Three Gorges Dam: Potential differential drivers and trend in the spatio-temporal evolution of the change in snail density based on a Bayesian spatial-temporal model and 5-year longitudinal study

BACKGROUND

Snail abundance varies spatially and temporally. Few studies have elucidated the different effects of the determinants affecting snail density between upstream and downstream areas of the Three Gorges Dam (TGD). We therefore investigated the differential drivers of changes in snail density in these areas, as well as the spatial-temporal effects of these changes.

METHODS

A snail survey was conducted at 200 sites over a 5-year period to monitor dynamic changes in snail abundance within the Yangtze River basin. Data on corresponding variables that might affect snail abundance, such as meteorology, vegetation, terrain and economy, were collected from multiple data sources. A Bayesian spatial-temporal modeling framework was constructed to explore the differential determinants driving the change in snail density and the spatial-temporal effects of the change.

RESULTS

Volatility in snail density was unambiguously detected in the downstream area of the TGD, while a small increment in volatility was detected in the upstream area. Regarding the downstream area of the TGD, snail density was positively associated with the average minimum temperature in January of the same year, the annual Normalized Difference Vegetation Index (NDVI) of the previous year and the second, third and fourth quartile, respectively, of average annual relative humidity of the previous year. Snail density was negatively associated with the average maximum temperature in July of the previous year and annual nighttime light of the previous year. An approximately inverted "U" curve of relative risk was detected among sites with a greater average annual ground surface temperature in the previous year. Regarding the upstream area, snail density was positively associated with NDVI and with the second, third and fourth quartile, respectively, of total precipitation of the previous year. Snail density was negatively associated with slope.

CONCLUSIONS

This study demonstrated a rebound in snail density between 2015 and 2019. In particular, temperature, humidity, vegetation and human activity were the main drivers affecting snail abundance in the downstream area of the TGD, while precipitation, slope and vegetation were the main drivers affecting snail abundance in the upstream area. These findings can assist authorities to develop and perform more precise strategies for surveys and control of snail populations.

[Molluscicidal effects of different formulations of niclosamide ethanolamine salt in marshlands]

OBJECTIVE

To evaluate the molluscicidal effects of different formulations of niclosamide ethanolamine salt in marshlands.

METHODS

The molluscicidal effects of spraying with 25% suspension concentrate of niclosamide ethanolamine salt (25% SCN) and 50% wettable powder formulation of niclosamide ethanolamine salt (50% WPN), and dusting with 4% niclosamide ethanolamine salt dustable powder (4% DP) for controlling Oncomelania hupensis snails were investigated and compared in the fields, and the cost-effectiveness was analyzed.

RESULTS

The corrected mortalities and the reduced rates of density of snails were 54.37%, 91.70%, 92.76%, 79.50%, and 59.55%, 95.93%, 97.63%, 94.15%, respectively, on 3, 7, 15, 30 d after spaying with 25% SCN, those on 3, 7, 15, 30 d after dusting with 4% DP were 59.10%, 91.83%, 95.56%, 93.34% and 65.03%, 94.93%, 97.61%, 97.28%, respectively; and those on 3, 7, 15, 30 d after spraying with 50% WPN were 76.29%, 91.68%, 93.12%, 81.59% and 81.24%, 97.02%, 97.84%, 95.27%, respectively. The cost of spraying with 25% SCN was 0.21 Yuan/m², that of dusting with 4% DP was 0.39 Yuan/m², and that of spraying with 50% WPN was 0.23 Yuan/m² for snail control in the marshland. The cost of reduced one percentage of the corrected mortalities and the density of snails in controlling snails by 25% SCN, 4% DP and 50% WPN on 15 d were 22.68, 40.63, 25.17 Yuan and 21.54, 39.78, 23.95 Yuan, respectively.

CONCLUSIONS

The three different formulations of niclosamide are reliable and effective for snail control in marshlands. There are some differences among the different molluscicides in start time, pharmacodynamic characteristics, spraying methods in the field, cost of snail control, and influencing factors. Therefore, we need reasonably select the suitable molluscicides according to the environmental characteristics and working condition.

[Surveillance of schistosomiasis in People's Republic of China in 2015]

OBJECTIVE

To analyze the investigation data of the national schistosomiasis surveillance sites in 2015, so as to provide scientific evidences for schistosomiasis control, elimination and surveillance.

