Digenean Trematodes Research Articles

A volunteer-populated online database provides evidence for a geographic pattern in symptoms of black spot infections.

Infections of parasitic digenean trematode metacercariae may lead to a visually observable syndrome in fish commonly called black spot disease. While black spot has been noted from various locations throughout North America, patterns in prevalence across the continent remain unknown. Funding to investigate continental-wide prevalence of low-mortality parasitic infections represents a barrier to such studies. I utilize iNaturalist.org's photograph database to examine fish for signs of black spot infections across North America. Fish targeted include blacknose dace, creek chub, chubs (Nocomis spp.), and stonerollers (Campostoma spp.). Photos were visually examined for symptomatic black spots indicative of infection by trematode species linked to black spot disease. Regardless of fish species group, symptoms of black spot pathogens were highly prevalent (27.1% of 314 fish) in watersheds of southern Ontario Canada, whereas mean prevalence was comparatively low elsewhere (7.8%). In one instance, a user uploaded a higher number of photos, with a higher percentage exhibiting signs of infection than other users in the watershed. However, it is difficult to tease apart if that user fished in waterbodies with high infection rates, uploaded more photos of symptomatic fishes, or some other explanation for the differences in user-reported fish with symptoms. Beyond this exception, geographic patterns in the frequency of black spot symptoms do not appear to be related to solely the users, suggesting the observed pattern is biological or ecological. While causative explanations remain conjectures, the data reported herein provides evidence that across four groups of fish, signs of black spot infections are more common in southern Ontario than other areas studied in North America. This work also represents an initial and unexpected utility of volunteer-population databases such as iNaturalist. Further data contributions could lead to better understanding of the causative agents to variation in black spot pathogens' occurrences.

Untangling the effects of size, habitat and invertebrate biodiversity on parasite prevalence in the Caribbean spiny lobster

The spiny lobster Panulirus argus is an important benthic mesopredator and a major fishing resource across the Wider Caribbean region. This species is host to the pathogenic virus PaV1 and metacercariae of Cymatocarpus solearis, a digenean trematode whose first intermediate host remains unknown. Previous studies found that the probability of infection with PaV1 was higher in juvenile lobsters and in densely vegetated habitats (suggesting that marine vegetation can be an environmental reservoir for PaV1), whereas the probability of infection with C. solearis was higher for larger lobsters and in poorly vegetated habitats. To increase insight into the role of habitat and body size in the ecology of lobster diseases, the presence of both C. solearis and PaV1 in P. argus was investigated across three contrasting zones in Bahía de la Ascensión, Mexico (19°35′27″N, 87°38′06″W): reef, lagoon and shallow habitat. Additionally, habitat complexity, cover of benthic components, and macroinvertebrate biodiversity were characterized in each zone. Consistent with previous studies, probability of infection with PaV1 (both at a clinical and infective level) decreased with increasing lobster size and was highest in the seagrass-rich lagoon, supporting the idea that marine vegetation could be an environmental reservoir for PaV1. In contrast, the probability of infection with C. solearis increased significantly with lobster size but did not vary with zone, suggesting no relationship with benthic substrate type. However, based on results of macroinvertebrate diversity, the gastropods Cerithium litteratum and Tegula fasciata are put forward as potential candidates for the first intermediate hosts of C. solearis.

Black spot syndrome in reef fishes: using archival imagery and field surveys to characterize spatial and temporal distribution in the Caribbean

Recently, observations of black spot syndrome (BSS) in Caribbean fishes have been linked to infection by a digenean trematode parasite, Scaphanocephalus expansus. Recently, This study examined the distribution of BSS over multiple spatial and temporal scales: at the island scale in Bonaire, Dutch Caribbean, using field surveys of 4885 fish belonging to three species, and across the wider Caribbean through analysis of 2112 images from Google Images searches (1985–2013). The field surveys in Bonaire indicated that the prevalence (% of individuals affected with BSS) and intensity (severity of BSS measured in 3 stages) were highest in Acanthurus tractus (prevalence 61.8%, including 30.1% in stage 3) followed by Sparisoma aurofrenatum (prevalence 48.3%, 24.1% in stage 3) and lowest in Caranx ruber (prevalence 38.5%, 3.3% in stage 3). Prevalence and intensity of BSS decreased significantly with survey depth (e.g., 2 m: prevalence 68.0%, 22.0% in stage 3; 18 m: prevalence 36.2%, 4.0% in stage 3) and were significantly lower in 2012 than in 2017 (prevalence: 59.3% in 2012, 68.7% in 2017; stage 3: 16.3% in 2012, 25.1% in 2017). The Southeast of Bonaire had significantly lower prevalence of BSS (16.4%) than the other four regions and lower intensity (11.7% in stage 3) than all regions but the Southwest. The Google Images searches querying for ocean surgeonfish, A. tractus and A. bahianus, identified pictures from 26 wider Caribbean locations; BSS was detected in 14 locations of which 13 were new, with the first detection dating back to 1985 in Bonaire. The Southern Caribbean had significantly higher prevalence of BSS (78.1%) than other ecoregions (0–34.6%), and Bonaire was identified as a hotspot, highlighting the utility of mining websites for archival imagery to quantify spatial and temporal patterns in disease phenomena. This study demonstrates how visible signs of parasite infection can be used to find differences in parasite prevalence and loads on a reef, island and sea scale.

