Species-specific primers designed from RAPD products for Bithynia funiculata, the first intermediate host of liver fluke, Opisthorchis viverrini, in North Thailand

Current assessment of the systematics and population genetics of Opisthorchis viverrini sensu lato (Trematoda: Opisthorchiidae) and its first intermediate host Bithynia siamensis sensu lato (Gastropoda: Bithyniidae) in Thailand and Southeast Asia

The group 1 carcinogen, the liver fluke Opisthorchis viverrini is the causative agent of opisthorchiasis and subsequent bile duct cancer (cholangiocarcinoma; CCA), which is an important public health problem in Southeast Asia. Bithynia snails are known to be the sole intermediate host of O. viverrini, and distributed throughout endemic areas of opisthorchiasis. Since 2001, the genetic variation investigation of O. viverrini has progressively been investigated. Comprehensive genetic variation studies of O. viverrini and Bithynia snails were undertaken and consecutively published in 2007 by Saijuntha and colleagues. These studies provided genetic evidence that O. viverrini and Bithynia snails are both species complex with evidence of co-evolution. Later, several studies have provided data in support of this finding, and have continuously to date reinforced that both O. viverrini and Bithynia are species complexes. Moreover, studies have shown that genetic variation of O. viverrini is related to geographical, temporal, fish host species including geographical genetic variation of its snail host, Bithynia siamensis sensu lato. This is significant and important in our understanding of the evolution and phylogenetic relationships between species within the O. viverrini and Bithynia species complexes. A comprehensive knowledge of the systematics and population genetics of O. viverrini and Bithynia snails provides a sound basis to instigate and develop effective prevention and control programs targeting opisthorchiasis and CCA in the endemic areas of Southeast Asia. Thus, this review examines the historical series of investigations of the systematics and population genetics of O. viverrini including Bithynia spp. in Southeast Asia since molecular genetic investigations commenced some 20 years ago.

Reservoir Animals and Their Roles in Transmission of Opisthorchis viverrini

Although any fish-eating mammals could be potential definitive hosts of Opisthorchis viverrini, only a few, especially cats and dogs, are actually known reservoir hosts for this parasite. Both animals usually get infected via consuming raw or undercooked contaminated fish, fish dishes or food remains from households. The infected animals sustain parasite egg spread via open environment defecation. Cats are the most important reservoir with higher prevalence rates of O. viverrini infection than dogs in endemic areas. Usually Opisthorchis-infected animals do not exhibit apparent clinical symptoms or specific abnormalities in laboratory examinations. Pathological findings in these animal reservoirs are basically similar to those seen in humans and experimental animals, namely periductal inflammation, biliary hyperplasia and periductal fibrosis. However, O. viverrini-associated cholangiocarcinoma has not yet been reported in the reservoir animals at present. Praziquantel is a treatment of choice not only for humans but also for animal reservoirs. Integrated control of opisthorchiasis in animal reservoirs is based on holistic approaches such as EcoHealth/One Health concepts. In fact integrated control of opisthorchiasis in humans in ecosystem has also proved successful, for example, the Lawa model for opisthorchiasis control in the endemic area of Khon Kaen, Thailand. Other feral and wild animals in endemic areas might also be potential reservoirs, and this requires more investigation. In addition, genetic diversity and evolution of the flukes might also influence zoonotic capability.

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.

Reprint of "An overview of freshwater snails in Asia with main focus on Vietnam"

Freshwater snails have received much attention for their role as intermediate hosts for trematodes causing disease in people and animals such as schistosomiasis and various food-borne trematodes. While effective medical treatment exists for some of these diseases there is need for preventive measures to reduce transmission, e.g. control of intermediate hosts because transmission patterns are often complicated due to presence of reservoir final hosts. In order to implement control measures against the intermediate host snails with minimal impact on the freshwater ecosystems and their biodiversity, a profound knowledge on transmission patterns of the trematodes is required and this is partly related to distribution, habitat preferences, and seasonal variation in density of the intermediate host species. Identification of snail species can be problematic on the basis of morphological and anatomical characters alone as some species show morphological plasticity and similarly morphological differentiation of cercariae found in snails may be difficult and this could lead to biased perceptions of intermediate host spectra and transmission patterns. In this paper, we give an overview of the snail families and their medical and veterinary importance in Asia but with main focus on Vietnam.

An overview of freshwater snails in Asia with main focus on Vietnam

Freshwater snails have received much attention for their role as intermediate hosts for trematodes causing disease in people and animals such as schistosomiasis and various food-borne trematodes. While effective medical treatment exists for some of these diseases there is need for preventive measures to reduce transmission, e.g. control of intermediate hosts because transmission patterns are often complicated due to presence of reservoir final hosts. In order to implement control measures against the intermediate host snails with minimal impact on the freshwater ecosystems and their biodiversity, a profound knowledge on transmission patterns of the trematodes is required and this is partly related to distribution, habitat preferences, and seasonal variation in density of the intermediate host species. Identification of snail species can be problematic on the basis of morphological and anatomical characters alone as some species show morphological plasticity and similarly morphological differentiation of cercariae found in snails may be difficult and this could lead to biased perceptions of intermediate host spectra and transmission patterns. In this paper, we give an overview of the snail families and their medical and veterinary importance in Asia but with main focus on Vietnam.