Seasonal transmission of Opisthorchis viverrini sensu lato and a lecithodendriid trematode species in Bithynia siamensis goniomphalos snails in northeast Thailand

Clonorchiasis and opisthorchiasis: epidemiology, transmission, clinical features, morbidity, diagnosis, treatment, and control

Clonorchis sinensis, Opisthorchis viverrini, and Opisthorchis felineus are important liver flukes that cause a considerable public health burden in eastern Asia, southeastern Asia, and eastern Europe, respectively. The life cycles are complex, involving humans, animal reservoirs, and two kinds of intermediate hosts. An interplay of biological, cultural, ecological, economic, and social factors drives transmission. Chronic infections are associated with liver and biliary complications, most importantly cholangiocarcinoma. With regard to diagnosis, stool microscopy is widely used in epidemiologic surveys and for individual diagnosis. Immunologic techniques are employed for screening purposes, and molecular techniques facilitate species differentiation in reference laboratories. The mainstay of control is preventive chemotherapy with praziquantel, usually combined with behavioral change through information, education and communication, and environmental control. Tribendimidine, a drug registered in the People's Republic of China for soil-transmitted helminth infections, shows potential against both C. sinensis and O. viverrini and, hence, warrants further clinical development. Novel control approaches include fish vaccine and biological control. Considerable advances have been made using multi-omics which may trigger the development of new interventions. Pressing research needs include mapping the current distribution, disentangling the transmission, accurately estimating the disease burden, and developing new diagnostic and treatment tools, which would aid to optimize control and elimination measures.

Examining ecosystem (dis-)services on liver fluke infection in rural Northeast Thailand

BACKGROUND

The direct reliance of humans on and their interactions with freshwater ecosystems in the Lower Mekong Basin have given rise to parasitic infections, which is particularly prevalent in Northeast Thailand where raw fish consumption is practiced. This study examined the interactions between environments, ecosystem (dis-)services, human raw fish consumption habits, and raw fish dish sharing on liver fluke infection risk.

METHOD

Water fecal contents and the first intermediate snail host were sampled between June and September of 2019. One hundred twenty questionnaires were surveyed in two villages of different environmental surroundings, one next to a river and the other located inland, in Northeast Thailand. Multivariate regression analyses using linear mixed effect models assessed the influence of social, behavioral and perceptual factors on raw fish consumption frequency, willingness to avoid consumption and liver fluke infection status. Social network analysis compared the degree of raw fish dish sharing between the villages and assessed the probable influence of connections to fish procurement locations and sharing activities on liver fluke infection risk.

RESULTS

High abundance of the first intermediate snail host and presence of fecal contamination in water could endanger both villages to ecosystem disservices of parasitic transmission. The river-side village relied more on provisioning ecosystem services than the inland village (29.7% vs. 16.1% of villages) to consume raw fish as their main source of protein. Males in both villages (64.5 and 40.4 days/year for the respective villages) are also likely to consume koi pla and pla som, higher risk fish dishes, more frequently than females (4.1 and 4.3 days/year for the respective villages). The consumption habits of both villages were driven mostly by deriving cultural ecosystem services. Participation in raw fish dish sharing activities significantly reduced the odds of an individual being willing to avoid the consumption (Odds ratio = 0.19). Network analysis suggested that river-side villagers had a more direct raw fish dish sharing interaction and they procured fish from multiple locations; these characteristics might potentially account for more liver fluke infected households in the village.

CONCLUSION

Villagers' raw fish consumption is driven by deriving cultural ecosystem services, and the geographic settings of the villages potentially affect villagers' fish procurement locations and infection risk. The findings underscore the linkages between villagers and their surrounding ecosystem environments as pertinent determinants for foodborne parasitic disease risk.

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.

