3. REVIEW OF LITERATURE Schistosomiasis is a chronic parasitic disease caused by the three main species: Schistosoma mansoni, Schistosoma haematobium and Schistosoma japonicum. In addition, Schistosoma mekongi has been considered as a pathogen in some areas of South East Asia while, Schistosoma intercalatum is identified as a pathogen in West Africa (Despommier et al., 1995). 3.1. Historical Background In 1852, Theodor Bilharz (figure 1) a German physician working at Medical School of Cairo, noticed