In spite of remarkable chemotherapeutic progress and the existence of highly effective molecules such as the acylated quinoline-pyrazine praziquantel (PZQ), there is, as previously noted, still a spreading of schistosomiasis into new areas. After over 20 years of experience, it is generally agreed that chemotherapy, although the mainstay of current schistosomiasis control programs (13, 48, 79, 149), does have some limitations. In particular, mass treatment does not prevent reinfection. This occurs rapidly in exposed populations in most areas of endemicity such that within a period of 6 to 8 months following chemotherapy, the prevalence returns to its baseline leve