Light and electron microscopy observations of embryogenesis and egg development in the human liver fluke, Opisthorchis viverrini (Platyhelminthes, Digenea)

Susceptibility patterns of Bithynia siamensis siamensis and Bithynia funiculata to Opisthorchis viverrini infection: an indication of the risk of opisthorchiasis transmission in non-endemic areas

Among the snail species acting as hosts for medically significant trematodes, only three taxa of Bithynia are responsible for transmitting the carcinogenic liver fluke Opisthorchis viverrini to humans in different geographical areas. Although B. siamensis goniomphalos is the primary species responsible for O. viverrini transmission in endemic areas, B. siamensis siamensis and B. funiculata remain potential hosts for transmission. This study objects to determine the susceptibility of B. siamensis siamensis and B. funiculata to O. viverrini to assess the risk of O. viverrini transmission in non-endemic areas. The snails of both species were first introduced to O. viverrini eggs, after which O. viverrini infection was investigated using specific PCR primers after a period of 1, 7, 14, 28, and 56 days post-infection (dpi). Opisthorchis viverrini infection in both B. siamensis siamensis and B. funiculata was high in the early period (1 and 7 dpi) while decreasing over time. It was also shown that the odds of susceptibility to O. viverrini infection in B. siamensis siamensis were 64.5% higher relative to the odds of susceptibility in B. funiculata (P < 0.05). Results of this study provide an early insight into the Bithynia-Opisthorchis relationship and thus have great potential to assess risk and raise awareness of opisthorchiasis in non-endemic regions, especially in regions endemic for B. siamensis siamensis.

Ultrastructure of digenean trematode eggs (Platyhelminthes: Neoophora): A review emphasizing new comparative data on four European Microphalloidea

Despite their tremendous diversity and their medical and veterinary importance, details of egg ultrastructure among the digenean trematodes has been studied rather little. The available literature is spread over several decades and several species, but has not been adequately reviewed to reveal patterns of similarity and divergence. We present this review to synthesize and analyse what is known from the available literature reporting studies using both transmission electron microscopy (TEM) and scanning electron microscopy (SEM). To support our general review of existing literature, we also have synthesized our own previously published descriptions, and present herein our new previously unpublished data. From these new electron micrographs, we provide a comparative analysis of the intrauterine eggs of four digenean species, representing four genera and three families of the superfamily Microphalloidea, collected from four different host wildlife species in four European countries: 1) Mediogonimus jourdanei (Prosthogonimidae) from Myodes glareolus (Mammalia: Rodentia), collected in France; 2) Maritrema feliui (Microphallidae) from Crocidura russula (Mammalia: Soricimorpha), collected in Spain; 3) Brandesia turgida (Pleurogenidae) from Pelophylax ridibundus (Amphibia: Anura: Ranidae), collected in Russia; and 4) Prosotocus confusus (Pleurogenidae) from Rana lessonae (Amphibia: Anura: Ranidae), collected in Belarus. All were studied by preparing whole worms by various techniques for TEM, so that eggs could be studied in situ within the uterus of the parent worm. Based on the literature review and the new data presented here, we describe basic similarities in patterns of embryogenesis and egg formation among all trematode species, but substantial variations in timing of larvigenesis, sculpturing of egg shell surfaces, and some other features, especially including accessory cocoon coverings outside the egg shells of B. turgida and P. confusus. In the future, many more studies are needed to explore egg ultrastructure in other digenean taxa, to explore potential phylogenetic patterns in egg development and structure, and to correlate structure with function in the life cycle.

Exploring Structural and Physical Properties of Schistosome Eggs: Potential Pathways for Novel Diagnostics?

In this era of increasing demand for sensitive techniques to diagnose schistosomiasis, there is a need for an increased focus on the properties of the parasite eggs. The eggs are not only directly linked to the morbidity of chronic infection but are also potential key targets for accurate diagnostics. Eggs were the primary target of diagnostic tools in the past and we argue they could be the target of highly sensitive tools in the future if we focus on characteristics of their structure and shell surface that could be exploited for enhanced detection. In this review, we discuss the current state of knowledge of the physical structures of schistosome eggs and eggshells with a view to identifying pathways to a comprehensive understanding of their role in the host-parasite relationship and pathogenesis of infection, and pathways to new strategies for development of diagnostics.

