Fish Intermediate Hosts Research Articles

Unregulated introduced fish (Perca fluviatilis Linnaeus, 1758) is host to zoonotic parasites in a small Mediterranean island

Introduced species have a major impact on freshwater ecosystems, particularly on islands. Numerous fish species have been introduced in Corsica (Mediterranean island, southern France) as part of planned programs or clandestinely. The introduction of non-native freshwater fish species can have a range of impacts on the recipient ecosystem, including through the co-introduction of its pathogens. A sample of introduced perch Perca fluviatilis Linnaeus, 1758 from the artificial reservoir of Padula was examined following a report of parasites by an angler. The analyses revealed the occurrence of Eustrongylides sp. (Nematoda) and Clinostomum complanatum (Digenea), two zoonotic parasites in P. fluviatilis. Both parasites are reported for the first time in France. Eustrongylides sp. and C. complanatum may have been introduced with their fish intermediate hosts or through their final bird hosts. The occurrence of the two parasites raises concerns from both a veterinary and human health perspective as they can use a wide range of amphibians as intermediate hosts and can be acquired in humans through the consumption of raw or undercooked fish.

Exploring the second intermediate hosts and morphology of human- and cat-specific Opisthorchis viverrini-like populations

Infection by the zoonotic fish-borne trematode, Opisthorchis viverrini, remains a crucial health issue in Thailand and neighboring countries. Recently, molecular analysis revealed two populations of putative O. viverrini: one found primarily in human hosts (“human-specific” population) and the other primarily in cats (“cat-specific” population). It is unclear how the infective stages (metacercariae) of these different populations circulate among definitive and reservoir hosts in nature. To gain an insight into this, mitochondrial cox1 and nad1 gene sequences of metacercariae from fish intermediate hosts were examined. None of 192 metacercariae from cyprinid fish in Lao PDR and Thailand had sequences typical of “cat-specific” O. viverrini, suggesting that cyprinid fish are not the main second intermediate hosts of this population. Interestingly, all 20 O. viverrini-like metacercariae from snakehead fish (Channa striata) shared 99.51–100% sequence identity with eggs from cats naturally infected in a previous study. Hence, we propose a modification of the known transmission dynamics of O. viverrini: consumption of metacercariae within snakehead fish provides another pathway for cats and (occasionally) humans to acquire infection. We also performed morphological comparisons of eggs, metacercariae, and adult flukes (raised in hamsters) of both Opisthorchis populations. The “cat-specific” population has eggs that are narrower and adults that are shorter and wider than in the human-specific population. The metacercaria of the “cat-specific” population is elliptical, while that of the “human-specific” population is oval, occasionally rounded. Our results confirmed that O. viverrini-like metacercariae from snakehead fish are the infective stages of the “cat-specific” fluke. This provides a new insight into the dissemination and transmission of each population in the second intermediate host. The identity of the cat-specific population is discussed.

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.

Invasive parasites and global change: Evidence for the recent and rapid spillover of a potential pathogen of tilapias with a complex, three-host life cycle

Biological invasions pose a serious threat to local flora and fauna and have negative impacts on ecosystems. Invasive parasites can also cause severe losses in aquaculture. In this article, we provide evidence of the recent spillover of an African parasite with a complex, three-host life cycle that has rapidly and successfully established itself in the Middle East, most likely due to the recent migration of its final hosts (great cormorant) from Africa. This case of parasite introduction into a country with intensive aquaculture is also important from an economic point of view, since large (up to 2 cm long) larvae of this parasite, the cyclophyllidean tapeworm Amirthalingamia macracantha (Cestoda) localised in the liver, can be pathogenic to their fish hosts, including farmed and wild fish, as shown by our histopathological examination of heavily infected fish. Since its first detection in Israel in November 2020, the parasite has spread rapidly and is currently found in both migratory (great cormorant, Phalacrocorax carbo) and non-migratory birds (pygmy cormorant, Microcarbo pygmaeus), as well as in fish intermediate hosts, including farmed tilapia in several farms in Israel and wild cichlids. There are numerous examples of the spillover of introduced parasites, including those that parasitise fish of commercial importance, but have a direct life cycle or use only a single intermediate host. Tilapines are the second most important group of farmed fish in the world after carps and are produced mainly in Southeast Asia, Central and South America. The global spread of great cormorants and the early evidence that pygmy cormorant may also harbour A. macracantha pose the risk of further spread of this invasive parasite to other countries and areas. In addition, global warming and reductions in foraging and resting areas near these waters may allow the parasite to complete its life cycle in new hosts.