Population dynamics and diversity of trematode infections in Bithynia siamensis goniomphalos in an irrigated area in northeast Thailand

Several trematodes including Opisthorchis viverrini utilize Bithynia siamensis goniomphalos as a snail intermediate host in their life cycles. In order to capture a comprehensive range of host–parasite interactions and their transmission dynamic patterns, B. s. goniomphalos were sampled monthly over 4 consecutive years in an irrigated paddy-field habitat in northeast Thailand. Using a standard cercarial shedding method, a high diversity of trematodes (17 types) was recovered. Virgulate xiphidiocercariae were the most prevalent (7.84%) followed by O. viverrini (0.71%). In addition to seasonal and environmental factors, the quantity of irrigation water for rice cultivation correlated with transmission dynamics of trematodes in B. s. goniomphalos. The peak prevalence of all trematode infections combined in the snails shifted from the cool-dry season in 2010–2012 to the hot-dry season in 2013 associated with an increasing quantity of water irrigation. A low frequency of mixed trematode infections was found, indicating that the emergence of virgulate cercariae, but not of O. viverrini, was negatively impacted by the presence of other trematodes in the same snail. Taken together, the observed results suggest that interactions between host and parasite, and hence transmission dynamics, depend on specific characteristics of the parasite and environmental factors including irrigated water for rice cultivation.

Effects of Environmental Factors and Infecting Trematodes on the Size and Inorganic Elements of Bithynia siamensis goniomphalos Snails in Northeast Thailand

Infection with the foodborne trematode, Opisthorchis viverrini, is a major public health issue in southeast Asia. The freshwater snail, Bithynia siamensis goniomphalos, is an intermediate host of O. viverrini and other trematode species. Understanding the effects of environmental conditions and infecting trematodes on B.s. goniomphalos snails is thus crucial for the potential influences on trematode transmission. This study measured environmental variables of water and soil properties, and analyzed B.s. goniomphalos snails for their trematode infection, snail shell length, and inorganic elemental concentration, from 30 localities in northeast Thailand. The results showed that prevalence of trematode infection in B.s. goniomphalos was 3.82%. Nine types of trematode cercariae were identified, with virgulate type 1 as the most common (1.23%). Opisthorchis viverrini-infected snails were mostly found in low-humic gley soils in Sakon Nakhon Province, and were associated with water dissolved oxygen and soil pH. Compared with uninfected snails, larger sizes were observed in virgulate type 1 and pleurolophocerca-infected snails, whereas hypercalcification was noticed in virgulate type 1, virgulate type 3, and pleurolophocerca-infected snails. Infected snails were more sensitive toward environmental conditions, possibly because of the dynamic parasitic processes between trematodes and hosts. Among the environmental factors, soil texture (i.e., sand, silt, and clay compositions) exhibited more significant correlations with B.s. goniomphalos shell characteristics regardless of the trematode infection types. The findings of this study underscore the need to consider the effects of environmental conditions and trematode species-specific pathogenic processes for a more effective and sustainable parasitic control and prevention effort.

Masculinity and misinformation: Social dynamics of liver fluke infection risk in Thailand

Liver fluke infection through the consumption of raw or undercooked freshwater fish is a major public health problem in the Mekong Region. Despite the extensive efforts of liver fluke health campaigns, Northeast Thailand still reports high human infection prevalence as consumption of raw fish dishes has diminished but not ceased. This study examines the roles of social-cultural factors, particularly the influences of masculinity and misinformation, on liver fluke infection risk. Participant observation, questionnaire surveys and semi-structured interviews were conducted in four villages in Kalasin Province, Thailand, to scrutinize reasons for raw fish consumption, gender differences in raw fish culture, processes of liver fluke information dissemination, and the extent of information mismatch. Our results show that one of the key reasons (76.9%) underlying continued raw fish consumption are deeply embedded cultural practices associated with ways of rural life. About 30% of the participants indicated that they would not avoid eating raw fish, regardless of knowing the health consequences. Gender difference is evident, with 75.6% of males consuming raw fish salad (koi pla), compared to 42.7% of females. Some male participants associate raw meat consumption with virility and strength. Such beliefs underscore the cultural linkage of koi pla consumption with masculinity. Misconceptions of liver fluke life cycle and risk of infection remain, as only 15.3% of the participants correctly selected raw fish as the food source for liver fluke infection while 84.2% misunderstood that other raw foods could lead to infection. The multi-layered and hierarchical structure of public health information dissemination from medical professionals to health officers and village health volunteers to villagers has contributed to information mismatch between different layers. Our study builds on others which call for multi-pronged scientific and social strategies, as well as culturally attuned approaches to public health messaging. The study raises masculinity and misinformation as relevant considerations in disease prevention. Incorporating grounded research and gendered perspectives are part of appreciating the cultural roots of raw fish consumption. Realizing the significant role of village health volunteers in information dissemination and in supplying coherent public health messages is vital for effective health campaigns.