METHODS

According to National Schistosomiasis Surveillance Programme (version 2014), 457 surveillance sites were selected, and the investigation data in residents, floating population, domestic animals and Oncomelania hupensis snails were collected and analyzed from four types of endemic counties.

RESULTS

A total of 4 468 seropositive cases were detected from 133 350 residents, among which 4 457 residents with seropositive results received the etiological tests, and 71 of them were identified with positive results. Most of them were fishermen and farmers in the middle and old-aged group. The schistosomiasis infection rate was 0.05% in local population. Totally 977 seropositive cases were examined from 85 047 migrant individuals, and 16 positive cases were found out from 966 individuals who took etiological tests, which showed the schistosomiasis infection rate was 0.02% in floating population. Imported cases were found among floating people in four provinces, namely Zhejiang, Hunan, Hubei and Anhui provinces. No acute schistosomiasis cases were reported. A total of 13 406 head of cattle received examinations and only 5 were determined as stool positives. The cattle infection rate was 0.04%. The snail survey covered an area of 22 295.13 hm² and snails were found in an area of 7 426.63 hm², including 3.47 hm² newly detected area with snails. No schistosome-infected snails were found.

CONCLUSIONS

Based on the collected data from 457 national schistosomiasis surveillance sites of China, the Schistosoma japonicum infection rate is 0.05% in local population which maintains a stably descending trend. In floating population, there are imported schistosome-in-fected persons. Cattle are still a vulnerable species infected with schistosome. Although no infected snails are found, snails are widely distributed in endemic areas. Some provinces detect areas with snails for the first time or the reproduction of snails. The staff in endemic provinces should carry out the surveillance work according to National Schistosomiasis Surveillance Programme (version 2014) to improve the surveillance system, and enhance the sensibility and effectiveness of surveillance work.

[Effects and cost of four formulations of niclosamide ethanolamine salt in Oncomelania hupensis snail control in field]

OBJECTIVE

To investigate the effects and cost of four formulations of niclosamide ethanolamine salt in Oncomelania hupensis snail control in the field in marshland and lake regions, so as to provide the evidence for drawing up the plan of molluscicide using in schistosomiasis endemic areas.

METHODS

One drainage channel and one channel without water in the same area with snails in Jiangling County, Jingzhou City were selected as the research fields. The drainage channel was divided into 9 sections, except one section as a blank control group where the natural death rate of snails was observed only, and the remaining 8 sections were taken as the observation groups, where different dosages of 4% niclosamide ethanolamine salt powder, 5% niclosamide ethanolamine salt granules, 25% niclosamide ethanolamine salt suspending agent, 26% metaldehyde and niclosamide ethanolamine salt suspending agent, and 50% niclosamide ethanolamine salt wettable powder were used respectively. The channel without water were divided into 4 sections, except one section as a blank control group, the other 3 segments were taken as the observation groups, where 4% niclosamide ethanolamine salt powder, 5% niclosamide ethanolamine salt granules, and 50% niclosamide ethanolamine salt wettable powder were used respectively. Before and after spraying molluscicide for 7 days and 15 days, the system sampling method was used to observe the effects of snail control. Meanwhile, the unit cost method was used to calculate the costs of the different mulluscicide formulations abovementioned in unit area (1 m²).

RESULTS

In the field at the drainage channel, the snail mortality rates of the groups spraying 4% niclosamide ethanolamine salt powder (50 g/m²), 5% niclosamide ethanolamine salt granules (40 g/m²), 25% niclosamide ethanolamine suspending agent, 26% metaldehyde and niclosamide ethanolamine salt suspending agent, and 50% niclosamide ethanolamine salt wettable powder (2 g/m² and 4 g/m²) for 7 days were 79.52%97.87%, while the rates after spraying for 15 days were 71.00%-96.30%, and compared with those before spraying, the differences were statistically significant (all P < 0.01). For the groups spraying with 2 g/m² or 4 g/m² suspending agent as well as wettable powder for 7 days, the snail mortality rates were significantly different (both P < 0.05). In the field at the channel without water, the snail mortality rates of the 3 observation groups after spraying molluscicide for 7 days were 97.14%-100%, while for 15 days were 94.32 %-100%, and compared with the rates before spraying, all the differences were statistically significant (all P < 0.01). The unit costs per 1 m² of the molluscicide abovementioned were ranged from 0.280 Yuan to 0.416 Yuan.