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.

Description of embryonic development and ultrastructure in miracidia of Cardiocephaloides longicollis (Digenea, Strigeidae) in relation to active host finding strategy in a marine environment

The functional ultrastructure and embryonic development of miracidia in naturally released eggs of the trematode Cardiocephaloides longicollis were studied using light and transmission electron microscopy. This species has operculated eggs and embryogenesis occurs in the marine environment before an actively infecting ciliated miracidium hatches. Six different developmental stages were identified. The lack of pores in the eggshell indicates its impermeability and the miracidium's dependency on glycogen nutritive reserves, contained in numerous vitellocytes in early embryos. As the development advances, these merge into larger vitelline vacuoles that encircle the miracidium and may aid its hatching. Tissue and primary organ differentiation were observed in advanced stages, i.e., terebratorium, glands, cerebral ganglion, peripheral sensory endings, and eyespots. The anterior part of the body contains a single apical and paired lateral glands, as well as two types of sensory endings, which permit location, adhesion, and penetration of the host. No previous studies describe the embryonic development and ultrastructure of miracidia in strigeids, however, some of the structural features shared with other, well described species with unknown life cycles are emphasised. This study highlights that ultrastructural data have to be interpreted in relation to parasite biology to understand the structural requirements of specific parasite strategies.

Experimental and modelling investigations of Opisthorchis viverrini miracidia transmission over time and across temperatures: implications for control

Transmissibility is a significant factor in parasite fitness. The rate and magnitude of parasite transmission affect prevalence and infection intensity in individual hosts and are influenced by environmental factors. In this context, the objectives of this study were: (i) to experimentally assess Opisthorchis viverrini miracidia survival and infectivity over time and across temperatures; and (ii) to combine these experimental results with environmental data to build a key component of a transmission model, identifying seasonal windows of transmission risk in hyper-endemic northeastern Thailand. Five replicates of 50 O. viverrini eggs were randomly distributed and maintained under four temperature conditions (25°C, 30°C, 35°C, 40°C). Microscopic observations were performed on all experimental units over a period of 3months to record miracidia motility and mortality trends. Six infection trials were also conducted to assess infectivity of miracidia over time and across temperatures, using observations of egg hatching success and infection rates. Upon completion of experiments, data were integrated into a transmission model to create a transmission risk index and to simulate seasonal transmission risk. Miracidia survival rate and motility decreased steadily with 50% mortality observed after 2weeks. Hatching and infection success also decreased significantly after 3weeks. Temperatures over 30°C were associated with increased mortality and decreased infectivity. When incorporating local environmental parameters into our model, we observed low transmission risk during the dry season and increasing transmission risk at the onset of the rainy season, culminating with the highest risk in September. We believe that our results provide the first estimates of O. viverrini miracidia survival and transmission potential under variable temperature conditions and suggest that high temperature treatment (>40°C) of fecal waste could be an efficient control strategy.

Differential Protein Expression in the Hemolymph of Bithynia siamensis goniomphalos Infected with Opisthorchis viverrini

Bithynia siamensis goniomphalos is a freshwater snail that serves as the first intermediate host of the human liver fluke Opisthorchis viverrini. This parasite is a major public health problem in different countries throughout the Greater Mekong sub-region (Thailand, southern Vietnam, Lao PDR and Cambodia). Chronic O. viverrini infection also results in a gradual increase of fibrotic tissues in the biliary tract that are associated with hepatobiliary diseases and contribute to cholangiocarcinoma (a fatal type of bile duct cancer). Infectivity of the parasite in the snail host is strongly correlated with destruction of helminths by the snail's innate immune system, composed of cellular (hemocyte) and humoral (plasma) defense factors. To better understand this important host-parasite interface we applied sequential window acquisition of all theoretical spectra mass spectrometry (SWATH-MS) to identify and quantify the proteins from the hemolymph of B. siamensis goniomphalos experimentally infected with O. viverrini and compare them to non-infected snails (control group). A total of 362 and 242 proteins were identified in the hemocytes and plasma, respectively. Of these, 145 and 117 proteins exhibited significant differences in expression upon fluke infection in hemocytes and plasma, respectively. Among the proteins with significantly different expression patterns, we found proteins related to immune response (up-regulated in both hemocyte and plasma of infected snails) and proteins belonging to the structural and motor group (mostly down-regulated in hemocytes but up-regulated in plasma of infected snails). The proteins identified and quantified in this work will provide important information for the understanding of the factors involved in snail defense against O. viverrini and might facilitate the development of new strategies to control O. viverrini infection in endemic areas.

