The Impact of Flooding on Snail Spread: The Case of Endemic Schistosomiasis Areas in Jiangxi Province, China.

Flooding is the main natural factor in snail diffusion, and it has a negative impact on schistosomiasis transmission. There are few studies on the spread and migration of snails following a flood; therefore, we aimed to investigate the influence of flooding on snail diffusion and explore the characteristics and laws of snail diffusion in Jiangxi Province. By using a retrospective survey and cross-sectional survey, the data on snail spreading in Jiangxi Province from 2017 to 2021 were collected. The distribution, nature, and area of snail spread were systematically analyzed in combination with the hydrological situation, types of region, and types of flood. From 2017 to 2021, a total of 120 snail-spread environments were found, including in 92 hilly areas and in 28 lake areas. The areas caused by flood and by other means numbered 6 and 114, respectively. The proportions of recurrence, expansion, and first-time occurrences were 43.42%, 38.16%, and 18.42%, respectively, and the 14 new snail environments were only distributed in the hilly areas. With the exception of 2018, the ratio of snail-spread areas in the hilly region was higher than that in lake region in other years. The average density of live snails was 0.0184-1.6617 no./0.1 m2 and 0.0028-0.2182 no./0.1 m2 in the hilly region. Among the 114 environments affected by floods, 86 consisted of hilly environments, including 66 spreading environments affected by rainstorm floods, and 20 rainstorm debris flow environments. There were 28 lake areas, of which 10 were in the Jiangxi section of Yangtze River and were affected by rainstorm floods. Snail spread following flooding has a certain 'lag effect,' and = simple annual changes in hydrological characteristics have little effect on the diffusion of snails or on their density = in the affected environment, but it is more closely related to local floods. The hilly environments are more susceptible to floods than the lake region, and the risk of snail spread is much higher in the hilly than in the lake region.

Impact of continuous low water stage on the breeding environment of Oncomelania hupensis: a case study of Poyang Lake area in China.

BACKGROUND: Oncomelania hupensis is the only intermediate host of Schistosoma japonicum and plays a decisive role in its transmission. The variation of water level greatly affects the reproduction and growth of snails. Therefore, in this paper, we analyze the variations of water level in the Poyang Lake region from 1993 to 2016 combined with satellite imagery to elucidate the evolution of the snail breeding environment. METHODS: By employing remote sensing data from 1993 to 2016 (April-June and September-November), the vegetation area of Poyang Lake and the vegetation area at different elevations were extracted and calculated. Moreover, the average daily water level data from the four hydrological stations (Hukou station, Xingzi station, Tangyin station and Kangshan station) which represent the typical state of Poyang Lake were collected from 1993 to 2016. The variance of the monthly mean water level, inundation time and the average area were analyzed by variance to find a significance level of α = 0.05. RESULTS: According to hydrological data before and after 2003, the average water level after 2003 is significantly lower than that before 2003 in Poyang Lake. After 2003, the time of inundateing the snail breeding period was later in April to June than that before 2003, while the time of wate-falling stage in September to November moved forward after 2003 than before 2003. Of them, the lowest water level affecting the breeding and growing period of O. hupensis in the northern part of Poyang Lake decreased from 11 m to 9 m. After 2003, the expansion of meadow area in the north part of Poyang Lake was mainly concentrated in the elevation of 9-11 m, and the newly increased infested-meadow in the lake area was mainly concentrated in the north part of Poyang Lake. CONCLUSIONS: By comparing the change of water level characteristics in different parts of the Poyang Lake area as well as changes in meadow area before and after 2003, it is found that the water level changes mainly affect the snail breeding area in the northern part of Poyang Lake. The results are helpful for improving scientific measures for snail control in Jiangxi Province. This approach could also be applicible to Dongting Lake area and other lake areas affected by water level changes and can bring significant guidance for snail control in lake areas.

Distribution pattern of the snail intermediate host of schistosomiasis japonica in the Poyang Lake region of China.

