In-feed Praziquantel treatment of grass carp (Ctenopharyngodon idella) infected with eye flukes.

Praziquantel (PZQ) is a drug effective against several parasitic diseases of fish caused by Platyhelminthes. The study assesses the efficacy of selected in-feed PZQ doses in eliminating metacercariae Diplostomum pseudospathaceum in grass carp. The fish were infected with D. pseudospathaceum in laboratory conditions and subsequently treated with PZQ at daily doses of 30 (PZQ1), 50 (PZQ2), and 100 (PZQ3) mg/kg bw for seven days. The treatment trial was followed by a 23-day observation period during which the fish were parasitologically examined on days 0, 2, 8 and 23. Smaller fish were infected with a significantly higher (P < 0.05) number of metacercariae than bigger fish among three weight groups as follows: up to 7 g > 7 to 12 g > 12 g. The in-feed PZQ at doses of 30, 50 and 100 mg/kg bw for seven days was 47%, 84% and 88% effective against metacercariae D. pseudospathaceum, respectively. A significant decrease (P < 0.01) in moving and an increase (P < 0.01) in the immotile metacercariae in the eye's lens were recorded in all the trial groups on each sampling day. An increasing proportion of motile metacercariae in the post-treatment observation period was seen in PZQ2 and PZQ3, indicating a decreasing trend in the actual efficacy in an environment with no reinfection possibility.

Host Developmental Stage Effects on Parasite Resistance and Tolerance.

AbstractHosts can defend themselves against parasites either by preventing or limiting infections (resistance) or by limiting parasite-induced damage (tolerance). However, it remains underexplored how these defense types vary over host development with shifting patterns of resource allocation priorities. Here, we studied the role played by developmental stage in resistance and tolerance in Atlantic salmon (Salmo salar). This anadromous fish has distinct life stages related to living in freshwater and seawater. We experimentally exposed 1-year-old salmon, either at the freshwater stage or at the stage transitioning to the marine phase, to the trematode Diplostomum pseudospathaceum. Using 56 pedigreed families and multivariate animal models, we show that developmental transition is associated with reduced resistance but does not affect tolerance. Furthermore, by comparing tolerance slopes (host fitness against parasite load) based on additive genetic effects among infected and unexposed control relatives, we observed that the slopes can be largely independent of the infection, that is, they may not reflect tolerance. Together, our results suggest that the relative importance of different defense types may vary with host development and emphasize the importance of including control treatments for more confident interpretations of tolerance estimates.

Eye flukes (Diplostomum spp) damage retinal tissue and may cause a regenerative response in wild threespine stickleback fish.

Fish rely upon vision as a dominant sensory system for foraging, predator avoidance, and mate selection. Damage to the visual system, in particular to the neural retina of the eye, has been demonstrated to result in a regenerative response in captive fish that serve as model organisms (e.g. zebrafish), and this response restores some visual function. The purpose of the present study is to determine whether damage to the visual system that occurs in wild populations of fish also results in a regenerative response, offering a potentially ecologically relevant model of retinal regeneration. Adult threespine stickleback were collected from several water bodies of Iceland, and cryosectioned eye tissues were processed for hematoxylin and eosin staining or for indirect immunofluorescence using cell-specific markers. In many of the samples, eye flukes (metacercariae of Diplostomum spp) were present, frequently between the neural retina and retinal pigmented epithelium (RPE). Damage to the retina and to the RPE was evident in eyes containing flukes, and RPE fragments were observed within fluke bodies, suggesting they had consumed this eye tissue. Expression of a cell proliferation marker was also observed in both retina and RPE, consistent with a proliferative response to the damage. Interestingly, some regions of infected retina displayed "laminar fusions," in which neuronal cell bodies were misplaced within the major synaptic layer of the retina. These laminar fusions are also frequently found in regenerated zebrafish retina following non-parasitic (experimental) forms of retinal damage. The stickleback retina may therefore respond to fluke-mediated damage by engaging in retinal regeneration.

Allee effect in a manipulative parasite within poikilothermic host under temperature change.