Cercarial trematodes in freshwater snails from Bangkok, Thailand: prevalence, morphological and molecular studies and human parasite perspective

We investigated the prevalence, morphological characters and molecular classifications of trematode cercariae in freshwater snails randomly collected from 59 sampling localities in Bangkok from May 2018 to March 2019. We used a crushing technique to observe the cercarial stage inside each snail body and amplified the internal transcribed spacer 2 regions of cercarial DNA using polymerase chain reaction methodology. The associated phylogenetic tree was reconstructed using Bayesian inference analyses. A total of 517 of 15 621 examined snails were infected with trematode cercariae, and the infected snails were classified into 11 species of seven families with a 3.31% overall prevalence of the infection. The Bithynia siamensis siamensis snail displayed the highest prevalence of infection (16.16%), whereas the Physella acuta snail exhibited the lowest prevalence (0.08%) of infection. Eight morphological types of cercariae were observed. The highest prevalence of infection was observed in mutabile cercaria (1.86%). Based on molecular investigations, the phylogram revealed eight cercaria types assigned to at least nine digenean trematode families, of which five belong to groups of human intestinal flukes. Although, with the exception of schistosome cercaria, trematode cercariae are not known to directly damage humans, understanding the general biology of trematode cercariae (including diversity, distribution, infection rates and host range) is important and necessary for the prevention and control of parasitic transmission that impacts aquatic cultivations, livestock farming and human health.

Very low prevalence of Opisthorchis viverrini s.l. cercariae in Bithynia siamensis siamensis snails from the canal network system in the Bangkok Metropolitan Region, Thailand

The liver fluke Opisthorchis viverrini s.l. is associated with a long-term public health problem in Thailand. However, O. viverrini s.l. infection in Bithynia snails in the canal network system (CNS) in the Bangkok Metropolitan Region (BMR) has never been assessed. This study aimed to investigate the occurrence of B. siamensis siamensis and the prevalence of O. viverrini s.l. infection in this snail in the CNS in BMR along with morphological examination and molecular analyses on O. viverrini s.l. cercariae. The snails were randomly sampled from the CNS in all BMR areas from January 2018 to July 2019. Snail specimens were identified and examined for digenean infection by shedding and dissection. The cercariae were identified using morphology and molecular methods, including PCR with a species-specific primer and a Bayesian phylogenetic analysis of ITS2 sequences. Bithynia siamensis siamensis was found in almost all sampling localities, with different quantities and detected frequencies. From a total of 7473 B. s. siamensis specimens, O. viverrini s.l. infections were detected in the Northern Bangkok, Muang Nakhon Pathom, Krathum Baen, and Lam Luk Ka areas with an overall prevalence of 0.05% (4/7473) and prevalence of 0.22% (1/455), 0.21% (1/469), 0.40% (1/253), and 0.16% (1/614) in individual localities with positive snails, respectively. This study is the first investigation of digenean infection in the canal network system-type habitat in Thailand and revealed extremely low O. viverrini s.l. prevalence.

Following the infection dynamics of the tropical trematode Oligogonotylus mayae in its intermediate and definitive hosts for 13 years

We present a time series of 13 years (2003-2016) of continuous monthly data on the prevalence and mean abundance of the trematode Oligogonotylus mayae for all the hosts involved in its life cycle. We aimed to determine whether annual (or longer than annual) environmental fluctuations affect these infection parameters of O. mayae in its intermediate snail host Pyrgophorus coronatus, and its second and definitive fish host Mayaheros urophthalmus from the Celestun tropical coastal lagoon, Yucatan, Mexico. Fourier time series analysis was used to identify infection peaks over time, and cross-correlation among environmental forcings and infection parameters. Our results suggest that the transmission of O. mayae in all its hosts was influenced by the annual patterns of temperature, salinity and rainfall. However, there was a biannual accumulation of metacercarial stages of O. mayae in M. urophthalmus, apparently associated with the temporal range of the El Niño-Southern Oscillation (five years) and the recovery of the trematode population after a devasting hurricane. Taking O. mayae as an example of what could be happening to other trematodes, it is becoming clear that environmental forcings acting at long-term temporal scales affect the population dynamics of these parasites.

