The Impact Of Water Level Changes On The Spread Of Schistosomiasis In Poyang Lake

The total area of Oncomelania snail habitats in China is now 3.42billion m~2,and among which more than 94 percent is located in marshland and lake regions.These regions are wet in spring,submerged in summer,exposed in fall and dry in winter. Every year when flood rises,the bottomlands change into a vast ocean,and after water secedes in fall,the bottomland is exposed and vegetation is flourishing.In general,marshland submerged for 3-5 months and keeps wet for a long time, especially bottomland covered with reeds,is fit for snail surviving.The snail habitat is strictly limited with water.Young snails cannot live without water during growth,and the survival rate of adult snails will decline if the snails submerged in water for a long time.And the distribution of water can also influence vegetation,which is necessary for the survival of snails;grass cannot cover dry land or bottomland submerged in water for a long time.Therefore,it is important to find out the impact of water level on the distribution of snails at bottomland regions.With the development of remote sensing techniques,it has been more and more applied in distilling water extents of lakes,and the areas of water is influenced by changes of water level to a large extent.Thus on the basis of understanding the impact of water level on the situation of snails,we can apply remote sensing technology to forecast the distribution of snails at bottomland.Part 1:The epidemic situation of schistosomiasis in experimental areaMajiawan and Menqian marshland is located in Zhuxi village,Liaonan township, Xingzi County.The density of live snails was 4.74,0.85 and 3.37/0.1m~2 in 2005, 2006 and 2007,respectively.The density of infected snails was 0.0 018/0.1m~2 in 2005, and no infected snails were checked out in 2006 and 2007.The infection rate of population was 3.35%,1.59%and 2.31%respectively in the three years,and the infection rate of cattle was 12.31%,3.23%and 3.03%,respectively.Part 2:Analyzing the change of areas of water surface of the Poyang Lake by applying remote sensing techniqueWe collected 12 TM images between 1998 and 2006,performed geometric correction,extracted MNDWI value,set threshold value,built mask and then chose regions of interest(ROI) to calculate the areas of the Poyang Lake.We did correlation analysis between water areas of the Poyang Lake and water level of Xingzi,Hukou and Kangshan,and the correlation is strong,the coefficient of correlation is 0.957, 0.893 and 0.939,respectively.We built a model to describe the relationship of water areas of the Poyang Lake and water level of Xingzi,and the logarithm model is better, R=0.959,R~2=0.920,and adjusted R~2=0.912.The model is S=5 181.51lln(h)-10 261.9, S stands for the water areas of the Poyang Lake extracted by TM images,h stands for water level of Xingzi.We checked the model by water areas of the Poyang Lake extracted from TM image of December 6th,2006,and the result showed that the difference between water areas extracted from TM images and water areas calculated by the model is small.The former is 1 658.73km~2,and the latter is 1 648.20km~2,and the error is less than 0.64 percent.These suggested that water areas extracted from TM images can well reflect the change of water level in the Poyang Lake,which provided a base for the application of remote sensing technique on forecasting the distribution of snails at bottomland.Part 3:The impact of water level change on snail situation with a small scaleWater is an essential carrier in the spread chain of schistosomiasis,which is also inseparable for the breeding and propagation of snails.The change of water level has an important influence on the distribution of snails.So it is necessary to study the influence on snails situation with the change of water level.We firstly analyzed the influence on the snails situation with the change of water level in small scale.We collected data about water level from 1997 to 2007,to analyse the impact on the distribution of snails in Zhuxi pilot and Xingzi County with the change of water level.The result of statistical analysis showed that there was a negative correlation between the percent of frames with living snails in Zhuxi pilot and average water level of last August(r=-0.748,p=0.013＜0.05).Besides,there is also a negative correlation between the percent of frames with living snails in Zhuxi pilot and average water level during wet period/the highest water level of last year(r=-0.652, p=0.041＜0.05/r=-0.771,p=0.009＜0.01).Thus,we can speculate that the higher water level is in wet period and the longer the flooding period lasts,the higher the mortality rate of snails is and the lower the percent of frames with living snails is.In addition,there is a negative correlation between areas of snails in marshland and lake regions of Xingzi county and the average water level of April(r=-0.761, p=0.011＜0.05),which suggests that if bottomland is submerged in advance or water level rises during spring tide period,the areas of snails in marshland and lake regions may reduce.Part 4:The impact of water level change on the areas of snails in large scalesIn order to further verify the results of part 2,we chose the average monthly water level of Xingzi Hydrometric Station to represent the average water level of the Poyang Lake,and did correlation analysis between the average monthly water level and the snails areas in marshland and lake regions in Jiangxi province.The results showed that there is a negative correlation between areas of snail habitats in marshland and lake regions and the average water level of April(r=-0.658, p=0.039＜0.05),which further suggested that the later the bottomland submerged and the lower the average water level of April was,the larger the areas of snails extended.In the second part,we analyzed the relationship between water areas extracted from TM images and the actual water level data.The results of the third and fourth part suggested that the average water level of last August and April this year influenced the distribution of snails and areas of snail habitats.Thus in practical application,we can collect images of April and August to forecast the distribution and diffusion of snails.