Temperature and intraspecific competition are important factors influencing the growth of all organisms, including parasites. The temperature increase is suggested to stimulate the development of parasites within poikilothermic hosts. However, at high parasite densities, this effect could be diminished, due to stronger intraspecific competition. Our study, for the first time, addressed the joint effects of warming and parasite abundances on parasite growth in poikilothermic hosts. The growth of the common fish parasite larvae (trematode Diplostomum pseudospathaceum) within the rainbow trout at different infection intensities and temperatures (15°C and 18°C) was experimentally investigated. The results showed that temperature was positively correlated with both parasite infection success and growth rates. The growth rates increased much more compared to those in many free-living poikilothermic animals. Atypically for a majority of parasites, D. pseudospathaceum larvae grow faster when abundant (Allee effect). The possible causes for this phenomenon (manipulation cost sharing, etc.) are discussed in this study. Importantly, limited evidence of the interaction between temperature and population density was found. It is likely that temperature did not change the magnitude of the Allee effect but affected its timing. The impact of these effects is supposed to become more pronounced in freshwater ecosystems under current climate changes.

Fatal Diplostomum phoxini infection in captive Atlantic puffin Fratercula arctica chicks following ingestion of infected European minnows Phoxinus phoxinus.

Conservation of endangered animal species is a major task of zoos. Husbandry and breeding of Atlantic puffins Fratercula arctica in captivity is challenging. In 2019, the entire chick population (n = 4 chicks) in Berne Animal Park's Atlantic puffin colony (Bern, Switzerland) died within 7 d. Due to supply constraints, the chicks had been fed with wild-caught European minnows Phoxinus phoxinus. At necropsy, the main pathological finding in all deceased puffin chicks was a multifocal, moderate to severe subacute heterophilic and granulomatous enteritis with intralesional adult trematodes and eggs. Metacercariae surrounded by few necrotic cells and scattered macrophages were found in the brain and spinal cord of the food fish. Additional microbiological analyses of both the puffin chicks and fish were unremarkable. Diplostomum phoxini DNA could be identified in formalin-fixed paraffin-embedded tissue from the small intestine of all puffin chicks and European minnows following PCR and sequencing of the 18S ribosomal RNA gene and the internal transcribed spacer (ITS1) region. This report illustrates the importance of intensive health checks of food fish for animal species kept in captivity.

Molecular and morphological characterisation of Diplostomum phoxini (Faust, 1918) with a revised classification and an updated nomenclature of the species-level lineages of Diplostomum (Digenea: Diplostomidae) sequenced worldwide.

We characterised morphologically and molecularly Diplostomum phoxini (Faust, 1918) based on cercarial isolates from the snail Ampullaceana balthica (L.) (Gastropoda: Lymnaeidae) and metacercariae from the Eurasian minnow, Phoxinus phoxinus (L.) (Cypriniformes: Leuciscidae), and provided molecular evidence for the identification of the snail intermediate host. Phylogenetic analyses based on the cytochrome c oxidase subunit 1 (cox1) gene depicted 44 molecularly characterised species and genetically distinct lineages of Diplostomum, and resulted in: (i) a re-identification/re-classification of 98 isolates plus D. baeri sampled in North America; (ii) re-definition of the composition of the D. baeri species complex which now includes nine molecularly characterised species/lineages; (iii) re-definition of the composition of the D. mergi species complex which now includes seven molecularly characterised species/lineages; and (iv) an updated nomenclature for the molecularly characterised species-level lineages of Diplostomum.

Intestinal Flukes Recovered from a Herring Gull, Larus argentatus, in the Republic of Korea.

Trematode specimens were collected from the intestine of a herring gull, Larus argentatus, which was found in a critical condition on the shore of a small island (Yubu-do, Seocheon-gun, Chungcheongnam-do) located at the western coast of the Korean peninsula. Total 11 specimens of intestinal flukes, including 3 Cryptocotyle lingua (Heterophyidae), 1 Himasthla alincia (Echinostomatidae), 5 Cardiocephaloides medioconiger (Strigeidae), and 2 Diplostomum spathaceum (Diplostomidae), were recovered. C. lingua was morphologically characterized by the presence of a large ventrogenital apparatus and 2 obliquely tandem testes. H. alincia had an elongated body and a head collar equipped with 31 collar spines. C. medioconiger had a bisegmented body and a voluminous copulatory bursa containing the seminal vesicle and ejaculatory duct. D. spathaceum also had a bisegmented body and its vitellaria extended up to the anterior border of the tribocytic organ. It is of note that C. lingua is potentially zoonotic that can occur in birds and humans. Three of them, i.e., C. lingua, C. medioconiger, and D. spathaceum, are new trematode fauna in Korea. Studies on trematode fauna of migratory birds should be continued in Korea.