Assessing the role of Filopaludina martensi martensi as a biocontrol agent of Bithynia siamensis goniomphalos, the first intermediate host of Opisthorchis viverrini

The freshwater snail Bithynia siamensis goniomphalos serves as the first intermediate host of liver fluke Opisthorchis viverrini, a foodborne parasite, of which human infection has persisted in Southeast Asia for decades. The snail Filopaludina martensi martensi has been proposed as a biological control agent against B.s. goniomphalos, but knowledge on the snail ecology and population dynamics between the two species remains rudimentary. This study investigated selected abiotic and biotic factors influencing the distribution and abundance of B.s. goniomphalos and F.m. martensi. Water quality, soils, and snails were collected from 34 localities in Northeast Thailand. Soil properties and snail soft tissue elemental concentrations were analyzed. Experiments were performed to examine interspecific competition. Statistical analysis was conducted to explore the associations between water and soil properties and soft tissue elemental concentrations. The results showed that B.s. goniomphalos had the highest mean dominance in streams and red-yellow podzolic soils, while F.m. martensi snails preferred ponds and latosol soils. Negative correlation in species abundances was found between the two species. Interspecific competition was detected, with B.s. goniomphalos growth rates hampered by the presence of F.m. martensi. Despite the possibility of using F.m. martensi to control B.s. goniomphalos, B.s. goniomphalos exhibited a greater adaptability to different water and soil properties, suggesting that the species could colonize a wide range of environmental conditions. This study provides further insights into the ecology of the two snail species, underscoring the importance of considering abiotic factors when assessing the possible biological control agent to control O. viverrini transmission.

Influence of Water Irrigation Schemes and Seasonality on Transmission Dynamics of Opisthorchis viverrini in the Snail Intermediate Host, Bithynia siamensis goniomphalos in Rice Paddy Fields in Northeast Thailand

Opisthorchis viverrini is a fish-borne zoonotic trematode that causes significant public health problems in Southeast Asia. Its life cycle requires Bithynia snails as the first intermediate hosts, fish, and human and/or carnivore hosts. This study assessed impacts of land use practice for rice cultivation and seasonality on the transmission dynamics of O. viverrini in Bithynia siamensis goniomphalos in rice paddy field habitats. The transmission of O. viverrini cercariae in B. s. goniomphalos was monitored at monthly intervals over a 4-year period from January 2010 to December 2013. From a total of 59,727 snails examined by standard cercarial shedding, the prevalence of O. viverrini was 0.7% (range, 0.0-4.1%). The prevalence of O. viverrini infection in B. s. goniomphalos varied with the amount of rainfall, with peaks of infection occurring in the cool-dry season, that is, after each rainy season. A shift of peak prevalence from cool-dry to hot-dry season observed in 2013 was associated with the increase in preceding water irrigation to support the production of second annual rice crop. Significant positive correlations were found between the prevalence and intensity of cercarial infection and the size of snails. Our results revealed substantial variation between years so that to have a clear understanding of the population dynamics of this complex system, studies should be conducted over an extended period (> 1 year). Results from this study highlight that water irrigation schemes in rice paddy cultivation and seasonality have a significant effect on the prevalence of O. viverrini in B. s. goniomphalos.

Infection Dynamics of Opisthorchis viverrini Metacercariae in Cyprinid Fishes from Two Endemic Areas in Thailand and Lao PDR

The infection dynamics of Opisthorchis viverrini metacercariae was analyzed in cyprinid fish from endemic areas in Mukdahan Province, Thailand, and Khammouane Province, Lao PDR. The fish were collected during the cool-dry (November-February), hot-dry (March-May), and rainy (June-October) seasons in 2017. They were examined by the digestion method, and the infection status was statistically analyzed by study area, season, and fish size. The prevalence (no. of fish positive/no. of fish examined × 100) and metacercarial intensities (no. of metacercariae detected/no. of fish positive) of O. viverrini in both study areas depended on season, being high in the cool-dry season and varying in the hot-dry and rainy seasons. In Mukdahan Province, the average prevalence was 18.3% (range 11.0-46.7%, n = 420) and the intensity was 4.07 ± 5.86 cysts/fish (mean ± SD), whereas in Khammouane Province, the prevalence was 51.9% (range 9.1-70.6%, n = 673) and the intensity was 6.67 ± 12.88 cysts/fish. Among the cyprinid fish species examined, the infection was associated with fish body size and predominantly found in Hampala dispar (86.5%), Cyclocheilichthys armatus (73.2%), and Puntius brevis (42.7%). The distribution of O. viverrini metacercariae in fish was skewed, with most of the fish having a low worm burden with an average of four to six cysts/fish. The findings that seasonality, sampling locality, fish size, and species of fish play roles in the risk of O. viverrini infection imply that these host and environmental factors are important for the transmission dynamics and control of O. viverrini.

