Impact of temporal changes and host factors on the genetic structure of a population of Opisthorchis viverrini sensu lato in Khon Kaen Province (Thailand)

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 is 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 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 CCA in endemic areas.

Opisthorchis viverrini Life Cycle, Distribution, Systematics, and Population Genetics

Opisthorchis viverrini plays a key role as the carcinogenic liver fluke causing bile duct cancer in Southeast Asia. A comprehensive understanding of its life cycle, distribution, systematics, and population genetics is critically important as they underpin the effective development and establishment of future prevention and control programs that center on opisthorchiasis and cholangiocarcinoma. This chapter provides detailed information concerning the basic biology and updated information of O. viverrini related to its host life cycle, transmission route via raw, partially cooked or fermented freshwater cyprinid fish, endemic areas, and the discovery of new foci. Previous sequential studies over the last two decades on the phylogenetic and systematic relationships, genetic variation, and population genetics of O. viverrini as well as its snail intermediate host Bithynia spp. are presented and discussed, which have led to the currently known complex species level systematics and population genetics framework of this host-parasite system. Additionally, further directions for comprehensive research are suggested to provide a more complete understanding of liver fluke, O. viverrini-related cholangiocarcinoma.

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.

Impact of geography and time on genetic clusters of Opisthorchis viverrini identified by microsatellite and mitochondrial DNA analysis

Infection by the small liver fluke, Opisthorchis viverrini, causes serious public health problems, including cholangiocarcinoma, in Thailand and southeastern Asian countries. Previous studies have reported that O. viverrini represents a species complex with varying levels of genetic differentiation in Thailand and Lao PDR. In this study, we re-examined population genetic structure and genetic diversity of O. viverrini using extensive samples of the parasite collected over 15 years from 12 geographical localities in Thailand and eight localities in Lao PDR. Parasite life-cycle stages of 721 individuals of O. viverrini (91 cercariae, 230 metacercariae and 400 adult worms) were genotyped using 12 microsatellite loci. Metacercariae exhibited genetic diversity comparable with that of experimentally raised adults: metacercariae can therefore be used to represent O. viverrini populations without the need for laboratory definitive hosts. Data obtained from larval as well as adult worms identified two distinct genetic clusters of O. viverrini. Sequences of a portion of the mitochondrial cox1 gene strongly supported the existence of these two clusters. One, the widespread cluster, was found at all sampled sites. The second cluster occurred only in Phang Khon District, Sakon Nakhon Province (SPk), within the Songkram River wetland in Thailand. A striking feature of our data relates to the temporal dynamics of the SPk cluster, which was largely replaced by representatives of the widespread cluster over time. If the SPk cluster is excluded, no marked genetic differences were seen among O. viverrini populations from Thailand and Lao PDR. The underlying causes of the observed population structure and population dynamics of O. viverrini are not known.

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.

Current Perspectives on Opisthorchiasis Control and Cholangiocarcinoma Detection in Southeast Asia

Similar to bile duct cancer or cholangiocarcinoma (CCA) in the western world, opisthorchiasis-associated CCA in Southeast Asia is an aggressive cancer with high mortality rates. It is known to cause a significant health burden in the opisthorchiasis region in Thailand and possibly throughout mainland Southeast. To reduce this health burden, a comprehensive prevention and control program for opisthorchiasis, as well as CCA, is required. In this review, our aim is to provide a brief update of the current situation regarding the natural history of opisthorchiasis and health burden of CCA in Southeast Asia. A comprehensive approach to tackling these issues being implemented in Thailand under the “Cholangiocarcinoma Screening and Care Program” is described. This comprehensive program consists of a three stage prevention and patient care program. The primary prevention component involves opisthorchiasis screening using a new and sensitive urine assay. The secondary prevention component involves screening for CCA and periductal fibrosis, with suspected CCA patients following the protocol for confirmation and appropriate treatment. Due to the eco-epidemiology of opisthorchiasis-induced CCA, the anticipated impacts and outcomes of the program include short-, medium-, and the long-term goals for the reduction of CCA incidence. To achieve long-term sustainable impacts, concerted efforts to raise social awareness and participating action by general public, non-government organizations, and government agencies are necessary. The strategic plans developed for this program can be expanded for use in other endemic areas as well as being a model for use in other chronic diseases.