The right response at the right time: Exploring helminth immune modulation in sticklebacks by experimental coinfection.

Parasites are one of the strongest selective agents in nature. They select for hosts that evolve counter-adaptive strategies to cope with infection. Helminth parasites are special because they can modulate their hosts' immune responses. This phenomenon is important in epidemiological contexts, where coinfections may be affected. How different types of hosts and helminths interact with each other is insufficiently investigated. We used the three-spined stickleback (Gasterosteus aculeatus) - Schistocephalus solidus model to study mechanisms and temporal components of helminth immune modulation. Sticklebacks from two contrasting populations with either high resistance (HR) or low resistance (LR) against S. solidus, were individually exposed to S. solidus strains with characteristically high growth (HG) or low growth (LG) in G. aculeatus. We determined the susceptibility to another parasite, the eye fluke Diplostomum pseudospathaceum, and the expression of 23 key immune genes at three time points after S. solidus infection. D. pseudospathaceum infection rates and the gene expression responses depended on host and S. solidus type and changed over time. Whereas the effect of S. solidus type was not significant after three weeks, T regulatory responses and complement components were upregulated at later time points if hosts were infected with HG S. solidus. HR hosts showed a well orchestrated immune response, which was absent in LR hosts. Our results emphasize the role of regulatory T cells and the timing of specific immune responses during helminth infections. This study elucidates the importance to consider different coevolutionary trajectories and ecologies when studying host-parasite interactions.

Examining the role of parasites in limiting unidirectional gene flow between lake and river sticklebacks.

Parasites are important selective agents with the potential to limit gene flow between host populations by shaping local host immunocompetence. We report on a contact zone between lake and river three-spined sticklebacks (Gasterosteus aculeatus) that offers the ideal biogeographic setting to explore the role of parasite-mediated selection on reproductive isolation. A waterfall acts as a natural barrier and enforces unidirectional migration from the upstream river stickleback population to the downstream river and lake populations. We assessed population genetic structure and parasite communities over four years. In a set of controlled experimental infections, we compared parasite susceptibility of upstream and downstream fish by exposing laboratory-bred upstream river and lake fish, as well as hybrids, to two common lake parasite species: a generalist trematode parasite, Diplostomum pseudospathaceum, and a host-specific cestode, Schistocephalus solidus. We found consistent genetic differentiation between upstream and downstream populations across four sampling years, even though the downstream river consisted of ~10% first-generation migrants from the upstream population as detected by parentage analysis. Fish in the upstream population had lower genetic diversity and were strikingly devoid of macroparasites. Through experimental infections, we demonstrated that upstream fish and their hybrids had higher susceptibility to parasite infections than downstream fish. Despite this, naturally sampled upstream migrants were less infected than downstream residents. Thus, migrants coming from a parasite-free environment may enjoy an initial fitness advantage, but their descendants seem likely to suffer from higher parasite loads. Our results suggest that adaptation to distinct parasite communities can influence stickleback invasion success and may represent a barrier to gene flow, even between close and connected populations.

Trematode cercariae as prey for zooplankton: effect on fitness traits of predators.

Removal of parasite free-living stages by predators has previously been suggested an important factor controlling parasite transmission in aquatic habitats. Experimental studies of zooplankton predation on macroparasite larvae are, however, scarce. We tested whether trematode cercariae, which are often numerous in shallow waters, are suitable prey for syntopic zooplankters. Feeding rates and survival of freshwater cyclopoids (Megacyclops viridis, Macrocyclops distinctus), calanoids (Arctodiaptomus paulseni), cladocerans (Sida crystallina) and rotifers Asplanchna spp., fed with cercariae of Diplostomum pseudospathaceum, a common fish trematode, were studied. In additional long-term experiments, we studied reproduction of cyclopoids fed with cercariae. All tested zooplankton species consumed cercariae. The highest feeding rates were observed for cyclopoids (33 ± 12 cercariae ind-1 h-1), which actively reproduced (up to one egg clutch day-1) when fed ad libitum with cercariae. Their reproductive characteristics did not change significantly with time, indicating that cercariae supported cyclopoids' dietary needs. Mortality of rotifers and cladocerans was high (25-28% individuals) when exposed to cercariae in contrast to cyclopoids and calanoids (<2%). Cercariae clogged the filtration apparatus of cladocerans and caused internal injuries in predatory rotifers, which ingested cercariae. Observed trophic links between common freshwater zooplankters and cercariae may significantly influence food webs and parasite transmission in lentic ecosystems.

