In the present study, fish obtained in the Lake 500 in May
and August, a dry season, had no metacercariae. In September,
the beginning of the rainy season, fish obtained in the lake and
also from flood plain have 15–20% prevalence. While the prevalence
of fish in the flood plain remained stable until December, the
prevalence of fish from the lake fluctuated by every examination.
Although fish were collected at the outlet three times (November,
December and February), those collected on February were positive
for metacercariae. In general, fish in this area harbour O. viverrini
metacercariae only in the rainy season. Similar to our results,
Vichasri et al. (1982) and Sithithaworn et al. (1997) reported that
the peak of metacercarial burden in fish in Thailand was observed
around October–February, the rainy season. Since the prevalence
of metacercariae in fish was constant in the flood plain in the rainy
season, and raw fish food (Trey Bok) consumption by the local people
is common in this season, the risk of human infections appeared
to be much higher in rainy season than dry season. More extensive
survey is required for the seasonal changes of the infection in fish
and the behaviour of local people in terms of fish consumption.