[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.