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The differen,t life cycles of the human digenetic parasitic trematodes Schistosoma japonicum. S. mansoni. S. haematobium. Clonorchis sinensis, various species of the genera Qpisthorchis and Paragonimus. and those of lesser pathogenic impbrtance, Fasciolopsis buski. Heterophyes heterophyes and Metagonimus yokogawai, illustrate the complexity of the clinic0=epidemiological profiles of these syndromes. The one factor common to all of infections is the necessity of a primary snail intermediate host to complete the biological life cycles: some additionally require a secondary intermediate host which may be several species of fresh-water fish, crustaceans or aquatic plants. Conversely, the mode of transmission to man varies froin dermal penetration of infective cercariae in the schistosome infections, to the consumption of infective metacercariae in raw or undercooked cyprinoid fresh-water fish in clonorchiasis and opisthorchiasis and the eating of fresh-water crabs or crayfish in paragonimiasis or the ingestion of water caltrop, water chestnut and bamboo in fasciolopsiasis. Yet, despite these variations, several general common features characterize the different epidemiological cycles:1 . the prevalences of the infections are highest in rural populations, frequently higher in males, commonly occurring in the younger age-groups and the distributions all follow the inverted binomial pattern, itself characteristic of those infections in which there is no replication of a parasite population within the human host; 2 . an important feature of their propagation is the indiscriminate disposal of human faeces in areas deficient in sanitary provisions or in the use of "night soil" where this practice remains part of the prevailing agricultural pattern; 3 . transmission in all of these infections is via human behavioural variableseither obligatory water-contact in the case of the schistosomes, or age-old eating habits in the other trematodiaseshuman attitudes and customs which are extremely difficult to alter and which are often resistant to conventional hea.Ith education; 4 . animal reservoirs of infection are, with the exception of S. haematobium, extremely common. It thus appears inherently improbable that complete eradication of these infections by the permanent interruption of transmission will be achieved on a global scaleat least in the immediate future and certainly not before the year 2000. Since schistosomiasis is the major globally-distributed example of the human trematode infections, it will be taken as an example of the linkage between epidemiological analysis and control strategy and will be dealt with in greater detail than the other trematode infections which, nowadays; are of greater regional than global Lately Director, Parasitic Diseases Programme, World Health Organization, Geneva, Switzerland. Address: Dr. A. Davis, 'Pantiles' Catcott, Bridgwater, Somerset TA7 9HU, United Kingdom

The epidemiology and chemotherapy of schistosomiasis and other snail-borne trematode infections.