Temperature dependence of Opisthorchis viverrini infection in first intermediate host snail, Bithynia siamensis goniomphalos

Determining of the success of a parasite's infectiveness in its snail host clearly depends on environmental conditions. Temperature, one of the most influential factors impinging on metabolism of cold-blooded animals, is believed to be an important factor in parasitic infection in snails. In order to elucidate the influence of temperature, sex and size of snails on infectivity of Opisthorchis viverrini to its first intermediate host, Bithynia siamensis goniomphalos, 960 snails were divided into 2 groups by sex. Each group was subdivided by their size into small and medium sub-groups. Each snail was fed with embryonated uterine-eggs of O. viverrini at different temperatures (16-37°C, 3°C intervals). Dissections were carried out 1, 7, 14, 28 and 56 days thereafter and detection of O. viverrini infection was undertaken by PCR using specific primers. Infection was strongly temperature-dependent, as temperature increases of 1°C resulted in increased odds of infection 5.4% (P<0.01). A temperature of 34°C gave the highest rate of infection of 44.14%. We also found that the odds of infection in small sized snails was 39.8% higher relative to medium sized snails (P<0.05). Relative to day 1, the decrease in the odds of infection was detected when the day post infection was longer (P<0.01). Proportion of infection in female was not different to male significantly.

Ultrastructural study of the egg wall surrounding the developing miracidia of the digenean Prosotocus confusus (Looss, 1894) (Plagiorchiida: Pleurogenidae), with the description of a unique cocoon-like envelope

Helminth eggs play a critical role in movement of the parasite from definitive to intermediate host. Eggs of the pleurogenid digenean trematode Prosotocus confusus (Looss, 1894), a parasite of naturally infected frogs Pelophylax lessonae (Amphibia: Ranidae) in Europe, are described here for the first time. Particular emphasis is placed on the ultrastructure on the egg wall and on the detailed description of a unique cocoon-like envelope. Each embryonating egg is composed of an early embryo surrounded by a four-layered egg wall: (1) an outer, anucleate layer external to the eggshell, which forms a thick cocoon; (2) the operculate eggshell; (3) not fully formed, a differentiating outer embryonic envelope containing large nuclei of macromeres; and (4) situated below, an undifferentiated layer of the future inner embryonic envelope containing mesomere nuclei. Layers enveloping the egg apparently play an important role in the protection, metabolism, and storage of nutritive reserves for the developing miracidium. The outer anucleate layer, or cocoon, is situated externally to the eggshell and composed of an electron-lucent substance with numerous electron-dense islands attached to its peripheral membrane. A cocoon envelope such as this has never been seen in previous TEM studies of the eggs of parasitic platyhelminths, with the exception of another pleurogenid Brandesia turgida. The origin, formation, functional ultrastructure, and chemical composition of this peculiar layer remain enigmatic, although its function appears to be protective. The thick, electron-dense eggshell resembles that of other trematodes, exhibiting a characteristic fissure zone around the operculum. Numerous lysosome-like structures observed in some eggs may be involved in the autolysis of both the embryonic envelopes (particularly the early degeneration of macromere nuclei of the outer envelope, characteristic for this species) and in the disintegration of several early micromeres. The inner envelope, which forms later from mesomeres, persists longer during embryogenesis.