CONCLUSIONS

In marshland area inside embankment, the molluscicide formulations of the powder and granule are suitable for the environments without water or with instability water level, while the molluscicide formulations of the suspended agents and wettable powder are suitable for the water environment. Though the unit cost of powder is the lowest, the molluscicide in this formulation flies away seriously.

[Risk analysis of imported schistosomiasis in Shanghai City from 2005 to 2015]

OBJECTIVE

To understand the risk situation of imported schistosomiasis and its main influencing factors in Shanghai City from 2005 to 2015.

METHODS

A retrospective survey was performed, and the database was established based on the data collection of imported schistosomiasis, Oncomelania hupensis snail situation and mobile population in Shanghai City from 2005 to 2015.

RESULTS

From 2005 to 2015, the mobile population in Shanghai City increased by 123.92%, which reached about 9 816 500, and 54.70% of them were located in the inner suburban districts. The accumulated areas with snails of 7.13 hm² were found in 16 towns of 4 outer suburbs (excluding Chongming District). A total of 23 cases of imported schistosomiasis were found in Shanghai City, and the number of the imported schistosomiasis cases was on the downward trend (rs = -0.782, P = 0.004). From 2005 to 2009, 11 out of 13 imported schistosomiasis cases (84.62%) were distributed in the inner suburban districts where no O. hupensis snails were found at the same time. From 2010 to 2015, 9 out of 10 imported schistosomiasis cases (90%) were distributed in the outer suburban districts where O. hupensis snails were found at the same time.

CONCLUSIONS

The number of imported schistosomiasis cases in Shanghai is on the downward trend. However, the threat of imported schistosomiasis to the prevention and control of schistosomiasis in Shanghai City should not be ignored.

[Molluscicidal effect of suspension concentrate of niclosamide ethanolamine salt]

Objective To evaluate the molluscicidal effect of suspension concentrate of niclosamide ethanolamine salt (SCNE) against Oncomelania hupensis snails in laboratory and field. Methods The experiment of SCNE against the snails by using the immersing and spraying methods was performed in laboratory and field, with control groups of wettable powder of niclosamide ethanolamine salt (WPN). Results In the laboratory, LC_50(s) of SCNE for 24, 48 h and 72 h by using the immersion method were 0.092 6, 0.062 9 mg/L and 0.054 9 mg/L, respectively. The mortality rates of snails for 24, 48 h and 72 h by using the immersion method were all 100% with the concentrations of 0.25 mg/L. The mortality rates of snails were all 100% while spraying SCNE for 3 d in the laboratory with the concentrations of 0.25 g/m². In Jiangling County, except 0.5 g/m³ SCNE immersing the snails for 24 h, the mortality rates of snails by using SCNE with the immersing method were all 100%. While the concentration of SCNE was 0.5 g/m³ or above, the mortality rates were all 100% after the use of it with the immersion method for 2 d in Gong'an County. In Jiangling County, the mortality rates of snails by using SCNE 0.5 g/m³ for 1 d, 3 d, and 7 d with the spraying method were 87.5%, 92.82% and 97.40% respectively. While the concentration of SCNE was 0.5 g/m³, the mortality rates were 85.94%, 86.78% and 94.21% respectively after the use of it with the spraying method for 1 d, 3 d, 7 d in Gong'an County, and the molluscicidal effect of SCNE (1.0 g/m²) was higher than that of WPN. Conclusion SCNE has a high molluscicidal effect in the laboratory and field, and it is a novel and simple formulation of niclosamide.

[Oncomelania hupensis snail distribution in working areas of Yangtze River hydrologic agencies located in middle and lower reaches of Yangtze River in 2016]

OBJECTIVE

To understand the Oncomelania hupensis snail distribution in the working areas of Yangtze River hydrologic agencies located in the middle and lower reaches of the Yangtze River in 2016, so as to provide the evidence for assessing the risk of schistosome infection of hydrological workers and establishing the control strategies.

METHODS

The suspicious environments with O. hupensis snails in the above working areas were selected as study areas, and the snail situation was surveyed by the system sampling method combined with the environmental sampling method. The survey data were collected and analyzed statistically.

