Abstract. Seasonal changes play roles in the transmission success of fish-borne zoonotic trematodes (FZT). This study examined the seasonal transmission patterns of Opisthorchis viverrini sensu lato (s.l.) and a virgulate cercaria (family Lecithodendriidae) in the snail intermediate host, Bithynia siamensis goniomphalos in northeast Thailand. Snail samples were collected monthly during the rainy, cool, and hot seasons during 2012-2013 to determine the prevalence and intensity of larval trematode infections. The prevalence of O. viverrini s.l. varied significantly with season, being 0.31%, 1.05%, and 0.37% in the rainy, cool, and hot seasons, respectively (P 0.05). Similarly, the prevalence of virgulate cercariae was 3.11%, 6.80%, and 1.64% in the rainy, cool, and hot seasons, respectively (P 0.05). The intensity of larval trematode infections also varied between seasons and peaked in the hot season (P 0.05) in both species. The snails infected with O. viverrini s.l. were significantly smaller (P 0.05) and those infected with virgulate cercariae were significantly larger (P 0.05) than uninfected snails. Seasonal variation and the different sizes of B. s. goniomphalos parasitized by O. viverrini s.l. and virgulate trematodes indicate complex host-parasite interactions with important implications for the epidemiology of O. viverrini s.l.

Transmission of the carcinogenic liver fluke Opisthorchis viverrini is ongoing across Southeast Asia. Endemic countries within the region are in different stages of achieving control. However, evidence on which interventions are the most effective for reducing parasite transmission, and the resulting liver cancer, is currently lacking. Quantitative modelling can be used to evaluate different control measures against O. viverrini and assist the design of clinical trials. In this article we evaluate the epidemiological parameters that underpin models of O. viverrini and the data necessary for their estimation, with the aim of developing evidence-based strategies for parasite control at a national or regional level. Assessing InterventionsControl initiatives against the liver fluke O. viverrini are driven by two related aims. Firstly, to reduce transmission of the parasite, leading to its elimination (see Glossary). Secondly, to halt the progression of O. viverrini-induced liver pathology, and thus prevent new cases of cholangiocarcinoma [1]. In endemic regions, definitive hosts become infected through consumption of raw or insufficiently cooked freshwater fish encysted with metacercariae and perpetuate the life cycle by defecating into water sources containing snails of the Bithynia genus. Programmes to control O. viverrini therefore have a number of interventions available to them, including health education to promote safe eating habits; case diagnosis and anthelmintic treatment; improvements to sanitation; and food safety controls [2]. Parasite-induced liver pathology can be tackled with ultrasound screening to detect periductal fibrosis, an early warning sign of cholangiocarcinoma; and operative surgery to improve survival rates [3]. With such a variety of tools on offer, policy makers, international health organisations, and affected communities are entitled to ask: what works? Which measures are most effective at interrupting transmission of the parasite and reducing mortality in a cost-effective, sustainable manner?The traditional method for answering this question is through randomised controlled trials, which are the gold standard for assessing interventions [4]. Such trials, however, are costly, take many years, and require substantial manpower and expertise. It is also unethical to conduct trials in contexts where public health initiatives are ongoing and would have to be withdrawn for participants allocated into the control arm of a trial. An alternative approach is to use quantitative models to make predictions on the effectiveness of different control strategies. These models take myriad forms, can be parameterised from routinely collected field data, and provide considerable insight for a fraction of the cost of a trial. Different types of models that can capture macroparasite dynamics are summarised in Box 1.When the need to conduct a clinical trial is unavoidable, for example on novel therapeutics or diagnostics, dynamic simulations can provide powerful insights into trial design, power HighlightsH...

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Throughout Southeast Asia and China, eating raw and or partially cooked cyprinid fish causes liver (hepatobiliary) disease and cancer (cholangiocarcinoma) due to fishborne zoonotic trematodes (FZT), in particular Clonorchis sinensis and Opisthorchis viverrini. The primary source of transmission is by native fish, but aquaculture fish are also reported to have high infective potential. Here, a cross-sectional survey of FZT in fish farms was conducted in an endemic area in Khon Kaen Province, Thailand. By using conventional and polymerase chain reaction (PCR) methods, we detected O. viverrini and FZT metacercariae (Centrocestus formosanus and Haplorchis taichui) in two popular fish species, Barbonymus gonionotus (silver barb) and Cirrhinus mrigala (mrigal), from aquaculture farms. Both species were infected in five of six farms examined by PCR but not by conventional methods, yet the prevalence of FZT metacercariae in aquaculture fish was high (46.9%). In addition to O. viverrini (17.1%), the native fish Cyclocheilichthys armatus and Hampala dispar had a prevalence of FZT of 81.4%, which included 5.7% for C. formosanus and 17.1% for H. taichui by conventional method. To our knowledge, this is the first discovery of O. viverrini in aquaculture fish in Thailand. More comprehensive studies are required to determine if human-induced disease transmission coupled with natural transmission cycle occurs throughout the aquaculture industry in the region. This has significant impact on food quality and safety, and provides the basis for the development of an effective strategy for the prevention and control of foodborne diseases.