distribution of S. japonica. The Three Gorges Dam (TGD) has substantially changed the ecology and environment in the Dongting Lake region. This study investigated the impact of water level and elevation on the survival and habitat of the snails.MethodsData were collected for 16 bottomlands around 4 hydrological stations, which included water, density of living snails (form the Anxiang Station for Schistosomiasis Control) and elevation (from Google Earth). Based on the elevation, sixteen bottomlands were divided into 3 groups. ARIMA models were built to predict the density of living snails in different elevation areas.ResultsBefore closure of TGD, 7 out of 9 years had a water level beyond the warning level at least once at Anxiang hydrological station, compared with only 3 out of 10 years after closure of TGD. There were two severe droughts that happened in 2006 and 2011, with much fewer number of flooding per year compared with other study years. Overall, there was a correlation between water level changing and density of living snails variation in all the elevations areas. The density of living snails in all elevations areas was decreasing after the TGD was built. The relationship between number of flooding per year and the density of living snails was more pronounced in the medium and high elevation areas; the density of living snails kept decreasing from 2003 to 2014. In low elevation area however, the density of living snails decreased after 2003 first and turned to increase after 2011. Our ARIMA prediction models indicated that the snails would not disappear in the Dongting Lake region in the next 7 years. In the low elevation area, the density of living snails would increase slightly, and then stabilize after the year 2017. In the medium elevation region, the change of the density of living snails would be more obvious and would increase till the year 2020. In the high elevation area, the density of living snails would remain stable after the year 2015.ConclusionThe TGD influenced water levels and reduced the risk of flooding and the density of living snails in the study region. Based on our prediction models, the density of living snails in all elevations tends to be stabilized. Control of S. japonica would continue to be an important task in the study area in the coming decade.

Partial Text: Schistosomiasis remains a serious public health problem worldwide, affecting more than 200 million people in approximately 76 countries with a loss of 1.53 million disability-adjusted life years (DALYs) [1]. Schistosomiasis japonica (S. japonica) is distributed in 12 provinces in China and 11.6 million people have been infected since 1949 [2–4]. S. japonica causes the most hazardous schistosomiasis, and is difficult to prevent and treat [5, 6]. After continued implementations of comprehensive control measures from the mid-1950s to 1980s, endemic regions were circumscribed in certain core areas in China, especially in the Dongting Lake region at the middle reaches of the Yangtze River [7–11]. Most of these core areas are in the downstream of Three Gorges Dam (TGD). The TGD is a world-class water conservancy project. It began to impound water and sediment discharge in 2003, and is one of several tremendous engineering projects transforming China’s ecology and natural environment. The construction and the operation of TGD have obviously affected the ecological environment. The water level of TGD in 2003 was 135m and it reached 160m in 2011. It is believed that the TGD project reduces the frequency of major flooding in the downstream areas from once every ten years to once every 100 years; however it threatens the living of aquatic animals in the Yangtze River including the river dolphin, or baiji, and finless porpoise, or jiangzhu [12]. Similarly the TGD project can also influence the survival of O. hupensis, the unique intermediate host of Schistosoma japonicum [13, 14].

After the impoundment of TGD in 2003, the dry season in Dongting Lake region was reported to arrive earlier and to be longer than before, and the water level was close to the lowest level in history for several times [31–34]. Anxiang County of Hunan Province is located in Dongting Lake region and at the downstream of TGD. Our results indicated that the water levels in all the elevation areas and number of flooding decreased after 2003 and that there was a severe drought in 2006.



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