RESULTS

Totally 19 working areas from 17 hydrological agencies were selected as the investigation sites, among which, 10 working areas from 9 agencies were found with O. hupensis snail distribution. The constituent ratio of the areas with snails reached to 38.81% of the investigation areas, the occurrence rate of frames with snails was 3.08%, and the average density of living snails was 0.07 /0.1 m². By comparison, the average density of living snails and occurrence rate of frames with snails in hydrological agencies under the jurisdiction of the Middle Reaches Administrative Bureau were the most serious among three administrative bureaus of the Yangtze River Water Resources Commission.

CONCLUSIONS

There are various degrees of O. hupensis breeding in the working areas of hydrological agencies located in the middle and lower reaches of the Yangtze River, and the hydrological workers are facing with the risk of schistosome infection.

[Oncomelania hupensis snail control effects of molluscicides with different formulations in field in marshland and lake regions]

OBJECTIVE

To observe the Oncomelania hupensis snail control effects of molluscicides with different formulations in the ditch with unstable water level in field of marshland and lake areas, so as to provide the evidence for formulating the prevention strategies.

METHODS

A drainage channel with O. hupensis snails in Jiangling County, Jingzhou City were selected as the study field, then it was divided into 6 segments, except 1 segment was treated as the blank control group for observing the natural mortality rate of the snails, the other 5 segments were treated as observation groups, where 4% niclosamide ethanolamine salt powder, 5% niclosamide ethanolamine salt granule, 25% suspension concentrate of niclosamide ethanolamine salt, 26% suspension concentrate of metaldehyde and niclosamide ethanolamine salt, 50% niclosamide ethanolamine salt wettable powder were applied respectively. Before and after the mollusciciding, the snail surveys were carried out through the systematic sampling method, and the short- and long-term effects of snail control were observed, and the mortality rates of the snails on the slope above the water level and those below the water level were compared.

RESULTS

When 7 d and 15 d after mullusciciding, the mortality rates of snails on the slope above the water level in each observation group were 79.52%-97.87% and 90.43%-96.30%, respectively, when compared with those before mollusciciding, all the differences were statistically significant (all P < 0.01). When 7 d after mullusciciding, the mortality rates of snails below the water level in each observation group were 17.11%-50.00%, which were all lower than those of the snails on the slope above the water level in the corresponding groups (all P ＜ 0.05); when 15 d after mullusciciding, the rates were 9.43%-95.24%, and those in 25% suspension concentrate of niclosamide ethanolamine salt, 26% suspension concentrate of metaldehyde and niclosamide ethanolamine salt, 50% niclosamide ethanolamine salt wettable powder groups were all lower than those of the snails on the slope above the water level in the corresponding groups (all P ＜ 0.01). Compared with the densities of living snails before mollusciciding, those 6 months after mollusciciding in each observation group decreased by 70.21%-78.98%.

CONCLUSIONS

The molluscicides in the formulation of powders and granule are suitable for both the snail environment with or without water, while those in the formulation of suspension agents and wettable powders are suitable only for environments with water.

[Analysis of soil components along water channel of east route of South-to-North Water Diversion Project]

OBJECTIVE

To explore the relationship between the soil components along the areas of the east route of the Southto-North Water Diversion Project and the survival situation of Oncomelania hupensis snails.

METHODS

Four study sites were selected along the project, and soil samples were collected to analyze the components. The amounts of components among the different study sites were compared statistically.

RESULTS

Except zinc, sodium, and phosphorus, the contents of the soil components where the snails survived were different from those where snails were dead (t = -19.150 to 12.810, all P < 0.01). However, the logistic regression analysis did not obtain a model with statistical significance.

CONCLUSIONS

O. hupensis snails live in the soil with adequate organic matter and neutral pH value. Along with the global warming and the operation of the South-toNorth Water Diversion Project, O. hupensis snails may survive in the northern area, and therefore, the surveillance work should be strengthened.

[Schistosomiasis surveillance after interruption of schistosomiasis transmission in Xiuzhou District, Jiaxing City]

OBJECTIVE

To analyze the endemic situation of schistosomiasis after its interruption of transmission in Xiuzhou District, Jiaxing City, Zhejiang Province, so as to provide the references for future surveillance work.

METHODS

The data of schistosomiasis and Oncomelania hupensis snails in Xiuzhou District were collected and analyzed statistically.

