Flooding will change the ecological environment. Water conservancy (dams, river embankments, channels, levees, etc.) and sanitary facilities can be damaged by severe floods. The damage of sanitary facilities may lead to a higher risk of fecal contamination. The main impact of flooding on the environment is the change in the density and distribution of
O. hupensis since the snail is the crucial link in the life cycle of
S. japonicum [51][42]. Without the existence of
O. hupensis, the transmission of schistosomiasis will be interrupted successfully. This is the reason why snail control is crucial for schistosomiasis elimination
[52][43].
The density of
O. hupensis could be influenced by flooding, through the impact on development, reproduction, and ability to survive under the condition of submergence
[53][44]. On the short-term individual level, continuous rainfall and the subsequent flooding facilitate the establishment of snail colonies on vegetation
[47][38]. During rainfall and flooding, the egg production of the snails significantly increases, with one female producing on average two eggs in five days
[54][45]. Yang et al. reviewed the data between 1995–2002 in Hunan province of China, and claimed that the annual rainfall, days of daily rainfall greater than 0.1 mm, and the days inundated with water (favorable for 2 to 7 months) were significantly associated with the reproduction of
O. hupensis [55][46]. On the long-term population level, there is a trend of the pattern of snail density in certain local snail habitats
[56][47]. Several studies reported that the rate of the frames with living snails fell in the first two years after flooding and then rose quickly from the third year
[38,57][29][48]. The decrease in the first two years can be explained as due to the considerable deaths of adult snails
[58][49] and the limited capacity of developing and hatching for snail eggs
[59][50] under the condition of submergence. However, some adult snails can survive through a period of natural drowning and young snails developed well under the drowning condition
[58][49]. This may be the reason for the subsequent increasing trend from the third year after flooding. A study reviewed the annual snail survey in Jiangsu province in China from 1998 to 2003 and reported similar trends in the rate of the frames with infected snails, the snail infection rate, and the density of infected snails
[57][48]. This provides an explanation for the higher risk of schistosomiasis transmission with a higher density of total living snails.
The influence is also demonstrated in the active and passive diffusion of the snails for the following expanded snail habitats. When flooding occurs, the snails drown, climb trees, and passively float down rivers
[60][51]. Along with the side-weir flow after the flood discharge, the snails stay in the places where flow velocity is small, such as the vortex areas
[61][52]. New decent potential habitats develop with the mud deposition due to the flood flow and the snails can actively and slowly move to neighboring new habitats. These active and passive dispersals can result in the enlargement of previous snail habitats, the emergence of new snail habitats, and the rebound in previously snail eliminated areas
[52][43]. During the period from 1979 to 2000, the re-emerging and newly discovered snail habitats in the flooding years accounted for up to 5.8% and 10.1% of the total snail habitats in the Yangtze River valley and were 2.6 and 2.7 times larger than the areas in years with normal hydrologic conditions, respectively
[38][29]. After the 2016 catastrophic flood in China, the re-emerging and the newly discovered snail habitat areas in Anhui province were 1375 hm
2 and 1288 hm
2, respectively
[62][53]. It should be noted that snail dispersal often presents as a retardation effect of flooding, since snails in the new habitats may not reproduce to a large amount that could be easily found by snail surveys and surveillance in one or two years after flooding.