Alternative host identity and lake morphometry drive trematode transmission in brook charr.

Both biotic and abiotic factors have been invoked to explain the large variations observed in the prevalence and abundance of parasites in aquatic ecosystems. However, we have only a poor knowledge of the potential interplay among these factors in natural systems. It is, therefore, important to analyze the effects of multiple potential environmental drivers together to get an integrated view of their influence on the prevalence and abundance of parasites. To this end, we selected two genera of digenean trematode parasites that require at least two hosts to complete their life cycle and use two different transmission strategies. Crepidostomum moves through a trophic pathway via consumption of infected prey by the host, while Apophallus infects its hosts via direct penetration of their skin. This study was conducted in 23 Canadian Shield lakes exhibiting orthogonal gradients of biotic (fish species richness and biomass) and abiotic (morphometry, physico-chemical) variables. We quantified prevalence and abundance of these parasites in the skin and intestine of brook charr (Salvelinus fontinalis). Our results show that biotic factors are key drivers of parasite abundance and prevalence, with Apophallus being negatively associated with the fish species richness-biomass gradient, and Crepidostomum responding more to identity of host than to the diversity gradient. Among the abiotic variables, lake areawas found to be positively related to both prevalence and abundance in Apophallus. Our results suggest that taking into account the interplay of both biotic and abiotic factors is crucial for understanding patterns of parasite transmission success in boreal lakes.

Evidence of Centrocestus formosanus (Nishigori, 1924) in Zebrafish (Danio rerio)

This study reports the presence of cartilaginous cysts in zebrafish gills, purchased at the end of 2012 before the application of the Italian Legislative Decree 26/2014 and used for experimental research. It highlights the importance of using specific pathogen-free organisms from reliable production establishments to avoid obtaining false results in experiments. Of 20 samples analyzed, 4 showed cysts in gills, with abnormal tissue modification and cartilaginous metaplasia induced by prolonged parasitic insult. The results obtained from the periodic health monitoring program showed the presence of metacercariae of digenean trematodes, specifically identified as Centrocestus formosanus.

Common Freshwater Fish Parasites Pictorial Guide: Digenean Trematodes

This publication is one in a series of pictorial guides that is designed to assist in the identification of common freshwater fish parasites. The information provided in this guide is not intended to be a complete, detailed description of each parasite or parasite group and its characteristics but rather is intended to assist in the visual identification of some of the most common species or groups of parasites seen in freshwater fish. For further information on each parasite, refer to publications in the “Recommended Reading” and “Reference” sections. This document is Fact Sheet FA-112, one of a series from the Tropical Aquaculture Laboratory, Department of Fisheries and Aquatic Sciences, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First published: July 2005. 
 FA-112/FA112: Common Freshwater Fish Parasites Pictorial Guide: Digenean Trematodes (ufl.edu)

Case 3775 – Megaperidae Manter, 1934 and Apocreadiinae Skrjabin, 1942 (Platyhelminthes, Digenea): proposed re-establishment of precedence of the older family-group name

The purpose of this application, under Articles 78.1 and 81.1 of the Code, is to suspend the application of Art. 35.5 so as to re-establish the precedence of Megaperidae Manter, 1934 over Apocreadiinae Skrjabin, 1942 for certain digenean trematodes of fish. Both names have been used as valid in recent literature, with some authors treating the former as a subfamily (Megaperinae Manter, 1934) of the latter, which is now treated also as a family (Apocreadiidae Skrjabin, 1942). While Apocreadiidae may possibly be regarded as enjoying prevailing usage, Megaperidae is the older name, and its supposed lack of precedence under Art. 35.5 is an aberrancy resulting from historical contingencies.

