| Oncomelania hupensis snail is the only intermediate host of Schistosoma japonicum.The effective control of O.hupensis is essential for blocking the transmission and prevalence of schistosomiasis.Suzhou,Wuxi and Changzhou,located in Lake Tai Basin,have a large area of water network environment,numerous historical snail habitats,which show the risk of schistosomiasis transmission.Based on the identification of the spatio-temporal distribution patterns of snail habitats in the three cities,this study carried out laboratory snail survival experiments to explore the impact of water quality on the survival of snails,and evaluated the suitability of water quality for the survival of snails in relevant districts(counties)along the Lake Tai,so as to provide theoretical basis for the formulation of accurate snail monitoring scheme.Firstly,based on the snail distribution data from 1950 to 2018 in Suzhou-WuxiChangzhou area,the spatial-temporal distribution patterns of snail habitat in this area were identified using nuclear density analysis and Sa TScan spatial-temporal scanning analysis methods.From 1950 to 2018,the number of snail habitats in the study area showed a single peak distribution,reaching maximum point in 1970,then decreasing rapidly.62.68% of all historical snail habitats were eliminated within 10 years since identified,of which 38.24% were eliminated at the same year of identification.Kernel density analysis and Sa TScan spatial-temporal scanning analysis revealed that high-density clusters of snail habitats mainly distributed in Kunshan City,Wuzhong District and Xiangcheng District from 1970 to 1980,and in Yixing City in 1990;since then,the clusters gradually shrank,and moved from the northeast to the west of Lake Tai.Among existing snail habitats,newly emerging snail habitats were mainly identified in Huqiu District(Dongzhu Town),Wuzhong District(Guangfu Town),Taicang City(Shaxi Town)and Jintan District,while re-emerging snail habitats were scattered.Historical snail habitats in Suzhou,Wuxi and Changzhou were numerous and presented spatio-temporal aggregation patterns.Therefore,it is necessary to strengthen the monitoring and prediction of emerging and re-emerging snail habitats in schistosomiasis prevention and control.Secondly,laboratory snail survival experiments were carried out to compare snail survival between groups with different water quality.Principal component analysis(PCA)and Cox regression were used to analyze the effects of different water physicchemical indexes on snail survival.In the infected snails group,there was a significant difference in the 6-month survival rate between the river water group and the tap water group(P < 0.05),and the survival rate in the tap water group was higher than that in the river water group;while in the non-infected snails group,there was no significant difference between the two groups(P > 0.05).In the infected snail group,the regression coefficient β of total phosphorus,p H value,five-day biochemical oxygen demand,conductivity and nitrite was less than 0,and the hazard ratio(HR)of total phosphorus was the lowest(0.27).The regression coefficient β of phosphate and total nitrogen was higher than 0,and the HR of phosphate was the largest(1.10).In the non-infectious snail group,the regression coefficient β of total phosphorus,p H value,five-day biochemical oxygen demand,conductivity and nitrite was less than 0,and the HR of conductivity was the lowest(0.26).The regression coefficient β of total nitrogen,ammonia nitrogen,phosphate and nitrate was greater than 0,and the HR of phosphate was the highest(3.38).Water physic-chemical indexes have an impact on the survival of snails and infected snails.Appropriate organic matter content and the increase of p H value(within the suitable range for snails survival)were beneficial to the survival of snails and infected snails;the increase of total nitrogen and phosphate content was not conducive to the survival of snails and infected snails,but the infected snails may have stronger tolerance to nitrogen.Finally,based on the analysis of laboratory snail survival experiments,Kriging interpolation and weighted overlay were applied to evaluate the suitability of snail survival in each district(county)along Lake Tai.The evaluation was divided into four levels.The higher the value is,the stronger the suitability is.Then the historical snail habitat density factor was combined to predict the existing snail habitats.Finally,the existing snail habitat data was used to verify the model prediction.The overall water quality environment in the study area was suitable for snail survival.There were six snail habitats,three of which were located in the area with grade 4,and the other three were distributed in the area with grade 2.The areas of Binhu District of Wuxi City,Huqiu District of Suzhou City and Wuzhong District close to Taihu Lake had higher suitability for snail survival.The areas of Huqiu District close to North Taihu Lake and the junction of Huqiu District and Wuzhong District had the highest suitability for snail survival.The water quality environment of the districts(counties)along Lake Tai was suitable for snail survival.In areas with high suitability for snail survival,environmental monitoring should be strengthened.In this study,spatial epidemiology and laboratory snail survival experiments were used to clarify the aggregation distribution of snail habitats in Suzhou,Wuxi and Changzhou areas of Lake Tai Basin,and the effects of various physic-chemical indicators in water quality on snail survival were discussed.Water quality had a significant effect on the survival of snails.The increase of p H value(within the suitable range for snail survival),and an appropriate amount of organic matter were beneficial to the survival of snails.The water quality environment in the study area was suitable for the survival of snail.Comprehensive water quality factors and historical snail distribution data have a certain predictive effect on snail distribution. |
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