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In this study of Metagonimus suifunensis (M. suifunensis) in the Russian Southern Far East, the variability of the full-length sequences of the cytochrome b (cytb) mtDNA gene was assessed for the first time. In addition, the cox1 mtDNA gene sequences were also obtained for this species from new localities. In total, 87 and 81 sequences of the cytb and cox1 genes, respectively, were used in the current study. The cytb gene proved more promising and revealed two haplogroups that are associated with the spatial distribution of the species: geographical isolation caused the fixation of differences between northern and southern populations. In addition, the results obtained for the cytb gene opened up new perspectives in the analysis of sequences of the cox1 gene, which was not sufficiently effective as a sole marker. Based on data for both mitochondrial genes, molecular processes influencing the formation of the modern population were analysed for M. suifunensis. The new data confirmed the previously expressed opinion that this species colonized the study territory from north to south and will form the basis for determining possible ways of its further expansion, which is important for predicting the emergence of new foci of metagonimosis.

Digenea usually use ventral sucker for sustainable attachment within intestine of their definitive vertebrate host. However, if the ventral sucker is absent or poorly developed, the means of attachment are unclear. We investigated attachment and locomotion in such digeneans: three species of the family Microphallidae (Microphallus piriformes, M. pygmaeus, and Levinseniella brachysoma) and two species of the family Heterophyidae (Cryptocotyle concava and C. lingua). Their tegumental spines and musculature were described with use of fluorescent actin staining, confocal microscopy, and scanning electron microscopy. Locomotion of living worms was observed and recorded. Wide serrated tegumental spines probably play the main role in attachment. Their firm contact with the host mucosa may be provided by the action of the ventral concavity—when the entire body or its part acts as a sucker. Dorsoventral muscle bundles act like radial musculature of the sucker generating negative pressure in the ventral concavity. The solid layer of longitudinal muscle fibers on the ventral body surface provides support for the bottom of the ventral concavity. In all microphallids, a U-shaped arrangement of body wall musculature (mostly originating from longitudinal fibers) outlines posterior part of the ventral concavity ridge. In all the studied species, tegumental spines, body wall musculature, and dorsoventral muscle bundles are better developed in the forebody which moves more actively than the hindbody.

For trophically transmitted parasites that manipulate the phenotype of their hosts, whether the parasites do or do not experience resource competition depends on such factors as the size of the parasites relative to their hosts, the intensity of infection, the extent to which parasites share the cost of defending against the host’s immune system or manipulating their host, and the extent to which parasites share transmission goals. Despite theoretical expectations for situations in which either no, or positive, or negative density-dependence should be observed, most studies document only negative density-dependence for trophically transmitted parasites. However, this trend may be an artifact of most studies having focused on systems in which parasites are large relative to their hosts. Yet, systems are common where parasites are small relative to their hosts, and these trophically transmitted parasites may be less likely to experience resource limitation. We looked for signs of density-dependence in Euhaplorchis californiensis (EUHA) and Renicola buchanani (RENB), two manipulative trematode parasites infecting wild-caught California killifish (Fundulus parvipinnis). These parasites are small relative to killifish (suggesting resources are not limiting), and are associated with changes in killifish behavior that are dependent on parasite-intensity and that increase predation rates by the parasites’ shared final host (indicating the possibility for cost sharing). We did not observe negative density-dependence in either species, indicating that resources are not limiting. In fact, observed patterns indicate possible mild positive density-dependence for EUHA. Although experimental confirmation is required, our findings suggest that some behavior-manipulating parasites suffer no reduction in size, and may even benefit when “crowded” by conspecifics.

By modifying the behaviour and morphology of hosts, parasites may strongly impact host individuals, populations and communities. We examined the effects of a common trematode parasite on its snail host, Batillaria cumingi (Batillariidae). This widespread snail is usually the most abundant invertebrate in salt marshes and mudflats of the northeastern coast of Asia. More than half (52.6%, n=1360) of the snails in our study were infected. We found that snails living in the lower intertidal zone were markedly larger and exhibited different shell morphology than those in the upper intertidal zone. The large morphotypes in the lower tidal zone were all infected by the trematode, Cercaria batillariae (Heterophyidae). We used a transplant experiment, a mark-and-recapture experiment and stable carbon isotope ratios to reveal that snails infected by the trematode move to the lower intertidal zone, resume growth after maturation and consume different resources. By simultaneously changing the morphology and behaviour of individual hosts, this parasite alters the demographics and potentially modifies resource use of the snail population. Since trematodes are common and often abundant in marine and freshwater habitats throughout the world, their effects potentially alter food webs in many systems.

