The Oncomelania hupensis is the only intermediate host of Schistosoma japonicum (S. japonicum) in China. Approximately 97.36% habitats infested by Oncomelania snails are in the middle and low reaches of the Yangtze River (1). The typical habitat of Oncomelania snails is marshland along the Yangtze River, its major branches, and the connected lakes (2). Such habitats are submerged in water in summer and emerge in winter. Therefore, rainfall and flooding impact the distribution of snails and the transmission dynamics of schistosomiasis (3). Acute schistosomiasis was frequently observed in patients who had contact with water due to rescue work in flooding (4). The water level hit a historical high at many hydrological stations in middle and lower reaches of Yangtze River in the summer of 2020. The most affected areas were in Jiangxi and Anhui provinces, which were at high risk of schistosomiasis transmission (1). A risk assessment in Jiangxi was conducted for schistosomiasis transmission and provided interventions after flooding. Human stool samples, Oncomelania snails, and animal feces released to the environment were collected in two counties, i.e., Lushan and Lianxi. Although no infected humans were found in the survey, infections in snails and animal feces were discovered from four field sites in Lushan. Our results indicated the intensive surveillance should be implemented post-flooding.
Two neighboring counties, i.e., Lushan and Lianxi, located in the northwest bank of Poyang Lake leading to Yangtze River were selected. The evidence showed that the density of Oncomelania snails increased significantly and exceeded that in the south lake according to survey in 2016 (5). The area drew attention because the only human acute schistosomiasis case and infected cattle case in 2019 was reported in this area (6). Two townships in each county were selected and one village was chosen in each township. About 100 residents who reported contact with water during flooding and another 100 participants without record of water contact in each village were randomly selected in the study. If there was no adequate eligible participant in the village, other potential individuals from neighboring villages in the same township were included. Stool samples of over 90 g per individual were collected for triple detection of S. japonicum miracidia hatching after nylon mesh bag concentration (7). At least three extant habitats of Oncomelania snails and suspected environments impacted by flooding were selected for the snail survey. More than 500 sampling frames (0.11 m2) were set for screening Oncomelania snails in each habitat or environment, and over 1,000 snails were collected. Every 50 snail specimens were pooled for DNA extraction and detected for S. japonicum by loop-mediated isothermal amplification (LAMP) (8). The animal feces released in the surveyed snail habitats were collected. At least 20 samples (all if less than 20) were collected in each habitat. Over 150 g per sample was collected for triple detection of S. japonicum using a miracidial hatching test with a plastic tube. The potential host animals of stool samples were recorded.
The sentinel-1A radar image was characterized around July 12, 2020, when Xingzi hydrological station in Lushan County experienced the highest water level and on October 19 when the present field survey was commenced. The water body was extracted from the image in ENVI 5.1 (Exelis Visual Information Solutions, Boulder CO, USA) and mapped in ArcGIS 10.1 (ESRI, Redlands CA, USA). The images in the same period in 2018 and 2019 were also collected to compare the water coverage between 2020 and the previous 2 years. The test statistic for the occurrence rate of Oncomelania snails in sampling frames, the LAMP-positive rate of snails, and overall schistosomiasis prevalence in animal feces samples in the 2 counties were performed in IBM SPSS Statistics 19.0 (IBM Corp; Armonk NY, USA).
A total of 452 and 403 stool samples from individuals from Lianxi and Lushan counties, respectively, were collected for the survey. No infection was detected in all 5 villages. A total of 19 field sites were visited during the survey, and 11 sites were infested by Oncomelania snails. The mean snail density was 0.09 per frame (0.11 m2) with a range between 0.01 and 0.28. The occurrence rate of Oncomelania snails in sampling frames was 10.17% in Lushan, which was higher than the 4.89% found in Lianxi, in which the difference was statistically significant (P<0.05). A total of 11 and 10 pooled samples of snails were detected for Schistosoma by LAMP in Lushan and Lianxi, respectively, in which only 2 samples from Lushan County were positive. A total of 71 animal-feces samples, including 46 cattle samples, 10 sheep samples, 7 dogs samples, and 8 unknown samples, were collected from 8 visiting sites. All cattle samples were collected from 6 out of 7 visiting sites in Lushan County in which 2 cattle samples were miracidial hatching positive from separated sites located in Jiaotang Township (Figure 1). However, no Oncomelania snail was found from the 2 positive cattle samples.Figure 1. Geographical distribution of visited sites with major findings in Lianxi and Lushan counties, Jiangxi Province, China, October 2020. Note: Snail: Oncomelania hupensis; Feces: animal feces released to environment; *: infected snails or feces with Schistosoma japonica eggs; Human feces: villages where residents’ stool samples were examined for Schistosoma infection; 2020 October: water distribution in October 2020.
Extant snail habitats according to the national survey on Oncomelania in 2016 were mapped. Most habitats were submerged in water until late October 2020 (Figure 1). The time of flood waters receding was remarkably later than that in 2018 (Figure 2A) and 2019 (Figure 2B). The radar image showed there was only little increase in water area in the study area, except for the area located in southwestern Lushan County (Figure 3). Few breaches were occurring in major dams along Poyang Lake during the flooding, although the water level obviously increased compared to the average level between 2018 and 2019.Figure 2. Comparison of water body distribution in north Poyang Lake during the same periods. (A) October, 2020 and October, 2018; (B) October, 2020 and October, 2019. Note: Overlap: the overlap of water body between October, 2020 and (A) October, 2018 or (B) 2019.