Appearance of Fasciola cercariae in rice fields determined by a metacercaria-detecting buoy.

The appearance of Fasciola cercariae in aquatic rice fields of fluky areas was investigated by using a metacercaria-detecting buoy. The buoy consisted mainly of a foam styrol disk 10 cm in diameter and 3 cm in thickness, and a polyethylene sheet 16 cm in diameter covering the bottom and side of the disk. Each buoy was floated at each corner of a section of rice fields. To determine the serial appearance of Fasciola cercariae, the polyethylene sheet was renewed at intervals of a week over a period of rice cultivation and the number of metacercariae was counted on each sheet. In most rice fields an evident peak of metacercarial detection was observed between the middle of July and the end of August. The time of the first detection of cysts in a small number varied from one section to another of the same field or from one area to another. Metacercariae decreased in number in accordance with drying up the fields. The metacercaria-detecting buoy method was applied to rice fields fertilized with cattle manure for 7 days in mid-summer, as well as to fields located closely to cattle pens, but not fertilized. Many cysts were detected from both groups of fields. The results obtained from weekly observation on the population of snails, particularly infected snails in their habitats, and those from the cercarial appearance detected by means of the buoy method were essentially the same.