An examination of nervous system revealed unexpected immunoreactivity of both secretory apparatus and excretory canals in plerocercoids of two broad tapeworms (Cestoda: Diphyllobothriidea).

Dibothriocephalus ditremus and Dibothriocephalus latus are diphyllobothriidean tapeworms autochthonous to Europe. Their larval stages (plerocercoids) may seriously alter health of their intermediate fish hosts (D. ditremus) or cause intestinal diphyllobothriosis of the final human host (D. latus). Despite numerous data on the internal structure of broad tapeworms, many aspects of the morphology and physiology related to host–parasite co-existence remain unclear for these 2 species. The main objective of this work was to elucidate functional morphology of the frontal part (scolex) of plerocercoids, which is crucial for their establishment in fish tissues and for an early attachment in final hosts. The whole-mount specimens were labelled with different antibodies and examined by confocal microscope to capture their complex 3-dimensional microanatomy. Both species exhibited similar general pattern of immunofluorescent signal, although some differences were observed. In the nervous system, FMRF amide-like immunoreactivity (IR) occurred in the bi-lobed brain, 2 main nerve cords and surrounding nerve plexuses. Differences between the species were found in the structure of the brain commissures and the size of the sensilla. Synapsin IR examined in D. ditremus occurred mainly around FMRF amide-like IR brain lobes and main cords. The unexpected finding was an occurrence of FMRF amide-like IR in terminal reservoirs of secretory gland ducts and excretory canals, which has not been observed previously in any tapeworm species. This may indicate that secretory/excretory products, which play a key role in host–parasite relationships, are likely to contain FMRF amide-related peptide/s.

Interspecies Phylogenetic Analysis of Clonorchis sinensis in High-incidence Areas of Hunan Province, China

ObjectiveThis study aims to investigate the infection of Clonorchis sinensis (C. sinensis) in high-incidence areas of Hunan Province, China. The phylogenetic analysis of the C. sinensis species in the highly infected areas was carried out. MethodInfection of the definitive human host and intermediate fish host by C. sinensis was investigated, and the mitochondrial genes cox1 and Nad1 were used as genetic markers for phylogenetic analysis. ResultsIn 2016–2020, the average population infection rate of Hunan was 1.38%, while in Tongdao County the rate was up to 26.90%, and the highest fish infection rate was detected in Qiyang County (99.44% in the dorsal fin of crucian carp). High genetic sequence similarity was observed in the samples from Qiyang and Lengshuitan which exhibited high homology with those from Guangdong and Gansu, whereas the parasitic species from Tongdao was highly homologous with those located in high-latitude areas. Moreover, no significant difference was found in the gene sequence of the parasitic species in definitive hosts dogs and cats. ConclusionThe systematically study of C. sinensis infection in the high-incidence areas will contribute greatly to the prevention and effectively controlling the spread of Clonorchis sinensis in Hunan Province The endemic of C. sinensis infection in Hunan Province is the result of co-action of local and foreign parasite species.

The parasite Schistocephalus solidus secretes proteins with putative host manipulation functions

BackgroundManipulative parasites are thought to liberate molecules in their external environment, acting as manipulation factors with biological functions implicated in their host’s physiological and behavioural alterations. These manipulation factors are part of a complex mixture called the secretome. While the secretomes of various parasites have been described, there is very little data for a putative manipulative parasite. It is necessary to study the molecular interaction between a manipulative parasite and its host to better understand how such alterations evolve.MethodsHere, we used proteomics to characterize the secretome of a model cestode with a complex life cycle based on trophic transmission. We studied Schistocephalus solidus during the life stage in which behavioural changes take place in its obligatory intermediate fish host, the threespine stickleback (Gasterosteus aculeatus). We produced a novel genome sequence and assembly of S. solidus to improve protein coding gene prediction and annotation for this parasite. We then described the whole worm’s proteome and its secretome during fish host infection using LC–MS/MS.ResultsA total of 2290 proteins were detected in the proteome of S. solidus, and 30 additional proteins were detected specifically in the secretome. We found that the secretome contains proteases, proteins with neural and immune functions, as well as proteins involved in cell communication. We detected receptor-type tyrosine-protein phosphatases, which were reported in other parasitic systems to be manipulation factors. We also detected 12 S. solidus-specific proteins in the secretome that may play important roles in host–parasite interactions.ConclusionsOur results suggest that S. solidus liberates molecules with putative host manipulation functions in the host and that many of them are species-specific.Graphical abstract