BACKGROUND: With the closure of the Three Gorges Dam in 2003 the hydrology of Poyang Lake was altered dramatically leading to significant changes in the environment. In order to assess the impact on schistosomiasis this study assessed the spatial and temporal patterns of the snail intermediate host, Oncomelania hupensis in the Poyang Lake tributaries. The results of the study have important implications for future snail control strategies leading to disease elimination. METHODS: The marshland area surrounding Poyang Lake was divided randomly into 200 × 200 m vector grids using ArcGIS software, and the surveyed grids were randomly selected by the software. The snail survey was conducted in each selected grid using a survey frame of 50 × 50 m with one sideline of each grid serving as the starting line. No less than ten frames were used in each surveyed grid with Global Positioning System (GPS) recordings for each. All snails in each frame were collected to determine infection status by microscopy. Altitude data for all frames were extracted from a lake bottom topographic map in order to analyze the average altitude. All snail survey data were collected and statistically analyzed with SPSS 20.0 software in order to determine the difference of the percentage of frames with living snails and mean density of living snails in different regions of Poyang Lake. The altitude of the snail-infested marshlands and snail dens were subsequently identified. RESULTS: A total of 1159 potential snail sampling grids were surveyed, of which 15 231 frames (0.1 m2/frame) were investigated. 1241 frames had live Oncomelania snails corresponding to 8.15% of the total number of frames. The mean density of living snails was 0.463/0.1 m2 with a maximum of 57 snails per frame. The percent of frames with snails in the southern sector (8.13%) of Poyang Lake did not differ statistically from the north (8.21%). However, the mean density of live snails in the northern sector (0.164/0.1 m2) of the lake was statistically higher (F = 6.727; P = 0.010) than the south (0.141/0.1 m2). In the south of the lake, the elevation of snail-inhabited marshland ranged between 11 - 16 m, and could be further subdivided into two snail-concentrated belts at 12-13 m of elevation and 15-16 m of elevation respectively. In the north of the lake, the elevation of snail-inhabited marshland ranged between 9- 16 m with the elevation of 12-14 m being the snail-concentrated zone. CONCLUSIONS: The elevation of snail-infested marshlands in the Poyang Lake region ranged from 9 to 16 m. The snail distribution and habitat has moved north of the lake and to a lower altitude due to changes in the water level post dam closure. Based on the current geological features of the snail habitant focused mollusciciding should occur in snail dense northern regions with frequent bovine and human traffic. Targeting these identified 'hotspots' of transmission will assist in elimination efforts.

[Growth and decline of Oncomelania hupensis status in Duchang County at North shore of Poyang Lake, 2005-2012].

OBJECTIVE: To understand the dynamic of Oncomelania hupensis snail status in Duchang County at north shore of Poyang Lake, so as to provide the evidence for formulating the strategy of schistosomiasis control. METHODS: The data about the snail status in Duchang County from 2005 to 2012 were collected, and the change trend of the snail status was analyzed. Results From 2005 to 2012, all the average density of living snails, the occurrence rate of frames with living snails, the density of infected snails and the area with infected snails showed a trend of sharp decline, and stayed at a low level. Compared to 2005, the above 4 indexes in 2008 decreased by 70.06%, 90.33%, 79.48%, and 37.88%, respectively, and those in 2012 decreased by 99.35%, 98.98%, 99.33%, and 93.84%, respectively. The infection rates of snails fluctuated between 0.06% and 0.53%, which showed a trend of first decrease and then increase. The snail area maintained at the level of 2052.5 hm2 since 2007. CONCLUSIONS: Most of the indexes of the snail status in Duchang County have decreased to the lowest level in history since 2012, but the snail area is stable and the infection rate of snails shows a fluctuation trend, which suggests that the risks of schistosomiasis transmission still exist, and the snail control still should be strengthened.

[Natural environment and schistosomiasis transmission in Poyang Lake region].

Schistosomiasis is closely related to natural environmental factors. The changes of environmental factors, such as temperature, hydrology, vegetation, soil etc., all impact the scope and extent of schistosomiasis transmission. Poyang Lake is the largest freshwater lake and one of the major endemic areas with schistosomiasis in China. With global warming, the implementation of the Three Gorges Dam operation, and the Poyang Lake Ecological Economic Planning, the natural environment in Poyang Lake area has been and will continue to change, especially, the water environment and climate environment, which are more closely related to the schistosomiasis transmission. These changes, to some extent, have affected and will continue to affect the prevalence and transmission of schistosomiasis. This article reviews the relationship between the natural environment and its changes and schistosomiasis transmission in the Poyang Lake region.

[Assessment and analysis of risks of realizing schistosomiasis transmission control in Jiangxi Province].

