Epidemiologic surveys were conducted in selected provinces throughout Thailand to delineate the endemic area of Fasciolopsis buski. A search for the snail hosts, water plants associated with them, and reservoirs of infection was undertaken in Ayuthaya Province, at the site of the most heavily infected villages. Fasciolopsis buski was found to be endemic only within central Thailand, with an estimated 100,000 persons infected. Significantly more females were found infected than males, the highest prevalence being among the 10 to 20 year age group. Pigs appear to be the only animal reservoir of any significance, and infection rates in them closely parallel that found in man, although pigs generally harbor fewer adult worms and egg production per worm is lower than in human beings. Two species of snail, Segmentina (Polypylis) hemisphaerula and Segmentina (Trochorbis) trochoideus, were found naturally infected; both were also infected experimentally. Five species of water plant were found to be important in transmission of the parasite: water morning glory (Ipomoea aquatica), water caltrop (Trapa bicornis), lotus (Nymphaea lotus), water cress (Neptunia oleracea), and water hyacinth (Eichhornia speciosa).
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