Proteomic profile of Bithynia siamensis goniomphalos snails upon infection with the carcinogenic liver fluke Opisthorchis viverrini

The snail Bithynia siamensis goniomphalos acts as the first intermediate host for the human liver fluke Opisthorchis viverrini, the major cause of cholangiocarcinoma (CCA) in Northeast Thailand. The undisputed link between CCA and O. viverrini infection has precipitated efforts to understand the molecular basis of host-parasite interactions with a view to ultimately developing new control strategies to combat this carcinogenic infection. To date most effort has focused on the interactions between the parasite and its human host, and little is known about the molecular relationships between the liver fluke and its snail intermediate host. In the present study we analyse the protein expression changes in different tissues of B. siamensis goniomphalos induced by infection with larval O. viverrini using iTRAQ labelling technology. We show that O. viverrini infection downregulates the expression of oxidoreductases and catalytic enzymes, while stress-related and motor proteins are upregulated. The present work could serve as a basis for future studies on the proteins implicated in the susceptibility/resistance of B. siamensis goniomphalos to O. viverrini, as well as studies on other pulmonate snail intermediate hosts of various parasitic flukes that infect humans.

BIOLOGICAL SIGNIFICANCE

Despite the importance and high prevalence of opisthorchiasis in some regions of Southeast Asia and the direct relationship between infection by Opisthorchis viverrini and the incidence of cholangiocarcinoma, little is known of the modifications induced by this parasite in its snail intermediate hosts. This time-course study provides the first in-depth quantitative proteomic analysis of experimentally infected Bithynia siamensis goniomphalos. We show how motor and stress-related proteins are upregulated in infected snails, while O. viverrini infection downregulates the expression of oxidoreductases and catalytic enzymes. This work serves as a basis for the development of new strategies, focused on the invertebrate intermediate hosts, to control parasite transmission.

Ultrastructural evidence for completion of the entire miracidial maturation in intrauterine eggs of the digenean Brandesia turgida (Brandes, 1888) (Plagiorchiida: Pleurogenidae)

Results of this TEM study provide ultrastructural evidence that miracidial morphogenesis is fully completed within the intrauterine eggs situated in the most posterior uterine regions of the pleurogenid trematode Brandesia turgida (Brandes, 1888). The ultrastructural characteristic of different larval organelles and cell types of these eggshell-enclosed, but fully formed, cilated miracidia is described. The body wall of the pyriform mature miracidium of B. turgida is composed of ciliated epidermis and underlying peripheral body musculature. Two miracidial flame cells of the protonephridial excretory system are localized in the central region of the ciliated larvae. Three types of miracidial glands were observed: a single apical gland, two lateral glands, and several small vesiculated glands; each gland type contains characteristic, but different types of secretory granules. The anterior end of each miracidium consists of an apical papilla on which are situated the exits of the three main larval glands: an exit of a single apical gland as well as the individual exits of two lateral glands. The exits of vesiculated glands, containing characteristic spherical membrane-bound and highly electron-dense granules, evidently different from the two other types of secretory granules of apical and lateral glands, were not identified. Germinative cells, grouped together in a sac-like germinative follicle, are situated in the medioposterior part of the larva, the germatophore. The germinative cells contain numerous electron-dense heterochromatin islands arranged in the form of a network or chain-like pattern and distributed mainly in the karyoplasm adjacent to the nuclear membrane. The thin layer of granular cytoplasm is rich in free ribosomes and contains a few small mitochondria. Both nuclear and cytoplasmic features if these cells indicate their great developmental potential for further growth and multiplication in postembryonic stages of the life cycle. In the mature eggs, the areas of focal cytoplasmic degradation were frequently observed and may be involved in the autolysis of some embryonic structures. Obtained results are compared with available literature data on the functional ultrastructure of the miracidia of other digeneans.