OBJECTIVE: To evaluate the situation of realizing schistosomiasis transmission control in Jiangxi Province. METHODS: The situation of reaching the criteria of schistosomiasis transmission control was evaluated by using the method of field surveys combined with retrospective investigations. RESULTS: The schistosome infection in human was kept at a stable low level, and the infection rate in residents was below 1% in 90.24% (536/594) of whole epidemic controlled villages. There were 45 spots with schistosome infected Oncomelania hupensis snails in 38 pieces of marshland. The epidemic situation in livestock showed less optimistic than that in human, and the infection rate in bovine was higher than 1% in 19.87% (118/594) of the whole epidemic controlled villages. CONCLUSIONS: Currently, the prevalence of schistosomiasis is at a low level in 9 infection-controlled counties of Jiangxi Province. More favorable situation has emerged to the realization of schistosomiasis transmission control. However, it is necessary to strengthen the infectious sources control with emphasis on bovine so as to achieve the goal of transmission control in whole province in 2015.

[Prevalence and control of schistosomiasis in Jiangxi Province during 2002-2012].

OBJECTIVE: To analyze the status of prevalence and control of schistosomiasis in Jiangxi Province from 2002 to 2012. METHODS: Endemic status and control effectiveness during the period from 2002 to 2012 was analyzed by using a combination of field survey, data collection and retrospective investigation. RESULTS: From 2002 to 2012, schistosomiasis control made significant achievements in Jiangxi Province. The number of counties which have reached the criteria of transmission interruption of schistosomiasis increased from 19 in 2002 to 22 in 2012. The number of patients with schistosome infection decreased from 128 331 in 2002 to 73 102 in 2012. The positive rate of serological test and fecal examination in residents decreased from 12.2% and 4.2% in 2002 to 5.2% and 0.4% in 2012, respectively. Acute infection reduced from 146 cases in 2002 to 3 cases in 2012, but advanced cases increased by 42.4%. The positive rate of fecal examination in cattle reduced from 4.7% in 2002 to 1.3% in 2012. The density of living snails and infected snails significantly decreased to 0.083 5/0.1 a? and 0.000 037/0.1 m2 in 2012, respectively. However, the snail-ridden areas slightly increased. Jiangxi Province reached the criteria of endemic control for schistosomiasis in 2008. During 2002-2012, control activities were intensified. Examination and chemotherapy for human increased by 65.2% and 65.5%, while 23.4% and 251.1% for cattle. Mean while, area with snail control activities increased by 617.8%. An integrated strategy of infection source control was implemented in the Province. CONCLUSION: During 2002 to 2012, endemic index of schistosomiasis in Jiangxi showed a steady decline after 2005. Currently, schistosomiasis prevalence shows a low level. The endemic indicators of infection in human and livestock as well as Oncomelania snails have been kept stable at low level.

[The changes of hydrological regime in Poyang Lake after runs of Three Gorges Project and its impact on prevalence of schistosomiasis in the lake region].

OBJECTIVE: To clarify new features of hydrological regime in the Poyang Lake after runs of the Three Gorges Project and its impact on the prevalence of schistosomiasis in this region. METHODS: It is collected information of Three Gorges Project, average daily water level from 2002 to 2012 at Duchang hydrological station in Poyang Lake and the data of prevalence and control of schistosomiasis in 12 endemic counties along the lake. It was analyzed the trends of prevalence of schistosomiasis under the changed condition of hydrologic situation. RESULTS: Effects of impoundment of Three Gorges reservoir running on water level of Poyang Lake are mainly shown in: Water levels of the lake are suppressed throughout the year which was characterized by high water level in flood season dropped, lower water level appeared in the spring, water withdrew earlier and quicker in the fall and to an even lower level in dry season. After impoundment of Three Gorges Project, infection rate of Schistosomajaponica in human and in bovine decreased by 92.65% and 76.37% respectively in endemic areas around the lake. The density of Oncomelania snails decreased by 94.21%. Prevalent of schistosomiasis in this region showed a significant downward trend. CONCLUSIONS: After runs of the Three Gorges Project, regularized changes hydrologic conditions has taken place in Poyang Lake which is characterized by lower water level and thus might lead to lower density of Oncomelania snails on marshlands and reducing of prevalence of schistosomiasis. Significant strengthening of schistosomiasis control efforts might also contributed to the stable and low level of prevalence of schistosomiasis in the region.

[Retrospective investigation on changes of endemic situation before and after reaching criteria of schistosomiasis transmission controlled or interrupted in hilly endemic areas of Jiangxi Province].

