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

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