Efficacy and toxicity of praziquantel in helminth-infected barbel (Barbus barbus L.).

This study evaluated efficacy and toxicity of the pyrazinoisoquinoline anthelmintic praziquantel (PZQ) in barbel infected with metacercariae of Diplostomum spathaceum and adult Pomphorhynchus laevis, and assessed antioxidant biomarkers and the lipid peroxidation response in juvenile barbel post-treatment. The estimated 96-hr LC50 of PZQ was 28.6 mg/L. For evaluation of efficacy, barbel naturally infected with D. spathaceum were exposed to a 10 and 20 mg/L PZQ 4-day bath treatment. Both concentrations were 100% effective against D. spathaceum and significantly (p < .01) affected the activity of catalase, superoxide dismutase, glutathione reductase and glutathione-S-transferase as well as levels of reduced glutathione in liver and muscle. The efficacy of orally administered PZQ was assessed in adult barbel naturally infected with P. laevis. Fish were administered 10, 30 and 50 mg/kg of body weight and examined via gut dissection after 6 days. The 50 mg/kg dose significantly decreased the intensity of infection. Praziquantel is a feasible bath treatment for barbel infected with D. spathaceum and has potential for oral treatment of broodfish infected with P. laevis.

Host infection history modifies co-infection success of multiple parasite genotypes.

Co-infections by multiple parasite genotypes are common and have important implications for host-parasite ecology and evolution through within-host interactions. Typically, these infections take place sequentially, and therefore, the outcome of co-infection may be shaped by host immune responses triggered by previous infections. For example, in vertebrates, specific immune responses play a central role in protection against disease over the course of life, but co-infection research has mostly focused on previously uninfected individuals. Here, we investigated whether sequential exposure and activation of host resistance in rainbow trout Oncorhynchus mykiss affects infection success and interactions between co-infecting parasite genotypes of the trematode eye-fluke Diplostomum pseudospathaceum. In accordance with earlier results, we show that a simultaneous attack of two parasite genotypes facilitates parasite establishment in previously uninfected hosts. However, we find for the first time that this facilitation in co-infection is lost in hosts with prior infection. We conclude that vertebrate host infection history can affect the direction of within-host-parasite interactions. Our results may have significant implications for the evolution of co-infections and parasite transmission strategies.

Experimental parasite community ecology: intraspecific variation in a large tapeworm affects community assembly.

Non-random species associations occur in naturally sampled parasite communities. The processes resulting in predictable community structure (e.g. particular host behaviours, cross-immunity, interspecific competition) could be affected by traits that vary within a parasite species, like growth or antigenicity. We experimentally infected three-spined sticklebacks with a large tapeworm (Schistocephalus solidus) that impacts the energy needs, foraging behaviour and immune reactions of its host. The tapeworms came from two populations, characterized by high or low growth in sticklebacks. Our goal was to evaluate how this parasite, and variation in its growth, affects the acquisition of other parasites. Fish infected with S. solidus were placed into cages in a lake to expose them to the natural parasite community. We also performed a laboratory experiment in which infected fish were exposed to a fixed dose of a common trematode parasite. In the field experiment, infection with S. solidus affected the abundance of four parasite species, relative to controls. For two of the four species, changes occurred only in fish harbouring the high-growth S. solidus; one species increased in abundance and the other decreased. These changes did not appear to be directly linked to S. solidus growth though. The parasite exhibiting elevated abundance was the same trematode used in the laboratory infection. In that experiment, we found a similar infection pattern, suggesting that S. solidus affects the physiological susceptibility of fish to this trematode. Associations between S. solidus and other parasites occur and vary in direction. However, some of these associations were contingent on the S. solidus population, suggesting that intraspecific variability can affect the assembly of parasite communities.

A new R package and web application for detecting bilateral asymmetry in parasitic infections.

When parasites invade paired structures of their host non-randomly, the resulting asymmetry may have both pathological and ecological significance. To facilitate the detection and visualisation of asymmetric infections we have developed a free software tool, Analysis of Symmetry of Parasitic Infections (ASPI). This tool has been implemented as an R package (https://cran.r-project.org/package=aspi) and a web application (https://wayland.shinyapps.io/aspi). ASPI can detect both consistent bias towards one side, and inconsistent bias in which the left side is favoured in some hosts and the right in others. Application of ASPI is demonstrated using previously unpublished data on the distribution of metacercariae of species of Diplostomum von Nordmann, 1832 in the eyes of ruffe Gymnocephalus cernua (Linnaeus). Invasion of the lenses appeared to be random, with the proportion of metacercariae in the left and right lenses showing the pattern expected by chance. However, analysis of counts of metacercariae from the humors, choroid and retina revealed asymmetry between eyes in 38% of host fish.

