Cercarial emergence patterns for Opisthorchis viverrini sensu lato infecting Bithynia siamensis goniomphalos from Sakon Nakhon Province, Thailand

Opisthorchis viverrini sensu lato is a food-borne trematode which is classified as a class 1 carcinogen, with infection potentially leading to cholangiocarcinoma. Snails of the genus Bithynia act as the first intermediate hosts and an amplifying point in the parasite life cycle. In order to investigate seasonal effect on transmission dynamics of O. viverrini in Bithynia siamensis goniomphalos , cercarial emergence and output profiles were monitored at different season. A total of 4533 snails originating from Sakon Nakhon Province, Thailand, collected during the three main seasons, were analyzed for O. viverrini s.l. infection. Emergence of O. viverrini s.l. cercariae from snails was monitored daily from 06:00 to 18:00 h for seven consecutive days. The prevalence of infection in the snails was highest in the hot-dry season and declined in the rainy and cool-dry seasons. Peak cercarial emergence occurred between 08:00 and 10:00 h during the rainy and cool-dry seasons and between 10:00 and 12:00 h during the hot-dry season. The cercarial output was highest in the hot-dry season, similar to a previous study from Lao People’s Democratic Republic (PDR). Average cercarial output/snail in Thailand was higher than in Lao PDR. The number of cercariae emerging from the snails was strongly related to snail size, but the relationship between prevalence of infection and snail size differed between seasons. Observed discrepancies in the emergence patterns and per capita cercarial release may reflect differences in environmental, snail, and/or parasite factors particularly biological characteristics between the cryptic species of O. viverrini s.l. and B. s. goniomphalos from Thailand and Lao PDR.