Three Gorges Dam: The changing trend of the density of Oncomelania hupensis, the intermediate host for Schistosoma japonicum, in reaches of the Yangtze River, 1990-2019

Abstract Background: Oncomelania hupensis ( O. hupensis ) snail is the sole intermediate host of Schistosoma japonicum . Environmental changes caused by Three Gorges Dam (TGD), flood and drought, affect the distribution of snail population, and better understanding the dynamics and spatial distribution of snail density is critical for schistosomiasis risk assessment and control for affected areas. Methods: Data of O. hupensis snail survey between 1990 and 2019 were collected from previous studies in four electronic databases (CNKI, Wanfang, Pubmed, and SCI) and from the national schistosomiasis surveillance. Meta-analysis was conducted to estimate the overall and annual snail densities and their 95% confidence intervals (CIs). Joinpoint model was used to identify the changing trend and inflection point of snail density between 1990 and 2019. Inverse distance weighted (IDW) interpolation was used to determine the spatial distribution of recent snail density. Results: A total of 3777 snail survey sites (872 for the upstream area and 2905 for the downstream area of the TGD) with a precise location of village or beach were identified. For the downstream area of the TGD, the snail density peaked in 1998 (1.635/0.11m2, 95% CI: 1.220–2.189) and fluctuated at a relatively high level before 2003, and declined steadily from 2003 (1.143/0.11m 2 , 95% CI: 0.905–1.397) to 2012 (0.127/0.11m 2 , 95% CI: 0.081–0.199). The snail density maintained lower than 0.150/0.11m 2 between 2012 and 2019. Joinpoint model identified that the inflection point of 2003 was statistically significant and the snail density showed a significant downward trend from 2003 to 2012 with an APC of -20.56% (95% CI: -24.15 to -16.80). For the Upstream area of the TGD, the snail density peaked in 2005 (0.760/0.11m2, 95% CI: 0.479–1.207) and was generally greater than 0.300/0.11m2 before 2005. The snail density steadily declined since 2006 and was generally lower than 0.150/0.11m2 after 2011. No inflection point was identified and the snail density showed a significant downward trend from 1990 to 2019 with an APC of -6.05% (95% CI: -7.97 to -7.09). The areas with a relatively high snail density were mainly distributed in Poyang Lake, Dongting Lake, Jianghan Plain, and the Anhui branch of the Yangtze River. Conclusion: The density of O. hupensis snails showed a fluctuating downward trend in the Yangtze River basin between 1990 and 2019. In the downstream area, the decline of snail density was accelerated after the operation of TGD, and then fluctuated at a relatively low level. Infected areas with a higher density of snails were distributed in Dongting Lake, Poyang Lake, Jianhan Plain, and the Anhui branch of the Yangtze River.