OBJECTIVE: To investigate the changing rules of schistosomiasis endemic situation before and after reaching the criteria of schistosomiasis transmission controlled or interrupted in hilly endemic areas of Jiangxi Province, so as to provide the evidence for reformulating the criteria of schistosomiasis control and eradication in the future. METHODS: In the hilly areas of schistosomiasis endemic in Jiangxi Province, 2 counties where the transmission has been interrupted and 1 county where the transmission has been controlled were selected and investigated with the retrospective research method. The endemic detailed data were collected and recorded 10 years before reaching the criteria of transmission interrupted/controlled, and several years after reaching the criteria (ending in 2008), and then a database was established. The changing rules of endemic situation before and after reaching the criteria of transmission interrupted/controlled were analyzed and compared. RESULTS: After reaching the criteria of transmission controlled, in the 3 counties, Guangfen, Shangrao and Dean, the declined rates of areas with Oncomelania hupensis snails were 96.79%, 98.99%, and 99.77% respectively. The snail density maintained a lower level, and 95% of infected persons and cattle were cured. The average time from transmission controlled to the transmission interrupted was 17 years in Guangfen County and 26 years in Dean County. However, in Shangrao County, the snail situation rebounded due to the snail re-found and spread although the schistosomiasis morbidity of population/animals maintained stably. CONCLUSIONS: After reaching the criteria of transmission interrupted/controlled, the remained snails were easy to re-find and spread under some certain condition, which is one of main obstacles for reaching the criteria of transmission interrupted. In an isolated snail unit, if the snail area and snail density are controlled in a very low level, it is still difficult to transmit and spread schistosomiasis even if there exist infectious sources.

[Retrospective investigation on national endemic situation of schistosomiasis. II. Analysis of changes of endemic situation in transmission-controlled counties].

OBJECTIVE: To summarize and analyze the process of the changes of schistosomiasis infection and Oncomelania snails in schistosomiasis transmission-controlled areas to explore the rules of the impact of changes of snail indexes on human infection before and after the stage of transmission control, so as to provide the evidence for editing snail indexes in the schistosomiasis controlled criterion and a more scientific, standardized control assessment and evaluation. METHODS: Twenty-one schistosomiasis transmission-controlled or transmission-interrupted counties in lake endemic areas and hilly endemic areas in 8 provinces were selected and investigated retrospectively to collect schistosomiasis epidemiological information 10 years before these counties reached the criterion of transmission control and the subsequent years until 2008 or 2009. A " Standard county schistosomiasis epidemic survey data review software" was developed to establish a national epidemic retrospectively database; the trends of changes of snail status, infected snail status and human infection status were analyzed in different years. The trends and rules of the changes of snail areas, emerging snail areas, living snail densities, infected snail densities, snail infection rates and human infection rates were analyzed and compared before and after the counties reached the schistosomiasis transmission-controlled criterion in different types of endemic areas. RESULTS: Before the transmission control, the human infection rate presented a declining trend in endemic areas. In lake endemic areas, the human infection rate declined to below 5% 4 years before the transmission control; in hilly endemic areas, the human infection rate declined to below 3% 8 years before the transmission control, and the human infection rates of the lake and hilly endemic areas declined to 2.10% and 1.45%, respectively, at the year of transmission control. However, 3 to 4 years after transmission control, the disease began to rebound, especially in lake endemic areas. The snail areas and snail densities declined year by year 10 years before the transmission control in the lake and hilly endemic areas, and the proportion of snail areas occupying the historic snail areas declined the lowest, being 2.75% and 0.55%, respectively, at the year of transmission control. The average densities of living snails kept a low level from 3-4 years before the transmission control to 9-10 years after the transmission control, and then rebounded. The new snail areas appeared 6-7 years after the transmission control and the snail areas and densities of living snails rebounded 9-12 years after the transmission control. CONCLUSIONS: The snail status can maintain a relatively low level during about 10 years after the transmission control in the lake and hilly schistosomiasis endemic areas, but the schistosomiasis status rebounded 5 years after the transmission control. While the comprehensive measures mainly including infectious source controlled are implemented, the snail surveillance and control should be still strengthened, and it is recommended that "the proportion of snail areas decreased" and "the control of infected snails" would be the indicators of the criterion of schistosomiasis transmission control.

[Retrospective investigation on national endemic situation of schistosomiasis I analysis of changes of endemic situation in transmission-interrupted counties].

