The geographical distribution of opisthorchiasis is constrained
by the presence of the requisite intermediate host snails and the
second intermediate host fishes, and dietary preferences of local
populations for uncooked or undercooked fish. O. viverrini infection
is prevalent in Thailand and the Lao People’s Democratic Republic
(PDR), and also occurs in Cambodia and Vietnam. There is a marked
geographical heterogeneity in the prevalence of O. viverrini infection
among the four regions of Thailand; nowadays the highest
prevalence occurs in the North and Northeast (below). In 1981, the
Thailand nationwide prevalence of O. viverrini infection was 14%,
but prevalence varied greatly by region – North (5.59%), Northeast
(34.60%), Central (6.34%) and South (0.01%) (Harinasuta and
Harinasuta, 1984). After intensive and continuous control activities
over the subsequent two decades, prevalence had declined
to 15.7% in the Northeast, leading to a corresponding decline
in the national prevalence to 9.4% in 2001