Metacercariae of Opisthorchis viverrini, a carcinogenic liver fluke, and Haplorchoides sp., a trematode maturing in catfish, are commonly found in cyprinid fish, the second intermediate hosts of both flukes. However, the specific identity of Haplorchoides sp. in Thailand and a precise assessment of the effects of co-infections with O. viverrini have never been clarified. Therefore, we aimed to identify the species of Haplorchoides and to investigate possible interactions of the two trematode species in cyprinid fishes. Based on the morphology and morphometry of the cercaria, metacercaria, and adult stages, the Haplorchoides species found was identified as Haplorchoides mehrai Pande and Shukla 1976. Thailand is formally recorded as a new locality for H. mehrai, where naturally infected hosts include the snail Melanoides tuberculata (first intermediate host), the cyprinid fishes Hampala dispar, Cyclocheilichthys apogon, Puntius leiacanthus, Labiobarbus burmanicus, and Cirrhina jullieni (second intermediate hosts), and a catfish, Mystus nemurus (definitive host). The co-infection rates of O. viverrini and H. mehrai were significantly associated with fish species and fish body region (P < 0.001), with an overall significantly higher average intensity of H. mehrai (126.26 metacercariae/fish) than that of O. viverrini (18.02 metacercariae/fish). Further work is required to demonstrate the extent and mechanisms of possible interactions between these trematode species in the fish host. These data may provide a better understanding of O. viverrini transmission dynamics, and help design integrated control interventions.

Abstract Pleurolophocercous cercariae found in the invasive gastropod Melanoides tuberculata (Müller, 1774) collected in a stream of the Vila do Abraão, Ilha Grande, Angra dos Reis, Rio de Janeiro, Brazil were used for experimental infection that enabled the identification of the heterophyid trematode Centrocestus formosanus (Nishigori, 1924). The parasite has been found in the locality since 2007, after two years of the introduction of M. tuberculata. Recently, from a sample of 483 specimens collected in June 2013, 101 (21%) were found infected with parasite. The potential environmental impacts caused by the parasite occurrence could be underestimated in the country, and actions to monitor and control both the parasite and the mollusk are necessary. Resumo Cercárias do tipo pleurolofocerca encontradas no gastrópode invasor Melanoides tuberculata (Müller, 1774) coletados em um riacho da Vila do Abraão em Ilha Grande, Angra dos Reis, Rio de Janeiro, Brasil foram utilizadas em estudos de infecção experimental que possibilitaram a identificação do trematódeo heterofiídeo Centrocestus formosanus (Nishigori, 1924). O parasito vem sendo encontrado na localidade desde 2007, dois anos após a introdução do molusco. Recentemente, de uma amostra de 483 espécimes coletados em junho de 2013, 101 (21%) apresentavam-se infectados pelo parasito. Os potenciais impactos ambientais ocasionados pela ocorrência deste parasito podem estar sendo subestimados no país, sendo necessárias ações visando o monitoramento e controle tanto do parasito quanto do molusco.

Parasites often use external cues to identify and move toward environments where they are likely to encounter suitable hosts. The trematode parasite Euhaplorchis californiensis produces cercariae that emerge from California horn snails (Cerithideopsis californica [= Cerithidea californica]) to infect California killifish (Fundulus parvipinnis) as second intermediate hosts. Based upon work on a congeneric Euhaplorchis species from Florida, and based on the ecology of its killifish host, we hypothesized that E. californiensis cercariae in southern California estuaries are positively phototactic and negatively geotactic, using both sunlight and gravity to guide their movement to the upper water column. To distinguish positive phototaxis from negative geotaxis, we first quantified E. californiensis movement in response to light along a horizontal plane and determined they were positively phototactic. In a second experiment, we quantified E. californiensis movement along a vertical plane in response to an overhead light, a light from below, or no light. We found that E. californiensis exhibit negative geotaxis in the absence of light, but will swim in the direction of gravity to move toward a light source from below. Thus, E. californiensis are both positively phototactic and negatively geotactic, but cercariae prioritize phototactic cues. These results suggest that E. californiensis cercariae aggregate in the open water, indicating that the pelagic zone represents an area of high infection risk for California killifish hosts.

