Title:
[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].

Abstract:
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.
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.
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.
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.