Didymozoids in Muscle of Atlantic Chub Mackerel (Scomber colias).

Digenean trematodes of the family Didymozoidae are tissue parasites that are particularly common in scombrid fish. These parasites can reduce the commercial value of fish, especially the ones occurring in the muscle. In the present study and for the first time, we report the occurrence of didymozoids in muscle tissue of Atlantic chub mackerel (Scomber colias) collected in northeast Atlantic (Portuguese coast). The entire musculature of 64 Scomber colias was removed and examined macroscopically and under a stereomicroscope to search for parasites. The prevalence (3.1%) and abundance (0.03) of infection detected were quite low, but the parasites cause muscle softening decreasing the quality of the infected fish. If the infection levels increase, this may constitute a cause of concern for the fisheries and canning industry. Didymozoids identified in this study are closely related to muscle parasites detected in other fish species of the genus Scomber, most probably the same species. Phylogenetic analysis also corroborates the hypothesis that muscle-parasitizing didymozoids are distinct from the ones parasitizing other fish organs.

Host-parasite life-histories of the diurnal vs. nocturnal chronotypes of Schistosoma mansoni: adaptive significance.

To optimise host-to-host transmission, digenean trematodes (parasites) synchronise their cercarial emission patterns with the aquatic activities of their vertebrate hosts. Schistosoma mansoni has two different circadian chronotypes: a diurnal shedding pattern with a mean peak occurring at 11:00h, and a nocturnal shedding pattern with a mean peak occurring at 20:00h. We analysed the life-history variations between these two chronotypes at the levels of the parasite and its hosts. For each chronotype, we quantified three life-history traits related to the parasite (prepatent period, infection rate and cercarial production) and analysed the morphometry and the morphology of the parasite eggs; we also quantified three life-history traits related to the snail intermediate host (shell diameter, fecundity and survival rate) and one life-history trait related to the experimental definitive host (survival rate). A phylogeny based on the mitochondrial cytochrome-oxidase gene was made on samples of both chronotypes. Life-history analysis revealed significant variations between the two chronotypes. Life-history traits were optimal for both the parasite and the snail host for the diurnal chronotype compared to the nocturnal one. The new chronotype behaved like an allopatric population towards its snail host. Phylogenetic analysis supports the hypothesis of a lateral transfer of S.mansoni from humans to Rattus rattus. These results were interpreted in terms of an ongoing sympatric speciation. The nocturnal chronotype of S.mansoni showed non-adapted life-history traits in its relation with the snail intermediate host Biomphalaria pfeifferi. The emergence of this new phenotype is probably linked to divergent natural selection.

In vivo fluorescent cercariae reveal the entry portals of Cardiocephaloides longicollis (Rudolphi, 1819) Dubois, 1982 (Strigeidae) into the gilthead seabream Sparus aurata L.

BackgroundDespite their complex life-cycles involving various types of hosts and free-living stages, digenean trematodes are becoming recurrent model systems. The infection and penetration strategy of the larval stages, i.e. cercariae, into the fish host is poorly understood and information regarding their entry portals is not well-known for most species. Cardiocephaloides longicollis (Rudolphi, 1819) Dubois, 1982 (Digenea, Strigeidae) uses the gilthead seabream (Sparus aurata L.), an important marine fish in Mediterranean aquaculture, as a second intermediate host, where they encyst in the brain as metacercariae. Labelling the cercariae with in vivo fluorescent dyes helped us to track their entry into the fish, revealing the penetration pattern that C. longicollis uses to infect S. aurata.MethodsTwo different fluorescent dyes were used: carboxyfluorescein diacetate succinimidyl ester (CFSE) and Hoechst 33342 (NB). Three ascending concentrations of each dye were tested to detect any effect on labelled cercarial performance, by recording their survival for the first 5 h post-labelling (hpl) and 24 hpl, as well as their activity for 5 hpl. Labelled cercariae were used to track the penetration points into fish, and cercarial infectivity and later encystment were analysed by recording brain-encysted metacercariae in fish infected with labelled and control cercariae after 20 days of infection.ResultsAlthough the different dye concentrations showed diverse effects on both survival and activity, intermediate doses of CFSE did not show any short-term effect on survival, permitting a brighter and longer recognition of cercariae on the host body surface. Therefore, CFSE helped to determine the penetration points of C. longicollis into the fish, denoting their aggregation on the head, eye and gills region, as well as on the dorsal fin and the lower side. Only CFSE-labelled cercariae showed a decreased number of encysted metacercariae when compared to control.ConclusionsOur study suggests that CFSE is an adequate labelling method for short-term in vivo studies, whereas NB would better suit in vivo studies on long-term performance. Cardiocephaloides longicollis cercariae seem to be attracted to areas near to the brain or those that are likely to be connected to migration routes to neuronal canals.