Opisthorchis viverrini metacercarial infection in cyprinid fish in Nakhon Phanom Province, Northeastern Thailand.

Opisthorchiasis is endemic in parts of Southeast Asia, including the northern and northeastern regions of Thailand. In these regions, the transmission by fish intermediate hosts has received little attention. We investigated the intensity of Opisthorchis viverrini metacercariae (OV MC) in wild cyprinid fishes from five districts within the Nakhon Phanom Province, Northeast Thailand. Fishes were procured from local markets in five districts throughout three different seasons (hot, rainy and cold) between February 2018 and January 2019. The samples were identified, counted and weighed before metacercariae detection was performed via the artificial digestion method. A total of 2,149 freshwater fishes, representing 20 species were collected. The fish most commonly contaminated with OV MC were Anematichthys repasson and Hampala dispar. The intensity of OV MC in Nakhon Phanom was 0.23 OV MC/fish and varied among districts, ranging from 0.07 to 0.52. A low intensity of OV MC/fish (defined as <=1 cyst) was found in all three different seasons in the Nakhon Phanom Province; hot season (0.55), cold season (0.22) and the rainy season (0.13). The intensity of OV MC/fish was moderate (defined as >1 cyst) in the Renu Nakhon district (2.5) in the hot season. By fish species, H. dispar yielded the highest, with a moderate intensity of 2.1. In natural freshwater cyprinid fish in Nakhon Phanom, OV MC infection is endemic with intensity rates varying according to district, season and fish species.

DNA barcoding reveals different cestode helminth species in northern European marine and freshwater ringed seals.

Three subspecies of the ringed seal (Pusa hispida) are found in northeastern Europe: P. h. botnica in the Baltic Sea, P. h saimensis in Lake Saimaa in Finland, and P. h. ladogensis in Lake Ladoga in Russia. We investigated the poorly-known cestode helminth communities of these closely related but ecologically divergent subspecies using COI barcode data. Our results show that, while cestodes from the Baltic Sea represent Schistocephalus solidus, all worms from the two lakes are identified as Ligula intestinalis, a species that has previously not been reported from seals. The observed shift in cestode communities appears to be driven by differential availability of intermediate fish host species in marine vs. freshwater environments. Both observed cestode species normally infect fish-eating birds, so further work is required to elucidate the health and conservation implications of cestode infections in European ringed seals, whether L. intestinalis occurs also in marine ringed seals, and whether the species is able to reproduce in seal hosts. In addition, a deep barcode divergence found within S. solidus suggests the presence of cryptic diversity under this species name.

Life history shifts in an exploited African fish following invasion by a castrating parasite.

Evolutionary theory predicts that infection by a parasite that reduces future host survival or fecundity should select for increased investment in current reproduction. In this study, we use the cestode Ligula intestinalis and its intermediate fish host Engraulicypris sardella in Wissman Bay, Lake Nyasa (Tanzania), as a model system. Using data about infection of E. sardella fish hosts by L. intestinalis collected for a period of 10 years, we explored whether parasite infection affects the fecundity of the fish host E. sardella, and whether host reproductive investment has increased at the expense of somatic growth. We found that L. intestinalis had a strong negative effect on the fecundity of its intermediate fish host. For the noninfected fish, we observed an increase in relative gonadal weight at maturity over the study period, while size at maturity decreased. These findings suggest that the life history of E. sardella has been shifting toward earlier reproduction. Further studies are warranted to assess whether these changes reflect plastic or evolutionary responses. We also discuss the interaction between parasite and fishery‐mediated selection as a possible explanation for the decline of E. sardella stock in the lake.

