Trematode Infection in a Freshwater Snail Hydrobioides Nassa (Gastropoda: Bithyniidae) in Thailand.

1,024 individuals of Hydrobioides nassa were collected from 12 different localities in eight provinces from north, west, and central regions of Thailand. The infection of parasitic trematodes was investigated using shedding and crushing methods to search for cercariae and metacercariae. Trematode infection was found at a relatively low prevalence of 5.57%. Five different morphological types of cercariae were detected; xiphidio, monostome, mutabile, ophthalmoxiphidio, and microcercous, and three different morphological types of unknown metacercariae were observed. Microcercous cercariae of the lung fluke genus Paragonimus is reported here for the first time in a bithyniid snail. Our current finding show that H. nassa can serve as intermediate host for a range of parasitic trematodes in Thailand.

Genetic structure and evidence for coexistence of three taxa of Bithynia (Gastropoda: Bithyniidae), the intermediate host of Opisthorchis viverrini sensu lato (Digenea: Opisthorchiidae) in Thailand examined by mitochondrial DNA sequences analyses.

The freshwater snails, Bithynia are the first intermediate hosts of the liver fluke, Opisthorchis viverrini, the causative agent of cholangiocarcinoma (CCA) in Southeast Asia. In Thailand, there are three traditionally recognized taxa of Bithynia: Bithynia funiculata; B. siamensis siamensis; B. s. goniomphalos. This study examines the geographical distribution and genetic structure of Bithynia species from five previously reported water catchments and six new catchments in Thailand. Of these, three new catchments Kok, Wang, and Nan are from the north and the remaining three new catchments are Phetchaburi, Prachuap Khiri Khan Coast, Mae Klong from the west of Thailand. We sampled 291 Bithynia snails from 52 localities in 11 catchment systems in the northern, western and central regions of Thailand. Mitochondrial cytochrome c oxidase subunit 1 (COI) and 16S ribosomal DNA (16S rDNA) sequences were used to examine genetic diversity of Bithynia snails which revealed 200 and 27 haplotypes of COI and 16S rDNA, respectively. However, as 16S rDNA is a conserved gene, it is not suitable to distinguish Bithynia at the species and sub-species levels in our study. The phylogenetic tree and haplotype network analyses included sequences of COI from GenBank. B. funiculata was found only in the north of Thailand and the genetic structure did not differ among populations. Genetic differentiation (ΦST) analyses showed that B. s. goniomphalos contained three distinct lineages. Lineage I contained B. s. goniomphalos from the vast majority of catchment systems in Thailand and Lao PDR. Lineage II contained all B. s. goniomphalos from the Prachin Buri and Bang Pakong catchment systems in eastern and central Thailand, including samples from all catchment systems in Cambodia. While lineage III contained B. s. goniomphalos from the Songkram and Nam Kam catchment systems in Thailand and the Nam Ngum and Huai Som Pak catchment systems in Lao PDR. Furthermore, results showed that all samples of B. s. siamensis were classified into one lineage and placed phylogenetically between B. s. goniomphalos lineages I and II. Thus, the taxonomic status of B. s. goniomphalos and B. s. siamensis requires reassessment, and they should be reclassified as belonging to the species complex "Bithynia siamensis sensu lato".

Bithynia siamensis goniomphalos, the first intermediate host of Opisthorchis viverrini in Thailand.

Opisthorchiasis caused by Opisthorchis viverrini (O. viverrini) remains as medically important problem in Thailand especially in the north-eastern part. Infection with this parasite can lead to cholangiocarcinoma improvement. The highest prevalence of O. viverrini infection has been found in the Northeast Thailand and is associated with the high incidence rate of cholangiocarcinoma. To complete the life cycle of O. viverrini, the freshwater snails namely Bithynia funiculata, Bithynia siamensis siamensis and Bithynia siamensis goniomphalos (B. s. goniomphalos) are required to serve as the first intermediate host. Within these snails group, B. s. goniomphalos is distributed concisely in northeast Thailand and acts as the majority snail that transmitted the opisthorchiasis in this region. This study described the information of B. s. goniomphalos which research are needed for understanding the biology, distribution, transmission and factors influencing on the infection of the snail vector of this carcinogenic parasite.