A fine-scale phylogenetic assessment of digenean trematodes in central Alberta reveals we have yet to uncover their total diversity.

Despite over 100 years of digenean trematode parasite species descriptions, from a wide diversity of vertebrate and invertebrate host species, our ability to recognize the diversity of trematode species within a single lake remains an incredible challenge. The most challenging aspect is the identification of species from larval stages derived from intermediate hosts, due to the disjointed data of adult worm morphological descriptions, from which species are named, and links to corresponding molecular identifiers in depauperate databases. Cryptic species also play a significant role in the challenge of linking trematode larvae to adults, species identifications, and estimating diversity. Herein, we utilize a large, longitudinal dataset of snail first‐intermediate host infection data from lakes in Alberta, Canada, to infer trematode larval diversity using molecular phylogenetics and snail host associations. From our assessments, we uncover a diversity of 79 larval trematode species among just five snail host species. Only 14 species were identified to a previously described species, while the other 65 species are either cryptic or otherwise unrepresented by mitochondrial genes in GenBank. This study currently represents the largest and most diverse singular molecular survey of trematode larval fauna composed of over one thousand mitochondrial sequences. Surprisingly, rarefaction analyses indicate we have yet to capture the complete diversity of trematodes from our sampling area.

Histopathological assessment of the health status of Mytilus chilensis (Hupé 1854) in southern Chile

Mussel (Mytilus chilensis) farming is an important economic and social activity in the south of Chile. With a landing of 302,642 tons in year 2016, Chile is the second mytilid producer after China, and the first exporter worldwide. To be better prepared to protect this industry, the health status of a total number of 478 mussels (M. chilensis) from the Los Lagos Region, in the south of Chile, was assessed. Baseline information on symbionts, parasites and pathological conditions of both cultivated and natural mussels was collected using histology. Organisms detected included: intracellular microcolonies of bacteria (IMC), located in the epithelium of the gill and of digestive gland tubules; a microsporidian resembling Steinhausia mytilovum hosted by oocytes; two gill ciliates; a copepod; and a digenean trematode. Pathological conditions included neoplasia, hemocyte aggregates within digestive epithelia, lipofuscin-like pigments in various tissues, and gill epithelium desquamation. The prevalence of each finding was assessed and compared statistically between sites and between cultured and natural populations. The infection intensity of them was low, and no OIE listed parasite was detected. Of all the findings of this study, those that could be of concern are the IMC, neoplasia, and the Steinhausia mytilovum-like microsporidian. This study provided baseline data necessary for the design of a larger, seasonal sampling, which will allow to assess the feasibility of a permanent monitoring program to protect the huge mussel cultivating industry of the Los Lagos Region in Chile.

Morphological and molecular characterization of invasive Biomphalaria straminea in southern China

BackgroundSchistosomiasis is a common parasitic disease designated as a neglected tropical disease by the World Health Organization. Schistosomiasis mansoni is a form of the disease that is caused by the digenean trematode Schistosoma mansoni, transmitted through Biomphalaria spp. as an intermediate host. Biomphalaria was introduced to Hong Kong, China in aquatic plants shipments coming from Brazil and the snail rapidly established its habitats in southern China. Earlier studies of Biomphalaria spp. introduced to southern China identified the snails as Biomphalaria straminea, one of the susceptible species implicated in S. mansoni transmission in South America. However, recent molecular investigations also indicated the presence of another South American species, B. kuhniana, which is refractory to infection. As such, it is important to identify accurately the species currently distributed in southern China, especially with emerging reports of active S. mansoni infections in Chinese workers returning from Africa.MethodsWe combined morphological and molecular taxonomy tools to precisely identify Biomphalaria spp. distributed in Guangdong Province, southern China. In order to clearly understand the molecular profile of the species, we constructed a phylogeny using mtDNA data (COI and 16S rRNA sequences) from six populations of Biomphalaria spp. from Shenzhen City in Guangdong Province. In addition, we examined the external morphology of the shell and internal anatomy of the reproductive organs.ResultsBoth morphological and molecular evidences indicated a close affinity between Biomphalaria spp. populations from Guangdong and B. straminea from Brazil. The shell morphology was roughly identical in all the populations collected with rounded whorls on one side and subangulated on the other, a smooth periphery, an egg-shaped aperture bowed to one side, and a deep umbilicus. The shape and number of prostate diverticula (ranged from 11.67 to 17.67) in Guangdong populations supports its close affinity to B. straminea rather than B. kuhniana. Molecular analysis did not conflict with morphological analysis. Little genetic differentiation was observed within Biomphalaria populations collected. Phylogenetic analysis of COI and 16S rRNA haplotypes from snails collected and B. straminea sequences from Brazil and China using Bayesian inference revealed that Guangdong populations were clustered in one clade with B. straminea from Hong Kong of China and B. straminea from Brazil indicating their close affinity to each other.ConclusionsData obtained in the current study clearly show that the populations of Biomphalaria spp. investigated are B. straminea, and we assume that those snails were either introduced via passive dispersal from Hong Kong of China or as a result of multiple introduction routes from Brazil.

