Assessment of the influence of levees along Yangtze River on Oncomelania hupensis, the intermediate host of Schistosoma japonicum.

BACKGROUND: Oncomelania hupensis is the exclusive intermediate host of Schistosoma japonicum in China. Snail control is an essential component of schistosomiasis elimination programme. With 70 years of continuous efforts, the range of O. hupensis had reduced significantly, but slowed down in last decades. A large number of levees against flooding were constructed along Yangtze River and its affiliated lakes in the middle and lower reaches, which influenced the hydrology and ecology in the alluvial plains. The purpose of this study was to assess the impact of levees on the distribution of O. hupensis in the middle and lower reaches of the Yangtze River. METHODS: The snail habitats were digitalised by hand-held GPS system. The years for discovery and elimination of snail habitats were extracted from historical records. The accumulated snail-infested range for each habitat was calculated on the basis of annual reports. The current distribution of O. hupensis was determined by systematic and environmental sampling. The geographical distribution of levees was obtained from satellite imagery. To assess the impact of levees, the data pertaining to O. hupensis were divided into two parts: inside and outside the Yangtze River. Joinpoint regression was utilised to divide the study time span and further characterise the regression in each period. The 5-year-period moving averages of eliminated area infested by snails were calculated for the habitats inside and outside Yangtze River. The moving routes of corresponding geographical median centres were simulated in ArcGIS. Hotspot analysis was used to determine the areas with statistical significance clustering of O. hupensis density. RESULTS: Three periods were identified according to Joinpoint regression both inside and outside Yangtze River. The area infested by O. hupensis increased in the first two periods. It decreased rapidly outside Yangtze River year over year after 1970, while that inside the Yangtze River did not change significantly. Furthermore, the latter was significantly higher than the former. It was observed that the present density of O. hupensis inside Yangtze River was lower than outside the Yangtze River. The median centre for eliminated ranges inside Yangtze River wavered between the east (lower reach) and the west (middle reach). In contrast, the median centre for eliminated ranges continuously moved from the east to the west. CONCLUSIONS: Our findings indicated that the levees had a considerable negative impact on the distribution of O. hupensis outside Yangtze River. Some hotspots observed in the irrigation areas need a sluice system at the inlet of branch for snail control. The major distribution of O. hupensis located in Hubei might be caused by severe waterlogging. The intensive surveillance should be implemented there. The biggest two freshwater lakes, the major endemic regions historically, were identified as cold spots. The long-term impact of Three Gorges Dam on the distribution of O. hupensis in the lakes should be monitored and evaluated.

[Simulation experiment of survival and reproduction of artificially imported Oncomelania snails in Qingpu District, Shanghai].

OBJECTIVE: To understand the survival of imported Oncomelania snails in new environments with different densities in waterway net region, Qingpu District, Shanghai. METHODS: The snails collected from Guichi, Anhui Province were put into the ponds of 4 square meters and each had 25, 50, 100, 200, 400 and 800 pairs of snails, respectively. During the next 2 years, the temperature, humidity, the activities of snails and their second birth snails were investigated each day. All the data were analyzed statistically. RESULTS: The highest number of snails appeared from May to June on the surface of soils in different densities. The average numbers of snails were 2.0%-12.7% of whole put in snails, and the numbers of offspring were less than 2% of the whole put in snails. CONCLUSION: The survival and reproduction of artificially imported Oncomelania snails from Guichi, Anhui Province is not good in Qingpu District, Shanghai.

[Retrospective investigation of formation of large-scale Oncomelania snail-ridden area in Maogang Town, Songjiang District, Shanghai City].

OBJECTIVE: To explore the influencing factors of the formation of large-scale Oncomelania snail situation in Maogang Town, so as to provide the scientific evidence for improving snail surveillance in Shanghai. METHODS: The data about the geography, construction and use of floodgate, and snail status of Maogang Town and its surrounding areas, as well as the meteorology and hydrology of Songjiang District were collected and reviewed, and the factors influencing the formation of snail status were analyzed. RESULTS: The condition of water, soil and temperature in Maogang Town was suitable for snail breeding. By 1993, 75% (18/24) of the mouths flowing to Huangpu River and its branches had been built. From 1991 to 2001, the average annual precipitation of Songjiang District was 7.44% higher than the usual condition. There were two basin-wide floods occurring in 1991 and 1999, respectively. The occurrence of tides reaching or exceeding the warning level (3.50 m) of Mishidu hydrology station was positively correlated with the rainfall in the flood season (between June and September, r = 0.903, P < 0.01). Snails were found in some adjacent areas surrounding Maogang Town, and the snail situation peak occurred in early 1990s. In 2002, a large scale area (26.67 hm2) of snail habitats was detected, which was mainly distributed in the west of Maogang Town in the upstream of the Huangpu River. CONCLUSIONS: The formation of large-scale snail-breeding area in Maogang Town probably is the result of the combined action of the construction and use of floodgates, increasing rainfall, floods, tides that frequently surpass warning level of Huangpu River and the snail status of upstream areas. It is necessary to pay more attention to the data collection, analysis and utilization about schistosomiasis-related natural and social factors. In addition, regular snail surveys are suggested in schistosomiasis non-endemic areas adjacent to snail-infested area.

[Potential risks for transmission of schistosomiasis caused by mobile population in Shanghai].

OBJECTIVE: To understand the potential risk for schistosomiasis transmission caused by introduction of infection source from mobile population in Shanghai. METHODS: Field investigation was conducted in the suburb of Shanghai City by screening the mobile population living in Shanghai for more than 1 month and over 1 years old in a procedure of interviewing, serum indirect hemagglutination (IHA) test, and then fecal examination to detect the eggs with nylon sedimentation approach for those IHA positives. RESULTS: Among 2,931 mobile people investigated, 1,575 were male (53.74%) and 1356 were female(46.26%); 138 out of 2931 were positive in IHA test (4.71%). 1938 (66.12%) out of 2 931 came from Schistosoma japonicum-endemic provinces and its positive rate in mobile population (5.99%) was significantly higher than those from the transmission-interrupted provinces (2.6%) (chi2=10.28, P<0.01), and those from non-endemic provinces (1.68%) (chi2=12.86, P<0.01). The 138 IHA positives all showed negative in fecal examination. In accordance with the serum positive rate and egg-infection rate in the national reporting system in 2004, it was estimated that there would be about 13 356 and 1 699 potential serum positive cases respectively from endemic area and transmission controlled area, and about 2,168 and 255 egg-positive cases from the two kind areas respectively, majority of the cases were from Anhui Province. CONCLUSION: Schistosomiasis transmission risks potentially exist in Shanghai suburb due to the introduction of infected mobile people from other endemic provinces, and a surveillance system and quick response are needed for the possible re-emergence of the disease.