Change patterns of oncomelanid snail burden in areas within the Yangtze River drainage after the three gorges dam operated.

Background: An "integrated control" strategy has been implemented within seven provinces at highest risk for schistosomiasis along Yangtze River in Peoples' Republic of China (P. R. China) since 2004. Since Oncomelania hupensis is the only intermediate host of the blood fluke (Schistosoma japonicum), controlling the distribution of snails is considered an essential and effective way to reduce the risk of schistosomiasis infection. The study aimed to determine the snail area burden and annual trend among provinces with potential risk for schistosomiasis along the Yangtze River, above and below the Three Gorges Dam (TGD).
Methods: This retrospective study utilized data previously collected from the National Parasitic Diseases Control Information Management System (NPDCIMS) on annual snail surveys from 2009 to 2017. Descriptive statistics were performed for analyzing the snail burden by provinces, counties, type of environmental location and year, and mapping was conducted to present the snails distribution.
Results: From 2009 to 2017, the total snail infested area decreased by 4.22%, from 372 253 hm ² to 356 553 hm ² within the seven high risk provinces. The majority of snails were found in the marshland and lake regions, outside of control embankments. The total snail burden trend remained relatively stable in upstream regions above the TGD from 2010 to 2015, while the trend decreased within downstream regions during this period. In 2016 and 2017, the total snail burden trend increased in both upstream and downstream provinces, however, upstream saw a larger increase. From 2009 to 2017, there were a total of 5990 hm ² of newly developed snail areas in the seven study provinces and the majority were concentrated in regions below the TGD, accounting for 5610 hm ² (93.70%).
Conclusions: There has been a decline in total snail counts from 2009 to 2017. Meanwhile, new snail breeding areas were formed mainly within provinces downstream the TGD due to spread of snails, indicated that the oncomelanid snail would be difficult to completely eliminate. We suggest that the national schistosomiasis integrated control strategy, including mollusciding and environmental modification, will need to be enhanced significantly going forward to achieve a greater reduction in snail burden and ultimately to achieve elimination.