Agents of swimmer\u2019s itch\u2014dangerous minority in the Digenea invasion of Lymnaeidae in water bodies and the first report of Trichobilharzia regenti in Poland

Trichobilharzia spp. have been identified as a causative agent of swimmers’ itch, a skin disease provoked by contact with these digenean trematodes in water. These parasites have developed a number of strategies to invade vertebrates. Since we have little understanding of the behavior of these parasites inside the human body, the monitoring of their invasion in snail host populations is highly recommended. In our research, lymnaeid snails were collected from several Polish lakes for two vegetation seasons. The prevalence of bird schistosomes in snail host populations was significantly lower than that of other digenean species. We were the first to detect the presence of the snails emitted Trichobilharzia regenti (potentially the most dangerous nasal schistosome) in Poland. In addition, by sequencing partial rDNA genes, we confirmed the presence of the snails positive with Trichobilharzia szidati in Polish water bodies, showing that swimmer’s itch is more frequent during summer months and that large snails are more often infected with bird schistosomes than small ones.

Effect of tidal level on abundance of symbionts in the White Sea blue mussel.

In the White Sea, the blue mussel Mytilus edulis occupies a wide range of biotopes and is associated with numerous symbiotic organisms. At some sites, mussel cover spreads continuously from the intertidal to the subtidal zone. We checked whether the patterns of infection by different associated organisms differed among the upper subtidal, zero-depth and lower intertidal zones at 3 sites in the Kandalaksha Gulf and the Onega Bay of the White Sea. Organisms belonging to 13 taxa were found in mantle cavities and tissues of blue mussels. Parasitic green algae, a sporocyst and metacercariae of 5 species of digenean trematodes occupied mussel tissues; commensal ciliates, rhabdocoelans and some free-living invertebrates were found in mantle cavities. Quantitative composition of symbiotic communities of mussels was not the same at different tidal levels: Urastoma cyprinae (commensal rhabdocoelans) were more abundant in the subtidal and zero-depth zones, while encysted metacercariae of Renicola roscovita and Himasthla sp. were more abundant at the zero-depth and intertidal zones. We suggested several hypotheses to explain this heterogeneity.

Use of qPCR-Based Cercariometry to Assess Swimmer\u2019s Itch in Recreational Lakes

Swimmer’s itch (cercarial dermatitis) is a nuisance encountered by bathers and recreational water users worldwide. The condition is caused by the penetration of larval digenean trematodes (cercariae) of the family Schistosomatidae, into the skin, following their release into freshwater from pulmonate snails that serve as the intermediate hosts for these parasites. This study utilizes qPCR-based cercariometry to monitor and quantify cercariae from water samples collected at 5 lakes in northern Michigan. The resolution provided by qPCR facilitated assessment of the environmental and biological drivers of swimmer’s itch-causing cercariae concentrations, allowing us to demonstrate that cercarial abundance is greatest at the top of the water column, in locations with prevailing on- and alongshore winds.

First report of Euryhelmis parasites (Trematoda, Heterophyidae) in Africa: conservation implications for endemic amphibians.