Interactions among bacterial strains and fluke genotypes shape virulence of co-infection.

Most studies of virulence of infection focus on pairwise host-parasite interactions. However, hosts are almost universally co-infected by several parasite strains and/or genotypes of the same or different species. While theory predicts that co-infection favours more virulent parasite genotypes through intensified competition for host resources, knowledge of the effects of genotype by genotype (G × G) interactions between unrelated parasite species on virulence of co-infection is limited. Here, we tested such a relationship by challenging rainbow trout with replicated bacterial strains and fluke genotypes both singly and in all possible pairwise combinations. We found that virulence (host mortality) was higher in co-infections compared with single infections. Importantly, we also found that the overall virulence was dependent on the genetic identity of the co-infecting partners so that the outcome of co-infection could not be predicted from the respective virulence of single infections. Our results imply that G × G interactions among co-infecting parasites may significantly affect host health, add to variance in parasite fitness and thus influence evolutionary dynamics and ecology of disease in unexpected ways.

Trematode Metacercariae in Freshwater Fish from Water Systems of Hantangang and Imjingang in Republic of Korea.

The infection status of freshwater fish with digenetic trematode metacercariae was examined in water systems of Hantangang and Imjingang (River), the Republic of Korea. A total of 877 (594 from Hantangang and 283 from Imjingang) fishes were examined by the artificial digestion methods. Clonorchis sinensis metacercariae were detected in 7 (1.2%) fishes (in 3 spp.) from Hantangang in Cheorwon-gun, Gangwon-do, and 40 (14.1%) fishes (in 7 spp.) from Munsancheon in Paju-si, Gyeonggi-do. The average densities were 1.9 and 35.6 per fish infected, respectively. Metagonimus spp. metacercariae were detected in 312 (52.5%) and 113 (39.9%) fishes from Hantangang and Imjingang, and their average densities were 47.5 and 9.6 per fish infected, respectively. Centrocestus armatus metacercariae were found in 161 (27.1%) and 70 (24.7%) fishes from Hantangang and Imjingang, and their average number per fish infected was 694 and 82, respectively. Echinostoma spp. metacercariae were detected in 50 (8.4%) and 94 (33.2%) fishes from Hantangang and Imjingang, and their average densities were 9.6 and 23.1 per fish infected, respectively. The infection status of fishes with metacercariae of Stephanoprora spp., Diplostomum spp., Clinostomum complanatum, Metorchis orientalis, and Metorchis taiwanensis were analyzed by surveyed regions. Conclusively, it was confirmed that C. sinensis metacercariae were quite commonly detected in fishes from Munsancheon but rarely from other localities, whereas the metacercariae of other digenetic trematodes were relatively prevalent in fishes from water systems of Hantangang and Imjingang in Korea.

Experimental evidence for parasite-induced over-winter mortality in juvenile Rhodeus amarus.

In this study, the effects of the eye fluke Diplostomum pseudospathaceum (Trematoda) infection on over-winter survival of young-of-the-year (YOY) European bitterling Rhodeus amarus (Cyprinidae) were examined between September 2010 and April 2011. The fish were reared in semi-natural conditions to ensure that results were not confounded by other parasite infections. The cumulative mortality of R. amarus from November until April was significantly higher in D. pseudospathaceum-infected fish (57·3%) compared to controls (42·1%). Infection of the parental generation did not have any effect on the mortality of juveniles. The results indicate that D. pseudospathaceum infection increases over-winter mortality of YOY R. amarus. The possible mechanisms causing mortality are discussed.

Is vision deterioration responsible for changes in the host's behavior caused by eye flukes?

