Schistosomiasis mansoni in the Baixada Ocidental Maranhense, state of Maranhão, Brazil: cross-sectional studies performed in 1987 and 1993 - Cutrim, Raimundo Nonato Martins; Chieffi, Pedro Paulo; Moraes, José Cássio de (Revista do Instituto de Medicina Tropical de São Paulo, 1998)