Exceptionally high prevalence of infection of Bithynia siamensis goniomphalos with Opisthorchis viverrini cercariae in different wetlands in Thailand and Lao PDR

Abstract

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.

Figure 1.

The study area showing sampling localities for B. s. goniomphalos in different wetlands in Thailand and Lao PDR. Codes for sampling localities are as follows: Khon Kaen (KK), Buri Ram (BR), Surin (SuR), Chaiyaphum (CP), Mahasarakham (MS1 and MS2), Kalasin (KS1, KS2, and KS3), Mukdaharn (MD1 and MD2), Sakon Nakhon (SK), Nakho Phanom (NP), La Ha Nam (LH), Na Seng (NS), Pon Sa-ard (PS), Hau Maung Neang (HM), Bueng Wa (BW), and Vientiane (VT). Expanded map of Vientiane (VT): Veiang Jarean (VJ), That Luang (ThL), Na Hae (NH), Dongnatong (DT), Sa Pang Muek (SM), Tanmi Xai (TM), Nong Pra Ya (NY), Naxaithong (NX), Thalad (TL), Phonhong (PH), Tha Heur (TH), and Vang Vieng (VV). Expanded map of Khon Kaen (KK): Ban Phai (KBp1 and KBp2), Phon (KP1, KP2, and KP3), Sa-ard (KBs), Lerngpleuy (KLp1 and KLp2), and Phu Wiang (KPv1 and KPv2). ⋆ = Sites positive for O. viverrini; • = sites negative for O. viverrini.

Figure 2.

Relationship between the prevalence of O. viverrini cercariae and shell size of B. s. goniomphalos. Black bars represent size class based on shell length (small ≤ 8.0 mm, medium = 8.1–10.0 mm, large ≥ 10.0 mm); white bars represent size class based on shell width (small ≤ 5.0 mm, medium = 5.1–6.0 mm, large ≥ 6.0). *P < 0.05, **P < 0.01, ***P < 0.001.

Figure 3.

Number of O. viverrini cercariae produced per day in different size classes of B. s. goniomphalos. Black bars represent size class based on shell length (small ≤ 8.0 mm, medium = 8.1–10.0 mm, large ≥ 10.0 mm); white bars represent size class based on width of shell (small ≤ 5.0 mm, medium = 5.1–6.0 mm, large ≥ 6.0). Data shown are mean ± SD calculated from cercaria-positive snails (two-sample LSD test; *P < 0.05, **P < 0.01).