Liver fluke infections by Amphimerus sp. (Digenea: Opisthorchiidae) in definitive and fish intermediate hosts in Manabí province, Ecuador.

Amphimerus sp. is a fluke that dwells in the biliary tracts of vertebrate definitive hosts including humans, domestic, and wild mammals in Latin America. Opisthorchiid liver infections are rarely studied in the Americas confirming its status as a neglected tropical disease. In Ecuador, small trematode eggs were reported in human cases from the province of Manabí in 1949, and recently, Amphimerus sp. adults were recovered from human and reservoir hosts in the province of Esmeraldas. Due to the lack of research on the infectious sources of Amphimerus sp. in the continent, we have developed a series of epidemiological studies with parasitological and molecular techniques to elucidate the endemicity of opisthorchiid fluke infections. We developed a cross-sectional study in three communities at Pedro Pablo Gómez parish in the province of Manabí, Ecuador. We examined a total of 176 fecal samples to detect opisthorchiid eggs, and four fish species to find opisthorchiid metacercariae. To study adult worms, we treated and purged seven patients in a family and dissected the livers of a dog and a cat infected. We observed morphological features of adults and metacercariae and used polymerase chain reaction with restricted fragment length polymorphism (PCR-RFLP) and DNA sequencing of a section of the ITS2 gene for identification. Small trematode eggs were detected in 63 (35.8%) out of 176 fecal samples of residents in the three study sites. Adult opisthorchiid flukes were recovered from human patients, a dog and a cat, and they were morphologically and molecularly identified as Amphimerus sp. Opisthorchiid metacercariae were also identified molecularly as Amphimerus sp. in four fish species, i.e., Rhoadsia altipinna, Bryconamericus bucay, Andinoacara rivulatus, and Piabucina aureoguttata. Metacercariae of the heterophyid Haplorchis pumilio were also found in the four fish species examined. This is the first study to confirm the current endemicity of Amphimerus sp. in Pedro Pablo Gómez, Manabí, Ecuador. The adult worms isolated here shared morphological characteristics with previous Amphimerus sp. descriptions and were molecularly similar to Amphimerus sp. described in the province of Esmeraldas. Moreover, this study is the first to document four fish species as infection sources of Amphimerus sp. detected via a molecular protocol targeting the metacercariae of the parasite. Fish species identified here should be targeted for public health campaigns to avoid further human liver-fluke infections by Amphimerus sp. or potential intestinal-fluke infections by H. pumilio or others.

Limited impacts of chronic eye fluke infection on the reproductive success of a fish host

AbstractParasitic infections may affect the reproductive success of the host either directly, through behavioural modification, or indirectly, by altering their reproductive investment in response to infection. We determined the effects of infection with the eye fluke Diplostomum pseudospathaceum (Trematoda) on the reproductive traits of European bitterling (Rhodeus amarus, Cyprinidae), an intermediate fish host with a resource-based mating system. Male bitterling infected by Diplostomum exhibited a larger but less pronounced red eye spot (sexually selected signal) than control males, suggesting that infected males were less preferred by females. The frequency of female ovulation and number of offspring were comparable between the infected and the control group, although there was a 1–2 week delay in the peak of ovulation and offspring production in infected fish, which is known to coincide with higher juvenile mortality. Chronic eye fluke infection had minimal metabolic costs (measured as oxygen consumption) and, consistent with these results, reproductive activity did not differ between infected and control fish in an experimental test of intersexual selection. Overall, the impact of eye fluke infection on the reproduction of European bitterling was limited. We consider the potential effect of favourable conditions during experiments (abundant food, access to spawning substrate and lack of predators and co-infections) on experimental outcomes and recognize that the effects of chronic eye fluke infection in natural conditions might be more pronounced.

Phylogenetic relationships, expanded diversity and distribution of Crassiphiala spp.(Digenea, Diplostomidae), agents of black spot disease in fish.

