Distribution of O. hupensis snails around Poyang Lake areas
Results of a snail survey carried out in in Jiangxi Province in 2016 found snail habitats mainly distributed in 13 marshland and lake counties (cities, districts), including Nanchang, Xinjian, Jinxian, High-tech Zone of Nanchang City, Yongxiu, Gongqingcheng, Lushan, Lianxi, Hukou, Duchang, Poyang, Yugan and Wannian. A total of 1,257 habitat settings were identified, with marshlands consisting of 74.94% of all identified habitat settings. Among 763 snail habitats identified, 99.48% were marshland areas. In a marshland area covering 126,756.75 hm2, snail habitats accounted for78,900.89 hm2. Snails were detected in 519 settings with a density of living snails ranging from 0.000,1 to 6.497,2 snails/0.1 m2 and 0.01% to 84.44% occurrence within surveyed frames. Among these, 12 settings were identified with a density of living snails at 1 snail/0.1 m2 and greater located in northern coast of the Poyang Lake (seven settings in Lushan, four in Hukou and one in Yongxiu), and eight settings with 50% and higher occurrence of frames with living snailslocated in northern coast of the Poyang Lake (five settings in Lushan, two in Yongxiu and one in Hukou) (Table 1).
Environmental identification for suitable snail habitats
NDVI values were estimated with a range of -1 to 0.61 in the Poyang Lake area based on Landsat 8 remote sensing image data. A total of 75% of snail distribution sites were randomly sampled from snail habitats identified by field surveys as a training dataset to extract the optimum thresholds of the maximum and minimum value of NDVI, with 95% confidential intervals from 0.08 to 0.59. Figure 2 shows the distribution of suitable snail habitats.
Extraction of flood-cased water body expansion
Radar echo intensity was determined by brightness in SAR data. Because of low echo intensity in water bodies and high echo intensity in corresponding to land areas, water body areas in SAR images appeared as dark or black and the land areas as grayish white or dark grey. Pre-processed SAR images from Sentinel-1B satellite images for the time period before and after flooding are shown in Figure 3. The speckle noise was effectively inhibited, and a more obvious water-land boundary was seen on original radar images in which the water and land were well differentiated and the water profile was more distinct.
Figure 4 shows a histogram of pre-processed SAR image scattering values before and after flooding. There are two apparent peaks in the histograms in images presented in Figure 4, with segmented SAR image thresholds of between -25.5 and -24 dB before observed by the visual interpretation before and after flooding.
Segmentation results were saved as classification results and transformed to a vector file. Following post-classification data processing, incorrect extraction images were removed through a human-computer interaction to generate a final water body dataset for the Poyang Lake area for the May 15 and July 14, 2020 time period.
Changes in water body areas before and after the flood disaster
The distribution of water bodies in the Poyang Lake area was overlapped before and after flooding. The blue areas in Figure 5 indicate the distribution of water bodies before the flood, and red areas describe the expansion of water bodies during flooding. Examination of the main body of the Poyang Lake and neighboring water areas from Sentinel-1B SAR images showed that the water area was approximately 2,207 km2 on May 15 and 4,403 km2 on July 14, an increase of 2,196 km2 compared to May and an increase of 25.4% on the historical mean level during the same period (3,510 km2). The water body expanded by approximately 99.5% after flooding relative to the water body areas before flooding, mainly identified in Xinjian, Duchang, Poyang, Yongxiu and Yugan (Figure 5).
Risk Predictions of potential snail diffusion and associated schistosomiasis transmission after the flood
Data indicating flood-affected water areas were transformed into a binary image of potential snail habitat distribution. Areas of predicted snail diffusion exhibited a patchy clustered distribution. After submersion of snail habitats following flooding, snail habitats were likely to be in neighboring settings. Snail distribution was predicted to cover an area of approximately 759 km2, mainly occurring in the east of Yongxiu, south of Lushan, southwestern Poyang, southwestern Duchang, northwestern Xinjian and northwestern Yugan. This suggested that areas of the possible snail diffusion were predominately concentrated in marshlands around Poyang Lake (Figure 6).
NDVI values of suitable snail habitats were calculated based on snail density data, with values ranging from 0.15 to 0.35 in high density snail habitats, 0.35 to 0.42 in medium density snail habitats, and from 0.08 to 0.15 greater than 0.42 in low density snail habitats. NDVI values of flood-affected areas were estimated and the risk of potential snail spread classified accordingly. We found areas at high risk of snail distribution predominantly located in northwestern Yongxiu, southwestern Duchang, south of Lushan and southwestern Poyang (Figure 7). These high risk areas are also indicative of neighboring areas suitable for snail breeding where snail habitats are likely to emerge following flooding, and with potential for schistosomiasis transmission. Validation of predicted snail habitats was carried out using observational data on snail breeding habitats in order to assess the predictive performance of models NDVI value of snail habitats ranged from 0.1 to 0.52 using the 25% validation dataset, with 87% prediction accuracy indicating that the NDVI values are a good predictor of snail habitats.
Multiple dykes were observed to collapse following flooding in Jiangxi Province. Three sites in Yongxiu and Poyang counties where dykes had collapsed were found to overlap with areas classified as snail distribution risk areas. The sites in Yongxiu and Poyang County where dykes were observed to have collapsed was predicted to be medium risk areas of snail distribution, (Figure 7), suggesting that snail habitats were likely to emerge in both sites.