Adaptation of the secretome of Echinostoma caproni may contribute to parasite survival in a Th1 milieu

Echinostoma caproni (Trematoda: Echinostomatidae) is an intestinal trematode, broadly employed to study the host-dependent mechanisms that govern the evolution of intestinal helminth infections. Resistance against E. caproni homologous secondary infections has been reported in mice and appears to be related to the generation of a local Th2 response, whereas Th1 responses promote the development of chronic primary infections. Herein, the ability of E. caproni to modulate its secretome according to the host environment is investigated. A two-dimensional differential in gel electrophoresis (2D-DIGE) analysis was performed to elucidate changes in the excretory/secretory products of E. caproni adults after primary and secondary infections in mice. A total of 16 protein spots showed significant differences between groups, and 7 of them were successfully identified by mass spectrometry. Adult worms exposed to a primary infection appear to upregulate proteins involved in detoxification (aldo-keto reductase), stress response (GroEL), and enhancement of parasite survival (acetyl-CoA A-acetyltransferase and UTP-glucose-1-phosphate urydyltransferase). In contrast, any protein was found to be significantly upregulated after secondary infection. Upregulation of such proteins may serve to withstand the hostile Th1 environment generated in primary infections in mice. These results provide new insights into the resistance mechanisms developed by the parasites to ensure their long-term survival.

Taxonomy, Ecology and Population Genetics of Opisthorchis viverrini and Its Intermediate Hosts

There have been considerable advances in our understanding of the systematics and ecology of Opisthorchis viverrini; however, this new knowledge has not only clarified but also complicated the situation. We now know that what was once considered to be a single species is, in fact, a species complex, with the individual species being confined to specific wetland areas. There is also a strong genetic association between the members of the O. viverrini species complex and their Bithynia snail intermediate hosts. Although this does not negate data collected before the recognition of this situation, it does lead to the caveat that regional and temporal variations in data collected may be related to the species examined. The advances in ecology have generally been spatially limited and have led, in part, to contradictory results that may well be related to nonrecognition of the species studied. It may also be related to natural temporal and spatial variation related, for example, to habitat characteristics. To understand the variation present, it will be necessary to conduct long-term (several years at least) sampling projects after defining the genetic characteristics of O. viverrini sensu lato and its Bithynia snail intermediate hosts.

Preliminary genetic evidence of two different populations of Opisthorchis viverrini in Lao PDR

Opisthorchis viverrini is a major public health concern in Southeast Asia. Various reports have suggested that this parasite may represent a species complex, with genetic structure in the region perhaps being dictated by geographical factors and different species of intermediate hosts. We used four microsatellite loci to analyze O. viverrini adult worms originating from six species of cyprinid fish in Thailand and Lao PDR. Two distinct O. viverrini populations were observed. In Ban Phai, Thailand, only one subgroup occurred, hosted by two different fish species. Both subgroups occurred in fish from That Luang, Lao PDR, but were represented to very different degrees among the fish hosts there. Our data suggest that, although geographical separation is more important than fish host specificity in influencing genetic structure, it is possible that two species of Opisthorchis, with little interbreeding, are present near Vientiane in Lao PDR.

The role of evolutionary biology in research and control of liver flukes in Southeast Asia

Stimulated largely by the availability of new technology, biomedical research at the molecular-level and chemical-based control approaches arguably dominate the field of infectious diseases. Along with this, the proximate view of disease etiology predominates to the exclusion of the ultimate, evolutionary biology-based, causation perspective. Yet, historically and up to today, research in evolutionary biology has provided much of the foundation for understanding the mechanisms underlying disease transmission dynamics, virulence, and the design of effective integrated control strategies. Here we review the state of knowledge regarding the biology of Asian liver Fluke-host relationship, parasitology, phylodynamics, drug-based interventions and liver Fluke-related cancer etiology from an evolutionary biology perspective. We consider how evolutionary principles, mechanisms and research methods could help refine our understanding of clinical disease associated with infection by Liver Flukes as well as their transmission dynamics. We identify a series of questions for an evolutionary biology research agenda for the liver Fluke that should contribute to an increased understanding of liver Fluke-associated diseases. Finally, we describe an integrative evolutionary medicine approach to liver Fluke prevention and control highlighting the need to better contextualize interventions within a broader human health and sustainable development framework.

