[Evolution of Schistosoma mansoni bilharziasis in Ampefy (Lake Itasy) from 1986 to 1993].

In 1976, 1983, 1986 to 1989 and 1993, the population of Ampefy, on the shores of lake Itasy, was invited to participate in an active detection programme of Schistosoma mansoni infection. An increase in the global prevalence was observed from 1976 (17.0%) to 1983 (40.0%). This prevalence level was then stabilized during the four years annual selective treatment (1986 to 1989) and the mean intensity of infection for positive patients fell from 393 eggs/g to 271 eggs/g and the proportion of severe infections, with a high risk of developing "hepatosplenic disease", fell from 8% to 4.2%. Selective treatment was interrupted from 1989 to 1993, during which time the global prevalence rose to 70.6% and mean intensity of infection to 594 eggs/g, with 22% of those examined classified as severely infested. This local control programme, associating active detection with annual treatment, seems effective in stabilizing prevalence and intensity of infection in spite of a regular arrival of infested, and never treated, immigrants. However, its impact throughout time is limited.