Recent advances on the immunobiology of Bithynia spp. hosts of Opisthorchis viverrini

This article reviews the past and present scientific reports regarding Bithynia spp. focusing on the biology, ecology and life cycle of Bithynia snails and their responses to Opisthorchis viverrini infection. Moreover, new data regarding comparative molecular genomics and proteomic approaches have recently revealed novel molecular components involved in the immune defence responses from Bithynia spp., providing additional perspectives for future studies. Studies on the specific interaction between Bithynia snails and their trematodes will contribute to further understanding the snail-parasite relationship with regards to epidemiology and control of Opisthorchiasis and broaden the scope on comparative immunology of gastropod snails.

Ecological Niche Model based on Maximum Entropy for mapping distribution of Bithynia siamensis goniomphalos, first intermediate host snail of Opisthorchis viverrini in Thailand

The snail Bithynia siamensis goniomphalos acts as first intermediate host of the liver fluke, Opisthorchis viverrini, which causes opisthorchiasis in humans. In this study, we used a geographic information system (GIS), remote sensing (RS) and software using the maximum entropy (MaxEnt) algorithm to predict the distribution of B. s. goniomphalos in Thailand on the basis of environmental and climatic factors. The MaxEnt model for B. s. goniomphalos was excellent, with average test AUC values of 0.89. The predicted distribution of B. s. goniomphalos was affected by altitude, land cover, normalized difference vegetation index (NDVI), precipitation in the driest month (BIO 14), land surface temperature (LST) and soil pH. The areas suitable for B. s. goniomphalos were mostly in Northeast Thailand and some northern parts of the country. The presence of B. s. goniomphalos decreases with increasing altitude and increasing NDVI value. Bithynia s. goniomphalos is most likely to occur in paddy fields and cropland. Opisthorchiasis prevalence was directly proportional to the likelihood of snail occurrence as predicted by the model. This is the first time this ecological niche model has been used to predict Bithynia snail distribution and hence to provide a basis for future work of opisthorchiasis prevention in opisthorchiasis-endemic countries.

Effects of aestivation on survival of Bithynia siamensis goniomphalos snails and the infection of Opisthorchis viverrini in the irrigation area of wet- and dry-season rice paddy

The snail Bithynia siamensis goniomphalos acts as the first intermediate host of the human liver fluke, Opisthorchis viverrini, a major cause of cholangiocarcinoma in Northeast Thailand. In this study, we investigated host-parasite interactions in wet- and dry-season rice paddy plantation. The snails that aestivated through 4-month-dried period after rice harvest had average mortality rate of 36.45% and monthly increasing from 17.76% to 54.57%. Surprisingly, at depths greater than 5 cm experienced a higher mortality rate than upper level. Average mortality rate at the depth more than 5 cm, was 42.97%. In the initial month of aestivation, mortality rate of 0-5 and 5-10 cm depth was 9.13% and 26.39% then increase to 57.58% and 51.97%, respectively in the last 4th month. The average prevalence O. viverrini infection in snails during the study period was 0.44%. The highest prevalence of O. viverrini infection was found in the cool dry and wet season. The odds of O. viverrini infection in female snails and large snails were higher relative to male snails and medium-sized snails. The physicochemical factors involved in increased prevalence of O. viverrini infection in snail hosts were mean daily air temperature and average monthly rainfall. Short aestivation period made lower mortality in irrigation area than the previous report of non-irrigation area, that is, the aestivation is one of snail population control.