Food-borne trematodiases still remain a public health problem world-wide, despite changes in eating habits, alterations in social and agricultural practices, health education, industrialization, environmental alteration, and broad-spectrum anthelmintics. Food-borne trematodiases usually occur focally, are still persistently endemic in some parts of the world, and are most prevalent in remote rural places among school-age children, low-wage earners, and women of child-bearing age. Intestinal fluke diseases are aggravated by socio-economic factors such as poverty, malnutrition, an explosively growing free-food market, a lack of sufficient food inspection and sanitation, other helminthiases, and declining economic conditions. Control programs implemented for food-borne zoonoses and sustained in endemic areas are not fully successful for intestinal food-borne trematodiases because of centuries-old traditions of eating raw or insufficiently cooked food, widespread zoonotic reservoirs, promiscuous defecation, and the use of “night soil” (human excrement collected from latrines) as fertilizer. This review examines food-borne intestinal trematodiases associated with species in families of the Digenea: Brachylaimidae, Diplostomidae, Echinostomatidae, Fasciolidae, Gastrodiscidae, Gymnophallidae, Heterophyidae, Lecithodendriidae, Microphallidae, Nanophyetidae, Paramphistomatidae, Plagiorchiidae, and Strigeidae. Because most of the implicated species are in the Echinostomatidae and Heterophyidae, emphasis in the review is placed on species in these families.

Metacercariae of an unidentified species of Apophallus Lühe, 1909 are associated with overwinter mortality in coho salmon, Oncorhynchus kisutch (Walbaum, 1792), in the West Fork Smith River, Oregon. We infected chicks with these metacercariae in order to identify the species. The average size of adult worms was 197 × 57 μm, which was 2 to 11 times smaller than other described Apophallus species. Eggs were also smaller, but larger in proportion to body size, than in other species of Apophallus. Based on these morphological differences, we describe Apophallus microsoma n. sp. In addition, sequences from the cytochrome c oxidase 1 gene from Apophallus sp. cercariae collected in the study area, which are likely conspecific with experimentally cultivated A. microsoma, differ by >12% from those we obtained from Apophallus donicus (Skrjabin and Lindtrop, 1919) and from Apophallus brevis Ransom, 1920. The taxonomy and pathology of Apophallus species is reviewed.

Fish-borne zoonotic trematodes (FZT) including heterophyids and opisthorchiids are prevalent in native and aquaculture fish in Southeast Asia. FZT are reported highly prevalent in juvenile aquaculture fish that belonged to the family Heterophyidae, particularly Haplorchis taichui. Aquaculture fish are reported to have varying levels of natural infection with H. taichui, but data for their susceptibility as well as resistance to infection are not available. The aim of this study was to determine the susceptibility of five aquaculture fish to H. taichui. Experimental infections were performed by exposing fish individually to 0 (control), 50, 100, or 200 H. taichui cercariae for 12 h. Metacercarial burden was measured at 45 days postinfection. Three out of five fish species, silver barb (Barbonymus gonionotus), common carp (Cyprinus carpio), and mrigal (Cirrhinus mrigala), were highly susceptible to H. taichui infection with percentage ranging from 93.33 to 100 %. The Nile and red tilapia were not susceptible to infection. Among the susceptible fish species, silver barb had higher intensity of H. taichui metacercariae than common carp and mrigal (P < 0.001). Metacercarial burden significantly correlated with the dose of cercarial infection (P < 0.001). Our findings that common aquaculture fish species have varying degrees of susceptibility to H. taichui infection provide important information to reduce parasite transmission in aquaculture fish.