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Annual prevalences of O.viverrini infection in each region in Thailand from 1957 to 2009. In the Northest, the prevalence tends to decline until 1996 but remains stable thereafter. A peaked prevalence in the north in 1996 may not entirely due to O.viverrini but mixed infection with the minute intestinal flukes which have similar egg morphology. (Data from the Ministry of Public Health).
