Opisthorchis viverrini in Thailand: The Life Cycle and Comparison with O. felineus

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Abstract
Of over 6,000 examinees in northeast Thailand, 79% were found to be infected with the human hepatic trematode Opisthorchis viverrini. Some 90% of all examinees over the age of 10 were positive, and it is estimated that over 3.5 million persons in that country harbor the parasite. The snail hosts are Bithynia goniomphalus, B. funiculata, and B. laevis. The oculate and pleurocercous cercaria resembles that of O. felineus. The flame cell pattern of the cercaria as it leaves the snail host is 2[(3 + 3) + (3+3 +3)], differentiating it from the cercaria of O. felineus, and establishing O. viverrini and O. felineus as separate species. The most important fish intermediate hosts are Punteus orphoides, Hampala dispar, and Cyclocheilicthys siaja, although a total of nine species were found to harbor the metacercariae. It is postulated, on the basis of seasonal fluctuations, that the largest number of human infections occur during the last portion of the rainy season and the first third of the dry (September-February). The characteristics which have been reported to differentiate adult O. viverrini from O. felineus have been studied but it has not been possible to separate the two species on the morphological characteristics of egg or adult. Thus, although not of practical laboratory significance, at present the only certain means of distinguishing between the two species is the flame cell patterns of cercariae or metacercariae.

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