Trematodes localizing in the lenses of fish change the behavior of their hosts. These behavioral changes are widely suggested to be parasitic manipulations of host behavior aimed at increasing the possibility of eye flukes completing their life cycle. It is often assumed that fish change their behavior due to the vision deterioration caused by trematode larvae. We checked this assumption by testing Salvelinus malma infected with eye flukes (Diplostomum pseudospathaceum) under different lighting conditions. We suggested that if the parasite alters the host's behavior through vision impairment, then in the dark (when fish do not rely on vision to navigate), the difference in the behavior of infected and uninfected fish would disappear. Eye flukes, indeed, changed fish behavior, making their hosts less vigilant. We believe this is the first evidence of possible parasitic manipulation in this study system. However, contrary to expectations, the difference in the behavior of infected and control fish was independent of the lighting conditions. Our results suggest that mechanisms of behavioral change other than vision impairment should be taken into account in this fish-eye fluke study system.

Molecular phylogeny of Diplostomum, Tylodelphys, Austrodiplostomum and Paralaria (Digenea: Diplostomidae) necessitates systematic changes and reveals a history of evolutionary host switching events.

The Diplostomidae Poirier, 1886 is a large, globally distributed family of digeneans parasitic in intestines of their definitive hosts. Diplostomum and Tylodelphys spp. are broadly distributed, commonly reported, and the most often sequenced diplostomid genera. The majority of published DNA sequences from these genera originated from larval stages only, which typically cannot be identified to the species level based on morphology alone. We generated partial large ribosomal subunit (28S) rRNA and cytochrome c oxidase subunit 1 (cox1) mtDNA gene sequences from 14 species/species-level lineages of Diplostomum, six species/species-level lineages of Tylodelphys, two species/species-level lineages of Austrodiplostomum, one species previously assigned to Paralaria, two species/species-level lineages of Dolichorchis and one unknown diplostomid. Our DNA sequences of 11 species/species-level lineages of Diplostomum (all identified to species), four species/species-level lineages of Tylodelphys (all identified to species), Austrodiplostomum compactum, Paralaria alarioides and Dolichorchis lacombeensis originated from adult specimens. 28S sequences were used for phylogenetic inference to demonstrate the position of Paralaria alarioides and Dolichorchis spp. within the Diplostomoidea and study the interrelationships of Diplostomum, Tylodelphys and Austrodiplostomum. Our results demonstrate that two diplostomids from the North American river otter (P. alarioides and a likely undescribed taxon) belong within Diplostomum. Further, our results demonstrate the non-monophyly of Tylodelphys due to the position of Austrodiplostomum spp., based on our phylogenetic analyses and morphology. Furthermore, the results of phylogenetic analysis of 28S confirmed the status of Dolichorchis as a separate genus. The phylogenies suggest multiple definitive host-switching events (birds to otters and among major avian groups) and a New World origin of Diplostomum and Tylodelphys spp. Our DNA sequences from adult digeneans revealed identities of 10 previously published lineages of Diplostomum and Tylodelphys, which were previously identified to genus only. The novel DNA data from this work provide opportunities for future comparisons of larval diplostomines collected in ecological studies.

RECRUITMENT AND SEASONAL OCCURRENCE OF PARASITES IN JUVENILE INVASIVE ROUND GOBIES (NEOGOBIUS MELANOSTOMUS) IN THE ST. LAWRENCE RIVER, QUEBEC, CANADA.

The round goby (Neogobius melanostomus) is an invasive species that has become one of the most abundant fish in the St. Lawrence River, Quebec, Canada over the past 15 yr. Since its introduction, the round goby has acquired a number of native parasites, yet little is known about the dynamics of parasite recruitment. To examine this question, young-of-the-year and juvenile round gobies were collected monthly from 2 localities in the river (Îles de la Paix, Île Dorval) from June through November 2012. At Îles de la Paix, round gobies (n = 180) were infected with 3 species of parasites, all larval stages (Diplostomum spp., Tylodelphys scheuringi, Neoechinorhynchus tenellus). Prevalence of the digenean Diplostomum spp. varied from 3.3 to 13.3%, and mean abundance from 0.03 to 0.53 from June through September, with a maximum in August. The digenean T. scheuringi was seen only in August, at a prevalence of 10.0% and a mean abundance of 0.53. The acanthocephalan N. tenellus was observed in June, August, and September, prevalence ranging from 3.3 to 10.0% and mean abundance from 0.03 to 0.27. Maximum infection for all 3 species occurred in August. All infected fish were ≥44 mm in total length (TL). Fish infected with more than 1 parasite species were >60 mm TL. No round goby (n = 178) was infected at Île Dorval. This study demonstrated that the invasive round goby starts to acquire parasite infections in the St. Lawrence River in the first year of life and may contribute to the transmission of some parasites within this ecosystem.