Seasonal cercarial emergence patterns of Opisthorchis viverrini infecting Bithynia siamensis goniomphalos from Vientiane Province, Lao PDR

BACKGROUND: Snail intermediate hosts play a pivotal role in maintaining the life cycles of trematodes, including Opisthorchis viverrini. We investigated the emergence patterns of O. viverrini cercariae infecting Bithynia siamensis goniomphalos at foci in an endemic area in Vientiane Province, Lao PDR. FINDINGS: Samples of B. s. goniomphalos collected during the hot-dry, rainy and cool-dry seasons were examined for O. viverrini infection by cercarial shedding. Emergence of cercariae from O. viverrini-positive snails was monitored daily from 06:00–18:00 h for seven consecutive days at 2 hourly intervals. Snail infections varied seasonally, being highest in the cool-dry season. Peak cercarial emergence was not consistent in different seasons, occurring between 08.00–10.00 h during the hot-dry season and between 12.00–14.00 h during the rainy and cool-dry seasons. The cercarial output was highest in the hot-dry season. The prevalence of infection and the emergence of cercariae were strongly dependent on snail size. CONCLUSIONS: This study shows that size of snails and environmental factors (such as season) may affect the emergence patterns of cercariae of O. viverrini in snails. These results have both fundamental and applied implications for opisthorchiasis epidemiology and control.