Relationships between uterus and eggs in cestodes from different taxa, as revealed by scanning electron microscopy

Uterine organization and interaction with developing eggs in Tetrabothrius erostris (Tetrabothriidea), Nippotaenia mogurndae (Nippotaeniidea), Arostrilepis tenuicirrosa, and Monocercus arionis (Cyclophyllidea), cestodes belonging to three different orders, were investigated by scanning electron microscopy. The interactions were traced from sexually mature to gravid proglottids for all species. Pieces of evidence of interactions among these species include specific tight contacts between microlamellae of the uterine epithelium and the egg capsule, networks of fibrils between eggs and uterus, or numerous branched diverticula of the uterine wall that surround eggs or combinations of these. The contacts between uterine epithelium and eggs take place in mature and post-mature proglottids, at a period of development when eggs are newly formed and the embryos are rapidly developing. The eggs grow and develop actively in tight contact with the uterine wall. The maximum diameter of eggs increases 1.5-2 times (or 3.5-4 times in M. arionis) during development. In all species, the intimate contacts between uterus and eggs have weakened or disappeared by the time the proglottids have become gravid. The association between uterus and eggs thus appears as strong evidence of active trophic interaction (or matrotrophy) between the parent organism and developing eggs.

Ultrastructure of the intrauterine eggs of the microphallid trematode Maritrema feliui: evidence of early embryonic development

Intrauterine embryonic development in the microphallid trematode Maritrema feliui is examined by means of transmission electron microscopy. Both fertilization and eggshell formation take place in the ootype. The eggshell is formed from a shell globule material derived from the vitelline cells combined with secretions of Mehlis' gland. The proximal uterus is packed with unembryonated eggs of the oligolecithal type, each composed of a fertilized oocyte and several vitelline cells, all surrounded by the shell. Intrauterine embryonic development of the egg is followed to the early stage of outer embryonic envelope formation, resulting in an embryo of ~20 blastomeres of three different types: macromeres, mesomeres and micromeres. The first equal cleavage division of the zygote produces two macromeres. The outer envelope is of cellular origin and formed by the cytoplasmic fusion of two macromeres, which become situated at opposite poles in the peripheral layer of the embryo just beneath the eggshell. Simultaneously, other blastomeres multiply and differentiate, whereas several micromeres exhibit clear signs of degeneration or apoptosis. These results show that the embryonic development of M. feliui starts in utero and represents an example of early stage ovoviviparity. A reduction in the number of blastomeres results from a continued degeneration of micromeres, which after autolysis and re-absorption, appear to represent an important source of nutritive reserves for the embryo. The embryonic development of this digenean is discussed in relation to its life cycle.

Production and deformation of Clonorchis sinensis eggs during in vitro maintenance

Clonorchis sinensis is a carcinogenic human liver fluke. The present study monitored eggs produced by long-term maintained adult worms of C. sinensis to confirm their egg productivity in vitro. The worms from infected rabbits were incubated in vitro in 1× Locke’s solution and broth media (RPMI-1640, DMEM and IMDM). Numbers of expelled eggs were counted sequentially and their morphological changes were monitored by microscopy after 1, 30, 60, and 90 days of cultivation. On the 1–3 days of cultivation, the eggs counted maximum 4,756±202 eggs/worm/day in IMDM medium. The number of eggs gradually decreased less than 1,000 at 7–14 days and below 100 at 21days but continued to pass eggs after 56 days in all media. Length of the eggs were reduced about 1 µm at 30 days, and the length/width ratio was maintained around 1.8 at 30 days but decreased to 1.7 at 60 days and 1.5 at 90 days. Faust-Meleney index (FMI) decreased as the cultivation duration increased and lowest FMI (5662.9±974.7) observed in IMDM media at day 90 (P = 0.001). Microscopic findings of the eggs recognized the miracidium in most of eggs at 60 days but not in those at 90 days. Instead, the eggs contained dark granules or vacuoles in the deformed shell at 90 days. Scanning electron microscopy revealed partial loss of wrinkles on the deformed egg surface and prominent abopercular knob. Eggs viability decreased as the cultivation progressed and showed significant positive correlation with FMI and length/width ratio. In conclusion, the cultivated worms pass only the eggs which are preformed in their uterus before cultivation. One gravid C. sinensis contains about 37,000 eggs in its uterus and produces about 4,000 eggs every day. The deformed eggs with FMI less than 7,000 and length/width ratio lower than 1.7 are non-viable.