Crassiphiala is a monotypic genus of diplostomid digeneans and is the type genus of the subfamily Crassiphialinae. The type species Crassiphiala bulboglossa parasitizes kingfishers in the Nearctic and has a Neascus-type metacercaria that encysts on fish intermediate hosts, often causing black spot disease. While recent molecular phylogenetic studies included some members of the Crassiphialinae, no DNA sequence data of Crassiphiala is currently available. Our molecular and morphological study of adult and larval crassiphialines from the Americas revealed the presence of at least three lineages of Crassiphiala from the Nearctic and two lineages from the Neotropics. This is the first record of Crassiphiala from the Neotropics. Herein, we provide the first molecular phylogeny of the Diplostomoidea that includes Crassiphiala. Our data revealed 0.2-2.4% divergence among 28S sequences and 11-19.8% among CO1 sequences of lineages of Crassiphiala. The results of our analyses did not support the monophyly of Crassiphialinae. Our results clearly demonstrated that the diversity of Crassiphiala has been underestimated.

Results on search for the broad fish tapeworm Dibothriocephalus latus (Linnaeus, 1758), (syn. Diphyllobothrium latum) (Cestoda: Diphyllobothriidea), in the Danube River

SummaryDiphyllobothriosis is a fish-borne parasitic zoonosis caused by so-called “broad tapeworms” or “fish tapeworms” of different genera of the order Diphyllobothriidea. Dibothriocephalus latus (Linnaeus 1758), (syn. Diphyllobothrium latum), is a medically important type species of the genus, whose occurrence in various European regions is either regular, e.g. in the Alpine lakes region, or occasional and sporadic, e. g. in the Danube River region. For the latter, data on the detection of D. latus plerocercoids in the second intermediate fish host (European perch Perca fluviatilis), as well as in definitive hosts (human and dog), in which infection was directly linked to the consumption of infected fish from the Danube, were published more than 50 years ago. In order to assess the current situation, we aimed to find out whether D. latus is present in the natural environment of the Danube River. In total, 700 perch from five sampling sites in the Slovak part of the Danube River were examined. Plerocercoids were not detected in any fish examined, which leads to the conclusion that D. latus is currently not present in the studied aquatic environment.

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.

A case of hepatic anisakiasis caused by Pseudoterranova decipiens mimicking metastatic liver cancer

BackgroundAnisakid nematodes (Anisakis spp. or Pseudoterranova spp.) usually infect gastric or intestinal walls, while they rarely infect in extra-gastrointestinal sites of human body. Generally, Anisakis spp. larvae are highly infected in fish intermediate hosts, whereas Pseudoterranova spp. larvae are very rarely infected. To the best of our knowledge, there have been no reports which have documented cases of hepatic anisakiasis caused by Pseudoterranova spp. This report describes the first documented case of hepatic anisakiasis due to infection with Pseudoterranova decipiens and clinical features of the hepatic anisakiasis through literature review.Case presentationThe case was a 28-year-old man with prior history of malignancy who was found to have a hepatic mass mimicking metastatic liver tumor. A new low density area of 20 mm in diameter in liver segment 7 was found on follow-up CT. With suspicious diagnosis of metastatic liver cancer, laparoscopic partial hepatectomy was performed. A pathological examination revealed no evidence of malignancy, but showed necrotic granuloma with eosinophil infiltration and the presence of a larva with Y-shaped lateral cords, which are specific to anisakid larvae. The type of larva was identified as Pseudoterranova decipiens sensu lato using PCR of DNA purified from a fixed granuloma embedded in paraffin.ConclusionThe present report is the first to discuss the case of a patient with hepatic anisakiasis caused by Pseudoterranova decipiens. Hepatic anisakiasis is a potential differential diagnosis for hepatic tumors and genetic identification with the PCR method was reliable for obtaining final diagnosis even when the larvae body in the resected specimen collapses with time.

The tapeworm Ligula intestinalis alters the behavior of the fish intermediate host Engraulicypris sardella, but only after it has become infective to the final host

Ligula intestinalis is a tapeworm using copepods and cyprinid fish as intermediate hosts and fish-eating birds as final hosts. Since some parasites can increase their own fitness by manipulating the behavior of the intermediate host, we explored if this parasite affected predator avoidance, swimming activity and depth preference of the fish intermediate host, Engraulicypris sardella. We found that when L. intestinalis had reached a developmental stage that is able to establish in the bird host, it had a significant impact on E. sardella behavior, while the tapeworm that was not fully developed had little effect and fish hosts showed a behavior more similar to uninfected fish. These results are discussed with respect to two different processes: the manipulation hypothesis and the energy drain hypothesis.