Liver Flukes: Clonorchis and Opisthorchis

Clonorchis sinensis, Opisthorchis viverrini, and O. felineus are liver flukes of human and animal pathogens occurring across much of Europe and Asia. Nevertheless, they are often underestimated compared to other, better known neglected diseases in spite of the fact that many millions of people are infected and hundreds of millions are at risk. This is possibly because of the chronic nature of the infection and disease and that it takes several decades prior to a life-threatening pathology to develop. Several studies in the past decade have provided more information on the molecular biology of the liver flukes which clearly lead to better understanding of parasite biology, systematics, and population genetics. Clonorchiasis and opisthorchiasis are characterized by a chronic infection that induces hepatobiliary inflammation, especially periductal fibrosis, which can be detected by ultrasonography. These chronic inflammations eventually lead to cholangiocarcinoma (CCA), a usually fatal bile duct cancer that develops in some infected individuals. In Thailand alone, opisthorchiasis-associated CCA kills up to 20,000 people every year and is therefore of substantial public health importance. Its socioeconomic impacts on impoverished families and communities are considerable. To reduce hepatobiliary morbidity and CCA, the primary intervention measures focus on control and elimination of the liver fluke. Accurate diagnosis of liver fluke infections in both human and other mammalian, snail and fish intermediate hosts, are important for achieving these goals. While the short-term goal of liver fluke control can be achieved by praziquantel chemotherapy, a comprehensive health education package targeting school children is believed to be more beneficial for a long-term goal/solution. It is recommended that a transdisciplinary research or multisectoral control approach including one health and/or eco health intervention strategy should be applied to combat the liver flukes, and hence contribute to reduction of cholangiocarcinoma in endemic areas.

Phylogenetic relationships within the Opisthorchis viverrini species complex with specific analysis of O. viverrini sensu lato from Sakon Nakhon, Thailand by mitochondrial and nuclear DNA sequencing

The liver fluke Opisthorchis viverrini sensu lato causes serious public-health problems in Northeast Thailand and Southeast Asian countries. A hypothesis has been proposed that O. viverrini represents a species complex with varying levels of genetic differentiation in Thailand and Lao PDR. This study aimed to clarify whether O. viverrini populations can be genetically divided into separate taxa. We collected O. viverrini s.l. from eight different locations in Lao PDR and Thailand. The results of nad1, cox1, CF-int6, Pm-int9, ITS2 and 28S rDNA sequence analysis revealed that sub-structuring occurred between the eight populations. We found that O. viverrini s.l. from Sakon Nakhon (SK), Thailand, shows significant genetic differentiation (P < .05) from all other isolates from different localities in Thailand and Lao PDR. This was supported by haplotype and phylogenetic tree analyses in which the SK isolate was separated from all other isolates. This suggests that O. viverrini s.l. from SK is a cryptic species. The data, however, also confirm the association between genetic groups of O. viverrini s.l. and specific wetland systems, and raise important questions regarding the epidemiological significance of these genetic differences.

A Large Scale Study of the Epidemiology and Risk Factors for the Carcinogenic Liver Fluke Opisthorchis viverrini in Udon Thani Province, Thailand

Opisthorchis viverrini infection and cholangiocarcinoma are serious problems in South East Asia. This study aimed to find the prevalence of opisthorchiasis in various hosts in Udon Thani Province. Total fecal samples were collected from 14,766 participants. The epidemiological data collected and analysed included prevalence and intensity of infection. Odds ratios (OR) were calculated to determine the associations between cross sectional data and to predict possible risk factors. The prevalence of O. viverrini infection in Udon Thani Province averaged 15.3% (eggs per gram (epg.) = 48.9 and range; 12-1, 320), with differences between villages (range; 3.8%-79.8%). An age-dependence for infection was observed to increase from ages 25 to 50 years and then decrease for older participants. A univariate analysis identified risk parameters including age (p = 0.040; OR = 3.9 (95% CI = 1.2-7.5)), education (p < 0.0001; OR = 7.3 (95% CI = 1.8-21.6)) and eating habits (p = 0.032; OR = 1.6 (95% C = 0.5-3.7)). Interestingly, most participants were not aware of treatments such as praziquantel (p < 0.0001; OR = 3.5 (95% CI = 1.4-11.6)), had no history of parasitic treatment (p = 0.486; OR = 1.5 (95% CI = 0.5-3.5) and had eaten raw fish (p=0.04; OR = 7.4 (95% CI = 1.5-18.6)). Liver fluke infection in dogs (18.1%, epg. = 44.7, range; 32-96) was significantly higher than in cats (11.0%, epg. = 117.8, range; 44-372) (p < 0.05). A positive association between O. viverrini infection in dogs and their owners was found. In addition, cyprinid fish dominantly infected by metacercaria including Henicorhynchus siamensis (27.7%), Cyclocheilichthys repasson (21.9%), Hampala dispar (14.1%), and Barbonymus gonionotus (6.9%). This study provides basic information required for the development of future effective and sustainable strategies to reduces infection rates, mainly by providing health education and encouraging behavioural changes.

