Impact Of Water Level And Climatic Factors On The Distribution Of Schistosoma Japonicum Intermediate Host Oncomelania Hupensis And The Identification Of Snail Habitats In Eastern Dongting Lake Areas

Schistosomiasis is one of the most serious parasitic diseases threatening the health welfare of residents in Southern China. To a great extent, it remains endemic in lake and marshland regions in Anhui, Hubei, Hunan, Jiangxi, Jiangsu Provinces and mountainous areas in Sichuan and Yunan Provinces. At present, due to the global climate change, man-made drastic changes to nature, such as Three George Dam (TGD) and returning farmland to lakes, and the natural disasters including droughts and deluges, relative changes influencing the breeding and distribution of Oncomelania hupensis arise to happen on the water level, meteorological and environmental factors in lake areas of lower-middle reaches of Yangtze River. In presented study, we selected typical snail habitats in Eastern Dongting Lake region as our study area to explore the impacts of water level and relevant weather variables on the distribution of schistosome-intermediate host Oncomelania hupensis. Combining the satellite remote sensing images and field data of Meteorological, hydrological and related snail variables, GIS database was established to detect the potential snail habitats. The results in our study could provide strong evidence for snail controlling policy decision, schistosomiasis transmission-blocking and population health promotion. The whole study was divided into 3 parts:Part 1 Impact of the Changing Water Level on Oncomelania hupensis Populations in Eastern Dongting Lake areasObjective To explore the impact and regularity of snail population after changing of water level variables and to develop effective control and prediction programs. Methods A typical snail habitats closed to Changjiang Village in Yueyang City, Hunan Province was selected as the survey field. Data on water level and the changing trend of snail population during 2001-2009 including the average water level, maximum and minimum water level, snail and infected snail density, proportion of sampling frames with living and infected snails etc. were collected. The distribution of water level and snail indexes were described, simple regression models and General Additive Models (GAM) were also fitted for the relationships between water level parameters and snail parameters based on each variables independently. Results Impacts of the average water level and the watered-out days in current year in the surveyed field on the snail density were statistically significant (p<0.01), showing a quadratic curve association. The snail density remained at the lowest level when the average water level was at 25.0m and the survey field watered-out days was 120. The average water level in current year and the field watered-out days in past year showed statistically significant impacts on the infected snail density (p<0.001), the proportion of sampling frames with infected snails (p<0.001) and living snails (p≈0.05), presented a quadratic curve, a cubic curve or even a more complicated piecewise curve association. Both the infected snail density and the proportion of sampling frames with infected snails remained at the lowest level when the average water level was at 25.0m and the watered-out days in the past year was 110, while the proportion of sampling frames with living snails kept the lowest level when the average water level was at 25.0m and the watered-out days in the past year was 160. Conclusion The water level and the field watered-out days affected the development of snail population directly. The changing water level had an impact on snail population change, which mainly presented as nonlinear smooth function relation. Impact of the field watered out-days on the infected snail density and the proportion of sampling frames with living snails and infected snails showed hysteresis effects. The snail density was predicted to be retaining a high level when the water level was 24.0m and the field watered-out days was 3 months. Compared to the results of simple regression model fitting, it had obvious advantages to fit the relationship of the changing water level and the snail indexes with GAMs which could get closer to reality as well as easier to find and explain the potential associations and regulations.Part 2 Impact of climatic factors on distribution of Oncomelania hupensis population in Eastern Dongting Lake areasObjective To explore the impact of water level and relevant climatic factors on the distribution of schistosome-transmitting snail Oncomelania hupensis in Eastern Dongting Lake Areas in order to provide an evidence for snail control and people health protection. Methods Based on the field data of environmental variables:water level, temperature, humidity, precipitation, sunshine duration and snail population collected from a typical habitat in Eastern Dongting Lake region, the relationships between interactional effects of environmental variables and lagged or unlagged snail density were explored by Factor Analysis and General Additive Model (GAM) method. Results Temperature, precipitation, humidity, sunshine duration and water level had strong correlations with each other to varying degrees. Water level and submerged days had significant relation and hysteresis effects with snail density which showed an apparent seasonal pattern:high value in later Spring and earlier Autumn, low value in Winter. Two common factors:general environmental suitability index and wetness index were extracted from the 5 variables above by factor analysis. The smoothing components plots corresponding to the two indexes demonstrated that both parametric and nonparametric effects were exhibited for the suitability index on snail density, presenting as a positive linear pattern and a cubic pattern, however, only nonparametric effect showed as quadratic pattern was represented for the wetness index on snail density. Snail density was increasing when the suitability index value was above 1 or near 0 and decreasing when the value was during 0～1. It also rose up when the wetness index value was at 0～1, and declined when the value was above 1. Moreover, the wetness index had much stronger influence on snail density than suitability index. Conclusion The interactional effects of water and relevant weather variables could be divided into two patterns on general environmental suitability index and wetness index, while the former determined whether the circumstance was suitable for snail breeding, the latter determined the reproduction and survivorship of snail population. Therefore, snail density was mostly determined by the wetness index implied in humidity and precipitation in typical snail habitats of the Eastern Dongting lake regions. The suitability index only resulted into flat changes and weakly seasonal influence on snail density. Additionally, the inferences in our study could form the basis of management measures aimed at snail controlling and schistosomiasis transmission-blocking in lake regions, nonetheless, these approaches must be demonstrated by further study theoretically and experimentally.Part 3 Detection of potential snail habitats in the marshland of Eastern Dongting Lake AreaObjective To detect the snail habitats from the marshland of Eastern Dongting Lake Area, using the Remote Sensing(RS) and Geographic Information System(GIS) technology based on China-Brazil Earth Resources Satellite-02B (CBERS-02B) CCD images. Methods According to the two typical traits of snail habitats:"water in summer and land in winter" and "no grass, no snails", the "water in summer and land in winter" region and the vegetation coverage region were calculated respectively based on two RS images of the Eastern Dongting Lake Area in flood period and drought period by RS image processing. The two regions mentioned above were then overlapped to confirm the snail habitats which through comparing with the data from field survey under spatial overlapping of Arcgis as the last step. Results In Eastern Dongting Lake area, the "water in summer and land in winter" region and vegetation coverage region were predicted based on the formula "NDWI>0.01" and "NDVI>0.36", respectively. The snail habitat was determined by theme overlay of the two regions said above. The agreement rate between the prediction and the geospatial data of field survey was 93.55%, which demonstrated the final results were accurate and reliable. Conclusion CBERS-02B image could be used to detect the snail habitats and to monitor the changes of them, so as to find out the characteristics of distribution and the trends of diffusion on snail population in the marshland of Eastern Dongting Lake Area. The snail index (Discriminant 1 and 2) seemed to be suitable for the detection of snail habitats in the whole Lake area and to be used for the programs of snail control.