In this study, we report, through molecular identification, the first African records of a digenean trematode parasite of the genus Euryhelmis. We recovered metacercariae encysted in an anuran, the endemic Moroccan painted frog (Discoglossus scovazzi), and a vulnerable caudate, the North African fire salamander (Salamandra algira), from four localities in North Africa (Morocco). Our records go back to the past century and have been confirmed in successive fieldwork seasons thereafter. Metacercarial stages of these parasites require amphibians as the last intermediate host, but the exact identity of the primary hosts and predators of the infected animals in Africa remain unknown. Our searches with basic local alignment search tool (BLAST) from Genbank revealed that hosts were infected by parasites of Euryhelmis costaricensis, which showed almost the same genetic identity (with only one substitution) to previous reports from Costa Rica and Japan, suggesting a recent introduction in Morocco. We proceed to discuss the likely role of introduced mustelids as the potential definitive hosts of trematode adults. Under this assumption, we conclude that the infestation of Discoglossus scovazzi and Salamandra algira might pose a risk to these threatened species.

Occurrence of Paratanaisia spp. Freitas, 1951 in a domiciled cockatiel (Nymphicus hollandicus, Psittaciformes: Cacatuidae).

Digenetic eucotylids of the Paratanaisia genus are widely reported parasites of the avian renal system. The infection, commonly reported in free-living and zoo-housed birds, is described for the first time in a domiciled bird, a cockatiel kept as pet bird with free access to the yard outdoors. The bird was received at Veterinary Hospital, where clinical and radiographic evaluations suggested a case of heavy-metal poisoning. Although the bird received supportive care and a chelating agent, it died the next day. The necropsy showed friable kidneys and congestion of blood vessels in structures such as the skin, proventriculus, brain, and skullcap. The histopathological evaluation of the kidneys revealed multifocal hemorrhages, commonly found in cases of heavy-metal poisoning. Parasitic structures similar to those of the digenean trematodes Paratanaisia spp. were also observed inside dilated collecting ducts, which presented epithelial cell flattening and vacuolization. There was compression of adjacent tissue and discrete fibrotic areas. In the presence of intermediate hosts in the yard, the synanthropic nature of some wild free-living birds could make them a source of trematodes infection and dissemination for pet birds. Conversely, the infected cockatiel could also have served as a reservoir and carrier of trematodes to wild free-living birds.

Parasites in space and time: a novel method to assess and illustrate host-searching behaviour of trematode cercariae.

The transmission from one host to another constitutes a challenging obstacle for parasites and is a key determinant of their fitness. Due to their complex life histories involving several different hosts, the free-living dispersal stages (cercariae) of digenean trematodes show a huge diversity in morphology and behaviour. On a finer scale, we still have an extremely limited understanding of the inter- and intraspecific variation in transmission strategies of many trematode species. Here, we present a novel method to study the movement patterns of cercariae of four New Zealand trematode species (Coitocaecum parvum, Maritrema poulini, Apatemon sp. and Aporocotylid sp. I.) via automated video tracking. This approach allows to quantify parameters otherwise not measurable and clearly illustrates the individual strategies of parasites to search for their respective target hosts. Cercariae that seek out an evasive fish target hosts showed higher swimming speeds (acceleration and velocity) and travelled further distances, compared with species searching for high-density crustacean hosts. Automated video tracking provides a powerful tool for such detailed analyses of parasites' host-searching strategies and can enhance our understanding of complex host-parasite interactions, ranging from parasite community structure to the transmission of potential disease agents.

Digeneans of northern fur seals Callorhinus ursinus (Pinnipedia: Otariidae) from five subpopulations on St. Paul Island, Alaska.

A parasitological survey of 651 northern fur seals Callorhinus ursinus L. from five subpopulations was conducted on St. Paul Island, Alaska, during July-August 2012-2014. Digenean trematodes were found in 210 of 651 fur seals with a total prevalence of 32.3%. Intensity of infection varied from 1 to 1540 parasites with mean intensity 18.4 ± 111.1 SD and median intensity of 2 specimens per host. Significant differences in prevalence and intensity of infection in northern fur seals between separate rookeries was not observed (Mann-Whitney test; p > 0.05). Four species of digeneans belonging to the families Heterophyidae (Apophallus zalophi Price, 1932, Phocitrema fusiforme Goto and Ozaki, 1930, and Galactosomum ubelakeri (Dailey, 1969)) and Troglotrematidae (Nanophyetus salmincola (Chapin, 1926)) were found. Nanophyetus salmincola is reported from C. ursinus for the first time. We obtained partial 28S rDNA sequences for all digenean species and conducted molecular phylogenetic analysis to demonstrate their phylogenetic relationships.