Opisthorchiasis and cholangiocarcinoma in Southeast Asia: an unresolved problem

The prevalence of cholangiocarcinoma (CCA) in Southeast Asia is much higher than other areas of the world. Eating raw, fermented, or undercooked cyprinid fish, infected with the liver fluke, Opisthorchis viverrini sensu lato (sl), results in chronic biliary inflammation, periductal fibrosis, and increased cancer risk. There may be associated glomerulonephritis. The process of infection is difficult to disrupt because eating practices have proven extremely difficult to change, and the life cycle of the fluke cannot be broken due to high prevalence in canine and feline reservoir hosts. Fecal analysis and enzyme-linked immunosorbent assay tests can be used to diagnose opisthorchiasis. Diagnosis of CCA is complex, partly due to the lack of definitive imaging characteristics but also due to the difficulty of obtaining samples for cytology or histology. This cancer has proven to be resistant to common chemotherapy treatments and so the two avenues of treatment available are surgical resection and liver transplantation, both requiring early detection of the tumor for the best chances of success. Late presentation of symptoms reduces the chances of successful surgical intervention. While liver fluke infections can be treated with praziquantel, individuals will often become reinfected, and multiple reinfections can be more harmful than a singular, long-term infection. A key research on the detection and characterization of novel biomarkers in all parts of the carcinogenic pathway for early diagnosis is needed.

Prospects and Challenges towards Sustainable Liver Fluke Control

The liver fluke Opisthorchis viverrini (Ov) is endemic in Southeast Asia where more than 10 million people are estimated to be infected. The infection is associated with several hepatobiliary diseases, including cholangiocarcinoma (CCA). Northeast Thailand is a hotspot for Ov transmission, and, despite extensive public health prevention campaigns led by the government, the prevalence of Ov infection is still high. High infection rates result from cultural and ecological complexities where wet-rice agrarian habitats, centuries-old raw-food culture, and the parasite's complex biology combine to create an ideal transmission arena. Here we review the state of our knowledge regarding the social-ecological determinants underlying Ov transmission. We also describe an integrative research rationale for liver fluke control better aligned with sustainable health development.

Seasonal and Annual Variation in Trematode Infection of Stream Snail Elimia proxima in the Southern Appalachian Mountains of Virginia

Understanding temporal variation of host-pathogen dynamics can be important for predicting disease risks and anticipating how disease systems may change in response to natural or human disturbances. Seasonal changes in weather, especially those associated with changes in temperature or precipitation, are often a key component of temporal changes in infection risk and can have important impacts on disease systems. However, these patterns can be difficult to track due to interannual variation and the need for longer term, multi-year surveillance efforts. We assessed seasonal and annual changes in the trematode component community of first-intermediate host stream snail Elimia (= Oxytrema = Goniobasis) proxima across 5 streams in the southern Appalachian Mountains. Over 3 yr, we found no evidence of consistent seasonal peaks of trematode infection in E. proxima. There was some across-site consistency in infection prevalence over 4 yr, because high prevalence sites tended to maintain higher prevalence from year to year, relative to lower prevalence sites. In addition, we examined the relationship between prevalence of first-intermediate host infection, weather variables, and site-level factors, including snail density and water quality metrics. Trematode prevalence was negatively related to total precipitation, which may have been due to the movement of infectious parasite stages and hosts downstream during high flows. We found no strong relationships between trematode prevalence and snail density or any of the water quality metrics examined in this study, indicating that snail infection may be driven primarily by definitive host activity.

