Exceptionally High Prevalence of Infection of Bithynia siamensis goniomphalos with Opisthorchis viverrini Cercariae in Different Wetlands in Thailand and Lao PDR

The carcinogenic liver fluke, Opisthorchis viverrini, requires Bithynia snail intermediate hosts in its life cycle. However, the prevalence of O. viverrini in snail intermediate hosts is typically low (< 1%). Here, we examined B. siamensis goniomphalos from 48 localities in Thailand and The Lao People's Democratic Republic (Lao PDR) and reported high-prevalence levels of O. viverrini. The highest-prevalence levels per locality were 6.93% (mean = 3.04%) in Thailand and 8.37% (mean = 2.01%) in Lao PDR; 4 of 13 localities examined showed prevalence higher than any prevalence previously recorded. The number of cercariae infecting snails and their prevalence were positively correlated with the size of the snails. High prevalence occurred in the Songkram River wetland (Thailand) and the Nam Ngum River wetland (Lao PDR). Our results show that transmission of O. viverrini from humans as well as animal reservoir hosts to snail intermediate hosts is ongoing and potentially increasing in endemic areas across Thailand and Lao PDR.