RESULTS

From 1994 to 2015, totally 975 village-times were investigated for O. hupensis snails, and the accumulated area of 4 385.31 hm² was surveyed. Twenty former snail sites were reoccurring, with an area of 32.61 hm². An area of 57.71 hm² was supplied with snail eradication measures. Totally 11 941 snails were dissected and no schistosome infected snails were found. The serum and stool tests were performed to 221 794 and 3 731 residents respectively, and no local infection cases but four imported cases were found.

CONCLUSIONS

The endemic situation of schistosomiasis in Xiuzhou District is stable after the transmission was interrupted. However, there are imported schistosomiasis cases, and therefore, the prevention of imported infection source is the focus of surveillance work.

[Effect of wild feces detection in Oncomelania hupensis environments on surveillance of infection source of schistosomiasis]

OBJECTIVE

To evaluate the effect of wild feces detection in Oncomelania hupensis environments on the surveillance of infection source of schistosomiasis, and find the weakness in schistosomiasis control in Hubei Province, so as to put forward the targeted strategies and measures.

METHODS

Four environments with O. hupensis snails in endemic areas of Hubei Provinces, where human and livestock often haunted, were selected according to the river systems, namely the Juzhanghe River beach in Jingzhou City, Changjiang River beach in Jiayu County, Hanbei River beach in Tianmen City, and Changshou River beach in Zhongxiang City, then the snail survey and wild feces detection were implemented in the selected environments.

RESULTS

There were O. hupensis snails, livestock, wild feces of cattle or sheep as well as positive cattle feces found in all the 4 environments, and the positive rate of schistosome miracidium incubation was 47.62%.

CONCLUSIONS

The schistosome miracidium positive rate of wild feces of cattle is high in the environments with snails in endemic areas of Hubei Province, which has high risk for schistosomiasis transmission.

[Analysis of endemic status of schistosomiasis in Gaoyou City from 1970 to 2009]

OBJECTIVE

To explore the changing rule of schistosomiasis endemic situation in Gaoyou City, so as to provide the evidences for schistosomiasis control in lake and marshland regions.

METHODS

The endemic data of schistosomiasis of Gaoyou City from 1970 to 2009 were collected to analyze the endemic patterns of schistosomiasis in different control stages comprehensively.

RESULTS

In the first years of infection control stage (1970-1975), transmission control stage (1976-1994) and transmission interruption stage (1995-2009), the prevalence rates of schistosomiasis were 4.20%, 0.80% and 0 in human and 3.00%, 0.51% and 0 in cattle respectively. In 1984, the positive rate of crowd stool tests fell to 0.04%, which was reduced by 99.05% compared with that in 1970. From 1970 to 1980, the positive rates of stool tests of people and livestock were positively correlated significantly (r = 0.67, P < 0.05). After 1985, no local infection patients or cattle were found. No schistosome infected Oncomelania hupensis snails were found from 1970 to 2009.

CONCLUSIONS

The comprehensive control measures have been insisted in Gaoyou City in different prevention and control stages. Although the snail situation is undulate, the prevalence of schistosomiasis steadily declines and the interruption of transmission has achieved.

[Sample unit size effects of spatial pattern of Oncomelania hupensis population in marshland schistosomiasis epidemic areas]

OBJECTIVE

To explore the sample unit size effects of spatial pattern of Oncomelania hupensis population in the marshland schistosomiasis epidemic areas.

METHODS

The push-broom method was performed to survey the snails in the study site in a basic unit size of 0.33 m×0.33 m in Poyang Lake region of Jiangxi Province. After combining different amount of survey cells, the aggregation indicators, regression models and aggregating factor index were employed to study the spatial distribution pattern.

RESULTS

The snail population was in aggregation pattern and the smaller sampling unit could produce higher degrees of aggregation. The regression models showed that the snails distributed in the form of individual groups with attraction with each other and the aggregation was related with the snail density. The clustering factor index demonstrated that the aggregation was introduced by aggregating behavior habit of the snails and environmental factors, or only by snail habit itself, and the aggregating habit of snails could be expressed better by the smaller sampling unit.

CONCLUSIONS

The spatial pattern of O. hupensis population in the marshland schistosomiasis epidemic areas is in aggregation pattern, and the sample unit size can affect the calculation results of the related biological indices.