Trematode infections of the freshwater snail Biomphalaria pfeifferi from a south-east Nigerian community with emphasis on cercariae of Schistosoma

Knowledge of trematode infections in Biomphalaria pfeifferi from south-eastern Nigeria is scarce, due to the absence of Schistosoma mansoni infection in the region. Therefore, the present study sought to describe trematode infections in B. pfeifferi from the River Uzuru in the Nigeria Cement Factory area, Nkalagu, south-eastern Nigeria. Four hundred and sixty snails were checked for trematode infections, and mice were exposed to the Schistosoma cercariae shed from the snails. Adult worms were harvested from the mice 13 weeks post-infection, while sections of the liver and spleen were examined. Primary school children living in the area were screened for S. mansoni infection and assessed for activities involving water contact. The edges of the river were also searched for burrows and rodents. The five cercaria morphotypes found were armatae xiphidiocercariae, echinocercariae, Schistosoma cercariae, cystophorous cercariae and cercariaeum cercariae. The overall prevalence and mean intensity of trematode infections were 39.78% and 195.46, with the prevalence and mean intensity of most cercaria morphotypes higher in the hot-dry than in the cool-dry season. The infected mice showed S. mansoni-like characteristics but the stool samples of the schoolchildren were negative for S. mansoni eggs. Water-contact activities in the River Uzuru were minimal. Burrows were seen at the river edges but no Schistosoma eggs were recovered from captured rats. This is the first report of Schistosoma cercariae and other cercaria morphotypes in B. pfeifferi from south-eastern Nigeria. Additional molecular investigations are needed to identify correctly these Schistosoma cercariae, due to their public health implication.

Assessing the role of landscape connectivity on Opisthorchis viverrini transmission dynamics

Opisthorchis viverrini (Ov) is one of the most important human parasitic diseases in Southeast Asia. Although the concept of connectivity is widely used to comprehend disease dispersal, knowledge of the influences of landscape connectivity on Ov transmission is still rudimentary. This study aimed to investigate the role of landscape connectivity in Ov transmission between the human and the first intermediate snail hosts. Fieldwork was conducted in three villages respectively in Kamalasai District, Kalasin Province, Phu Wiang District, Khon Kaen Province, and Nong Saeng District, Udon Thani Province. Bithynia snails were collected to examine parasitic infections, water samples were analyzed for fecal contamination, and locations of septic tanks and connections between habitat patches with observable water movement were surveyed. Euclidean distance, topological link and distance, and graph measures were employed to quantify the connectivity between human and snail habitats. The findings showed that snail patches with higher fecal contents were generally located nearer to septic tanks. The statistically significant results for the topological link and distance measures highlighted the importance of water in functionally facilitating Ov transmission. Graph measures revealed differences in landscape connectivity across the sites. The site with the largest landscape component size and the most mutually connected snail patches coincided with the presence of Ov parasite, reinforcing its higher risk for human to snail transmission. The site with the dissected landscape structure potentially limited the transmission. This study underscored the potential effect of landscape connectivity on Ov transmission, contributing to the understanding of the spatial variation of Ov infection risk.

Untangling the Complexity of Liver Fluke Infection and Cholangiocarcinoma in NE Thailand Through Transdisciplinary Learning

This study demonstrates how a transdisciplinary learning approach provided new insights for explaining persistent Opisthorchis viverrini infection in northern Thailand, as well as elucidating problems of focusing solely on the parasite as a means of addressing high prevalence of cholangiocarcinoma. Researchers from diverse backgrounds collaborated to design an investigative homestay program for 72 Singaporean and Thai university students in five northeast Thai villages. The students explored how liver fluke infection and potential cholangiocarcinoma development are influenced by local landscape dynamics, aquatic ecology, livelihoods, food culture and health education. Qualitative fieldwork was guided daily by the researchers in a collaborative, co-learning process that led to viewing this health issue as a complex system, influenced by interlinked multidimensional factors. Our transdisciplinary experience has led us to believe that an incomplete understanding of these linkages may reduce the efficacy of interventions. Further, viewing liver fluke infection and cholangiocarcinoma as the same issue is inadvisable. Although O. viverrini infection is an established risk factor for the development of cholangiocarcinoma, multiple factors are known to influence the likelihood of acquiring either. Understanding the importance of the current livelihood transition, landscape modification and the resulting mismatch between local cultures and new socio-ecological settings on cholangiocarcinoma initiation and liver fluke transmission is of critical importance as it may help readjust our view of the respective role of O. viverrini and other socioeconomic risk factors in cholangiocarcinoma etiology and refine intervention strategies. As demonstrated in this study, transdisciplinary approaches have the potential to yield more nuanced perspectives to complex diseases than research that focuses on specific aspects of their epidemiology. They may therefore be valuable when designing effective solutions to context-sensitive diseases such as liver fluke infection and cholangiocarcinoma.