Population genetic structuring in Opisthorchis viverrini over various spatial scales in Thailand and Lao PDR

Khon Kaen Province in northeast Thailand is known as a hot spot for opisthorchiasis in Southeast Asia. Preliminary allozyme and mitochondrial DNA haplotype data from within one endemic district in this Province (Ban Phai), indicated substantial genetic variability within Opisthorchis viverrini. Here, we used microsatellite DNA analyses to examine the genetic diversity and population structure of O. viverrini from four geographically close localities in Khon Kaen Province. Genotyping based on 12 microsatellite loci yielded a mean number of alleles per locus that ranged from 2.83 to 3.7 with an expected heterozygosity in Hardy–Weinberg equilibrium of 0.44–0.56. Assessment of population structure by pairwise F_ST analysis showed inter-population differentiation (P<0.05) which indicates population substructuring between these localities. Unique alleles were found in three of four localities with the highest number observed per locality being three. Our results highlight the existence of genetic diversity and population substructuring in O. viverrini over a small spatial scale which is similar to that found at a larger scale. This provides the basis for the investigation of the role of parasite genetic diversity and differentiation in transmission dynamics and control of O. viverrini.

Infection with the liver fluke (Opisthorchis viverrini) is a risk factor for cholangiocarcinoma (CCA), which is highly prevalent in Khon Kaen Province, Thailand. Within this Province, there is considerable variation in the epidemiology of opisthorchiasis among districts. Preliminary allozyme and mitochondrial DNA data indicate that genetic variation in O. viverrini occurs even over a small endemic area within the province. Here, we used microsatellite DNA analyses to examine the population genetic structure of O. viverrini from four geographically close localities. Analyses of adult worms based on 12 microsatellite loci revealed varying levels of genetic diversity and population substructuring between the four localities. Worms originating from one locality (Phu Wiang) had significantly higher genetic diversity than the other three localities. Data on the population genetic structure observed in these localities are similar to those found at a larger geographic scale. This provides background data to further investigate the biological and epidemiological significance of these genetic variants.

Spatial and temporal genetic variation of Echinostoma revolutum (Trematoda: Echinostomatidae) from Thailand and the Lao PDR

A total of 314 individual Echinostoma revolutum were collected at different locations and times from domestic ducks from Khon Kaen Province, Thailand and Vientiane Province, the Lao People's Democratic Republic (PDR). Genetic variation of these parasites was analyzed using multilocus enzyme electrophoresis at three polymorphic loci namely, glucose-6-phosphate dehydrogenase (G6pd), malic enzyme (Me) and peptidase valine-leucine (PepA). High levels of genetic variability were found within and between populations. Significant heterozygote deficiencies compared with the predictions under Hardy-Weinberg equilibrium were detected in populations from Thailand and the Lao PDR for all loci except G6pd-1. Significant genetic differentiation was observed between spatially separated populations from Thailand and the Lao PDR. This as also true for some samples collected at different times in Thailand. The variability found may be consistent with a Wahlund effect, genetic drift and/or other factors such as the population structure of snail hosts. Our data provide further insight into the process of genetic divergence within and among geographically and temporally isolated populations of E. revolutum, and potentially other medically important echinostomes in Southeast Asia.

Genetic variation and relationships of four species of medically important echinostomes (Trematoda: Echinostomatidae) in South-East Asia

Multilocus enzyme electrophoresis (MEE) and DNA sequencing of the mitochondrial cytochrome c oxidase subunit 1 (CO1) gene were used to genetically compare four species of echinostomes of human health importance. Fixed genetic differences among adults of Echinostoma revolutum, Echinostoma malayanum, Echinoparyphium recurvatum and Hypoderaeum conoideum were detected at 51-75% of the enzyme loci examined, while interspecific differences in CO1 sequence were detected at 16-32 (8-16%) of the 205 alignment positions. The results of the MEE analyses also revealed fixed genetic differences between E. revolutum from Thailand and Lao PDR at five (19%) of 27 loci, which could either represent genetic variation between geographically separated populations of a single species, or the existence of a cryptic (i.e. genetically distinct but morphologically similar) species. However, there was no support for the existence of cryptic species within E. revolutum based on the CO1 sequence between the two geographical areas sampled. Genetic variation in CO1 sequence was also detected among E. malayanum from three different species of snail intermediate host. Separate phylogenetic analyses of the MEE and DNA sequence data revealed that the two species of Echinostoma (E. revolutum and E. malayanum) did not form a monophyletic clade. These results, together with the large number of morphologically similar species with inadequate descriptions, poor specific diagnoses and extensive synonymy, suggest that the morphological characters used for species taxonomy of echinostomes in South-East Asia should be reconsidered according to the concordance of biology, morphology and molecular classification.