Culture of fecal indicator bacteria from snail intestinal tubes as a tool for assessing the risk of Opisthorchis viverrini infection in Bithynia snail habitat

Current status of human liver fluke infections in the Greater Mekong Subregion

The Greater Mekong Subregion (GMS) is a trans-national region of the Mekong River basin in Southeast Asia comprising Cambodia, the People's Republic of China (specifically Yunnan Province and Guangxi Zhuang Autonomous Region), Lao People's Democratic Republic (Lao PDR), Myanmar, Thailand, and Vietnam home to more than 340 million people or almost 4.5% of the global population. Human liver fluke infections caused by Opisthorchis viverrini and Clonorchis sinensis have been major public health problems in this region for decades. Opisthorchiasis caused by O. viverrini is prevalent in Thailand, Lao PDR, Cambodia and central-southern Vietnam with more than 12 million people infected. Clonorchiasis caused by C. sinensis is endemic in northern Vietnam and Guangxi with estimated 3.5 -5 million infected. The infections can cause several liver and biliary diseases such as cholangitis, periductal fibrosis, gallstones, and cholangiocarcinoma (CCA), a fatal bile duct cancer. Key determinants of the geographical distribution differences of the two liver fluke species are snail species and geographic barriers. Main risk behaviour of the infections is the culture of eating raw fish "the raw attitude" of people in the GMS, especially the Tai/Dai/Thai/Laos ethnic groups, the major population in the GMS. Over the past 20 years, there is a big change in prevalence of the infections. Opisthorchiasis has long been endemic, particularly in northern and northeastern Thailand and Lao PDR with over 8-10 million cases estimated. However, after several rounds of national campaign against opisthorchiasis using integrated control approach in Thailand over the past three decades, the prevalence of O. viverrini infection has reduced from over 15% in 1996 to 2.2% in 2019. High prevalence of O. viverrini infection continues in Lao PDR and central Vietnam. Emerging high prevalence, up to a maximum of 47.5%, has been noted in Cambodia during the past 10 years possibly due to more studies being conducted rather than increasing prevalence. O. viverrini infection has now also been reported in Lower Myanmar in recent years. Clonorchiasis has been known in northern Vietnam and southern China for a long time. Several surveys have reported infections in Guangxi in the last 10 years, and until now liver fluke infected cases have not been reported in Yunnan. Overall, nowadays, there is a shift in high risk areas for GMS liver fluke infection from northeastern Thailand to Lao PDR, Cambodia, Vietnam, Myanmar, and Guangxi P.R. China. Urgent systematic disease mapping and integrated liver fluke control using One Health approaches should be implemented nationwide in GMS countries.

Development of a nest-PCR for detection of Fasciola hepatica DNA in the intermediate snail host, Radix cucunorica, and the prevalence in northwestern China

Fasciolosis, a foodborne zoonotic disease, caused by Fasciola species which is considered an important problem for human health and livestock husbandry development. Snails are intermediate hosts of F. hepatica, the epidemiological surveillance of snails can evaluate the transmission risk of this disease in human and livestock. In this study, we developed a nest-polymerase chain reaction (nest-PCR) to detect the DNA of F. hepatica in Radix cucunorica, a prevalent intermediate host of this parasite in northwestern China. The nest-PCR was used to amplify a 208 bp fragment of the second internal transcribed spacer (ITS-2) of F. hepatica with two pairs of primers. The method was able to detect up to 0.16 fg genomic DNA in a 25 μL PCR reaction system even effected with high concentrations of snail DNA, and no cross reaction was observed from the genomic DNA of Paramphistomum cervi, Clonorchis sinensis, Orientobilharzia turkestanicum, Metorchis orientalis, Dicrocoelium chinensis. To evaluate the transmission risk of this disease, 409 snail samples collected from different areas of Gansu province were used to detect and analyze the transmission risk of F. hepatica in this area. Of 409 snail samples, the overall prevalence is 43.76%. The prevalence was 92.75% in Gannan Tibetan Autonomous Prefecture, while no snail was positive for F. hepatica in Linxia Hui Autonomous Prefecture. The nest-PCR was firstly used to detect the infection of F. hepatica in snail. It is a novel, useful and convenient method with high sensitivity and specificity. This study is the first report about the epidemiological surveillance of snail infection by F. hepatica in northwestern China, which will help to evaluate the transmission risk of F. hepatica in northwestern China.