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Schistosomiasis remains a public health problem in developing regions, including Africa, Asia, and Latin America. Of three major human-infecting Schistosoma spp., Schistosoma japonicum (S. japonicum) is the only one distributed in China (ibuted in China (1-2). It was endemic in 12 provincial-level administrative divisions (PLADs) in the southern Yangtze River Basin. In 2019, Shanghai, Zhejiang, Fujian, Guangdong, and Guangxi continually maintained the status of schistosomiasis elimination; Sichuan and Jiangsu achieved transmission interruption; and the 5 provinces of Yunnan, Hubei, Anhui, Jiangxi, and Hunan maintained the status of transmission control (3). As the only intermediate host of S. japonicum, the snail’s distribution is strictly consistent with that of schistosomiasis japonica (4), and surveys are an important part of routine work for schistosomiasis prevention and control. To know the characteristics and trends of snail distribution after achieving the standard of transmission control in China, data on snail habitat surveys were collected and analyzed from 2015 to 2019. According to the results, the areas deemed as snail habitats did not decrease during the five years, fluctuating from 356,287.55 hectares (hm2) to 363,069.38 hm2, and newly-detected snail habitats and recurrent snail habitats were found from 2015 to 2019.
In the study, data of the annual snail survey at the provincial level and county level from 2015 to 2019 were collected through the National Parasitic Diseases Control Information Management System (NPDCIMS) among 12 PLADs. All data were transferred to Microsoft Excel software (version 2013, Microsoft Office, CA, USA) for data compilation. Variables reflecting the distribution of snails including the total area of snail habitats, area of newly detected snail habitats (environments with no snails initially), area of recurrent snail habitats (snails being found in environments that previously eliminated snails), and other data from the snail survey were described and analyzed. Descriptive statistics and mapping were conducted to analyze the snail distribution at the provincial and county levels, the environmental type of snail habitats, and the distribution of snail habitats at the county level.
From 2015 to 2019, all counties reporting schistosomiasis conducted snail surveys each year. There were 253 counties (55.85%) with snails infested among 453 counties with schistosomiasis endemic in 2015, compared with 270 out of 450 in 2019 (number declined for the changes of administrative divisions). The proportion of counties with snails increased slightly (Figure 1A). The total areas of snail habitats were 356,287.55 hm2 in 2015 and 362,367.87 hm2 in 2019 (Figure 1B). During the 5 years, the main environmental type of snail habitats was marshland and lake regions (94.65%−96.57%), mountainous and hilly regions (3.40%−5.31%), and plains with waterway networks (0.03%−0.06%). The proportion of snail habitats in mountainous and hilly regions increased gradually (Table 1). Hunan, Jiangxi, and Hubei were the top 3 PLADs for total area of snail habitats in 2019 (Figure 1D), accounting for 47.75% of the total area (173,027.27 hm2/362,367.87 hm2), 23.05% (83,530.52 hm2/362,367.87 hm2), and 18.61% (67,433.58 hm2/362,367.87 hm2), respectively.
Figure 1.Distribution characteristics of snail habitats among the 12 PLADs in China from 2015 to 2019. (A) The changes of county number with snail infested from 2015 to 2019; (B) The change trend of total snail habitat and newly-detected and recurrent snail habitat from 2015 to 2019; (C) The areas of snail habitat in each PLAD in 2019 (hm2); (D) The areas of snail habitat in 12 PLADs in 2015 and 2019 (hm2). Abbreviation: PLADs=Provincial-level administrative divisions.
Year Area of snail habitat Area of newly detected snail habitat Area of recurrent snail habitat Total Marshland and lake Plain Hilly and mountainous region 2015 356,287.57 344,076.05(96.57%) 112.69(0.03%) 12,098.83(3.40%) 666.04 1,094.16 2016 356,834.59 339,174.56(95.05%) 135.62(0.04%) 17,524.41(4.91%) 1,346.48 6,767.50 2017 363,069.47 344,337.41(94.84% 108.965(0.03%) 18,623.10(5.13%) 208.54 2,299.19 2018 363,014.40 343,874.04(94.73%) 220.92(0.06%) 18,919.44(5.21%) 61.28 1,281.48 2019 362,367.87 342,987.74(94.65%) 154.10(0.04%) 19,226.03(5.31%) 64.20 851.24 Note: the percentage in brackets stands for the percent of each type of snail habitat among the total snail habitat in the same year. Table 1. The changes of different environmental snail habitats, newly detected and recurrent snail habitats in China from 2015 to 2019 (hm2).
Area of snail
habitat(hm2)Number of counties Shanghai Jiangsu Zhejiang Anhui Fujian Jiangxi Hubei Hunan Guangdong Guangxi Sichuan Yunnan Total ≤1 7 39 44 12 13 14 16 8 12 18 18 8 209 1.01–100 1 19 10 8 3 9 9 7 2 2 20 7 97 100.01–1,000 0 6 0 21 0 5 20 10 0 0 24 3 89 1,000.01–10,000 0 0 0 9 0 7 18 11 0 0 1 0 46 >10,000 0 0 0 0 0 4 0 5 0 0 0 0 9 Total 8 64 54 50 16 39 63 41 14 20 63 18 450 Table 2. Levels on areas of snail habitat among counties with schistosomiasis endemic in 12 provincial-level administrative divisions in 2019.
From 2015 to 2019, 2,346.54 hm2 areas of snail habitats were newly found among the 12 PLADs. The newly detected areas with snails were found in 9 PLADs except Fujian, Guangdong, and Sichuan. Among the 9 PLADs, Anhui accounted for 64.06% (1,503.31 hm2/2,346.54 hm2) and Hunan accounted for 29.36% (689.02 hm2/2,346.54 hm2). A total of 12,293.57 hm2 of snail habitats were recurrent among the 12 PLADs at the same time. Sichuan accounted for 29.89% (3,674.79 hm2/12,293.57 hm2), Jiangxi accounted for 19.63% (2,413.37 hm2/12,293.57 hm2), Yunnan accounted for 17.69% (2,175.10 hm2/12,293.57 hm2), and Anhui accounted for 12.75% (1,568 hm2/12,293.57 hm2). In addition, more newly detected and recurrent snail habitats were found in 2016 than any other years during the 5 years.
According to the latest data in 2019, among the 450 counties with endemic schistosomiasis, there were 9 counties with area of snail habitats exceeding 10,000 hm2, including 5 counties in Hunan (Yuanjiang, Hanshou, Yueyang, Xiangyin, and Junshan) and 4 counties in Jiangxi (Duchang, Poyang, Nanchang, and Yugan). The total area of snail habitats among these 9 counties were 172,907.34 hm2, accounting for 47.72% (172,907.34 hm2/362,367.87 hm2) of the total area of snail habitats; the area of snail habitats in 46 counties were between 1,000 hm2 and 10,000 hm2, including 9 counties in Anhui, 7 counties in Jiangxi, 18 counties in Hubei, 11 counties in Hunan, and 1 county in Sichuan; the area of snail habitats in 89 counties were between 100 hm2 and 1,000 hm2, the area in 97 counties were between 1 hm2 and 100 hm2, and in 209 counties were less than 1 hm2 or no snails detected (Table 2).
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