OBJECTIVE: To analyze the changing rules of schistosomiasis endemic situation in the area of transmission that has been interrupted before and after they reach the criteria of transmission interruption, so as to offer the basis of amending to the criteria of schistosomiasis transmission interruption and a more scientific, standardized assessment and evaluation of the effects of schistosomiasis control in the future. METHODS: Nineteen counties of transmission that has been interrupted in 9 provinces nationwide were selected and investigated with the retrospective research method to collect and record the endemic detailed data 10 years before they reach the criteria of transmission interruption and several years later (ended in 2008 or 2009) and then a database was established. The changing rules of various disease indices in different endemic areas before and after reaching the criteria of transmission interruption were analyzed and compared. RESULTS: The average time from the transmission control to the transmission interruption was 17 years in the 17 counties. After reaching the criteria of transmission interruption, the infection rates of people turned down to a minimum level in various endemic areas, the infection rates in a few numbers of lake endemic areas and hilly endemic areas increased slightly after the transmission interruption 4-9 years later, but all of them were below 1%. The densities of living Oncomelania snails in lake endemic areas and water-network endemic areas were high and the changes were great each year, and the densities of living snails were much lower in hilly endemic areas than in the above two types of areas and they came to the lowest 4 years before and after the transmission interruption. The infected snails appeared occasionally in lake endemic areas, and they recurred 6 years and 10 years after the transmission interruption in water-network endemic areas and hilly endemic areas, respectively. The continued time without infected snails found before reaching transmission interruption was 2.71 +/- 1.10 years averagely in transmission-interrupted counties with endemic stable, and was 3. 80 +/- 1.43 years in lake endemic areas. CONCLUSIONS: It is possible to maintain a low level of population schistosome infection, and the endemic rebound is shown as snail rebound after the endemic areas reach the criteria of transmission interruption, The infected snails could be the comprehensive index reflecting the control achievement and the risk of schistosomiasis transmission. So the persistent control of infected snails is the fundament of schistosomiasis transmission interruption. It is suggested that in our current social and economic development and science and technology capacity conditions, the status of no infected snails found continuously for 5 years would be one of the criteria of schistosomiasis transmission interruption.

[Retrospective investigation on national endemic situation of schistosomiasis. III. Changes of endemic situation in endemic rebounded counties after transmission of schistosomiasis under control or interruption].

OBJECTIVE: To analyze the changes of schistosomiasis endemic situation before and after reaching the criteria of schistosomiasis transmission control or transmission interruption in endemic rebounded areas, so as to offer the information for modifying the criteria of schistosomiasis control and elimination, and consolidating achievements of schistosomiasis control in People's Republic of China. METHODS: Twelve counties with endemic rebound were selected from seven endemic provinces and investigated with the retrospective research method. The data including the epidemic information and relative materials 10 years before the counties reached the criteria of transmission control and several years later (until 2008 or 2009) were collected. The changes of epidemic situation of these counties before and after the transmission was under control or interrupted, and the factors causing the endemic rebound were analyzed. RESULTS: Counties consolidated the endemic situations with an average time of 5 +/- 3 years after the transmission control, with endemic rebound normally caused by single factor. The ratio of Oncomelania snail areas to historically accumulative snail areas increased over 10% in hilly endemic regions and lake endemic regions in 7 and 12 years separately, while it was stabilized in a low level in water-network endemic areas. The rebound peak time of infected snail densities was consistent with or later than that of densities of living snails. The prevalence of schistosome infection in humans in lake endemic regions rebounded 2 years after transmission under control. The transmission interruption counties consolidated the endemic situations with an average time of 7 +/- 4 years. The human prevalence of schistosome infection decreased to a very low level after the counties reaching the criteria of transmission interruption. The snail areas increased to over 2% of historically accumulative snail areas 3-6 years after transmission interruption in lake, water-network endemic areas successively, while densities of living snails and infected snails were increased at the same year or 2-3 years later. CONCLUSIONS: The endemic rebound in regions after the transmission under control or interrupted is caused by biological, natural and social factors and mainly presents as the rebound of snail status. The modification of criteria for schistosomiasis control and elimination should consider the influence of variation of snails, snail areas and density of infected snails on endemic situations in different endemic areas comprehensively. Sensitive and valid surveillance and forewarning system should be established to strengthen the monitoring and consolidating endemic situation in endemic areas after the transmission under control or interrupted.