[The impact of water level changes on the Oncomelania snail habitats in Poyang Lake regions before and after the impoundment of Three Gorges reservoir].

OBJECTIVE: To explore the impact of the impoundment of Three Gorges reservoir on Oncomelania snail habitats of Poyang Lake.
METHODS: Four counties with schistosomiasis endemic around the Poyang Lake, naming as Xingzi, Duchang, Jinxian and Xinjian, were selected as study areas. The average daily water level of four major hydrological stations near the above 4 counties from 1997 to 2008 were collected, as well as the elevation of 215 marshlands with snails in those 4 counties, the data of schistosomiasis endemic situation and snail survey of 4 counties in the year before the impoundment of Three Gorges reservoir (2002) and the 6th year after the impoundment of Three Gorges reservoir (2008). The fluctuations of water levels of Poyang Lake, flooding day of marshland with snails as well as the changes of the indicators in snail survey and prevention and control measures before and after the impoundment of Three Gorges reservoir were compared.
RESULTS: Before the impoundment of Three Gorges reservoir, the fluctuations of average water level of the four hydrological stations around Poyang Lake was 6.91-12.93 m; which reduced to 5.72 - 10.75 m after the impoundment. The D-value of the fluctuations of average water level was (1.06 ± 0.47) m. The date of the annual maximum water level delayed by an average of 12 d; while the date of annual minimum water level arrived an average of 12 d earlier than the impoundment of Three Gorges reservoir. The average flooding days of marshland with snails were separately (100.04 ± 42.06) d before the impoundment and (64.13 ± 22.30) d after the impoundment. The difference was statistically significant (t = 3.70, P < 0.05). The density of snails in the 4 counties around Poyang Lake was 0.8437/0.1 m(2) after the impoundment; declining by 31.04% from the snail density (1.2234/0.1 m(2)) before the impoundment. The density of infectious snails was 0.0014/0.1 m(2) after the impoundment; dropping by 67.34% from it (0.0042/0.1 m(2)) before the impoundment; the infection rate of snails was separately 0.34% after the impoundment and 0.16% before the impoundment. The difference was statistically significant (χ(2) = 53.25, P < 0.05). The coverage rate of chemotherapy increased from 4.45% (35 558/798 200) in 2002 to 9.80% (79 457/810 400) in 2008; the difference was statistically significant (χ(2) = 17 336.63, P < 0.05). The coverage rate of snail killing increased from 1.35% (389.04/28 824.66) in 2002 to 7.49% (2260.90/30 166.22) in 2008; the difference was also statistically significant (χ(2) = 28 824.66, P < 0.05).
CONCLUSION: The impact of the impoundment of Three Gorges reservoir shortened the flooding days of marshland with snails and helped the prevention and control of schistosomiasis in Poyang Lake regions as the environment went against the breeding of the snails.