Recent Advances in the Diagnosis and Detection of Opisthorchis viverrini Sensu Lato in Human and Intermediate Hosts for Use in Control and Elimination Programs.

Opisthorchiasis is a neglected tropical disease, caused by infection with the fish-borne trematode Opisthorchis viverrini sensu lato that afflicts more than 10million people in Southeast Asia, including Thailand, Lao PDR, Vietnam and Cambodia. The disease is characterized by a chronic infection that induces hepatobiliary inflammation, especially periductal fibrosis, which can be detected by ultrasonography. This chronic inflammation eventually leads to cholangiocarcinoma (CCA), a usually fatal bile duct cancer that develops in approximately 1% of O. viverrini-infected individuals. In Thailand alone, 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 O. viverrini-associated morbidity and CCA, the primary intervention measures focus on opisthorchiasis control and elimination. Accurate diagnoses of O. viverrini infection, in both mammalian, snail and fish intermediate hosts, are important for achieving these goals. Despite extensive efforts over several decades to find sensitive and specific diagnostics for opisthorchiasis, a simple and robust diagnostic method is still required. Here we review earlier and current developments in the search for new diagnostics for opisthorchiasis, with practical applications in the research laboratory, the clinic and the field. Of the methods currently available, the urine antigen assay shows considerable potential for the diagnosis and screening of opisthorchiasis. Nevertheless, these new assays require validation, determination of their cost-effectiveness when applied for mass screening in an endemic setting in support of policy decisions for national public health programs aimed at the control and elimination of opisthorchiasis.

Assessment of in vitro killing assays for detecting praziquantel-induced death in Posthodiplostomum minimum metacercariae

Control of parasitic infections may be achieved by eliminating developmental stages present within intermediate hosts, thereby disrupting the parasite life cycle. For several trematodes relevant to human and veterinary medicine, this involves targeting the metacercarial stage found in fish intermediate hosts. Treatment of fish with praziquantel is one potential approach for targeting the metacercaria stage. To date, studies investigating praziquantel-induced metacercarial death in fish rely on counting parasites and visually assessing morphology or movement. In this study, we investigate quantitative methods for detecting praziquantel-induced death using a Posthodiplostomum minimum model. Our results revealed that propidium iodide staining accurately identified praziquantel-induced death and the level of staining was proportional to the concentration of praziquantel. In contrast, detection of ATP, resazurin metabolism, and trypan blue staining were poor indicators of metacercarial death. The propidium iodide method offers an advantage over simple visualization of parasite movement and could be used to determine EC50 values relevant for comparison of praziquantel sensitivity or resistance.

Opisthorchis viverrini infection in the snail and fish intermediate hosts in Central Vietnam

Opisthorchis viverrini, a carcinogenic fish borne fluke, requires freshwater snails and fish as intermediate hosts. Opisthorchiasis is endemic in parts of Southeast Asia, including Central and South Vietnam. In this region the transmission by intermediate hosts has received little attention. Therefore, freshwater snails and wild fish from Bau My Tho, an opisthorchiasis endemic area in Binh Dinh Province were collected for examination of O. viverrini cercariae and metacercariae, respectively. A total of 12,000 snails belonging to six families, of which 1616 Bithynia snails representing Bithynia siamensis goniomphalos and Bithynia funiculata; as well as 754 fish representing 12 species were examined. Shedding of O. viverrini cercariae was observed only in B. s. goniomphalos and B. funiculata, at infection rates of 0.86% and 0.14%, respectively. O. viverrini infection in Bithynia spp. was significantly associated with the habitat but not with the species and the shell size of Bithynia spp. O. viverrini metacercariae were found in 10 fish species representing both Cyprinidae and non-Cyprinidae families. The prevalence of O. viverrini infection in fish was significantly associated with species. Carassius auratus, a fish species commonly eaten raw, Rasbora aurotaenia and Puntius brevis had the highest prevalence of 74.0%, 55.8% and 31.6%, respectively. Sharing of the same snail and fish intermediate host species was found for O. viverrini and a O. viverrini duck-genotype that are sympatric in the study region. This study is the first to report on the intermediate host species of O. viverrini in Central Vietnam and indicates a high risk of acquiring opistorchiasis when eating raw fish dishes.