Histopathological changes in the swimbladder wall of the European eel Anguilla anguilla due to infections with Anguillicola crassus

Teleost innate immunity, an intricate game between immune cells and parasites of fish organs: who wins, who loses

Fish, comprising over 27,000 species, represent the oldest vertebrate group and possess both innate and adaptive immune systems. The susceptibility of most wild fish to parasitic infections and related diseases is well-established. Among all vertebrates, the digestive tract creates a remarkably favorable and nutrient-rich environment, which, in turn, renders it susceptible to microparasites and macroparasites. Consequently, metazoan parasites emerge as important disease agents, impacting both wild and farmed fish and resulting in substantial economic losses. Given their status as pathogenic organisms, these parasites warrant considerable attention. Helminths, a general term encompassing worms, constitute one of the most important groups of metazoan parasites in fish. This group includes various species of platyhelminthes (digeneans, cestodes), nematodes, and acanthocephalans. In addition, myxozoans, microscopic metazoan endoparasites, are found in water-dwelling invertebrates and vertebrate hosts. It is worth noting that several innate immune cells within the fish alimentary canal and certain visceral organs (e.g., liver, spleen, and gonads) play active roles in the immune response against parasites. These immune cells include macrophages, neutrophils, rodlet cells, and mast cells also known as eosinophilic granular cells. At the site of intestinal infection, helminths often impact mucous cells number and alter mucus composition. This paper presents an overview of the state of the art on the occurrence and characteristics of innate immune cells in the digestive tract and other visceral organs in different fish-parasite systems. The data, coming especially from studies employed immunohistochemical, histopathological, and ultrastructural analyses, provide evidence supporting the involvement of teleost innate immune cells in modulating inflammatory responses to metazoan and protozoan parasitic infections.

Effects of the invasive swim bladder parasite Anguillicola crassus on health and condition indicators in the European eel

Parasites negatively affect biological processes within their hosts, which may alter for example health, growth, and reproductive ability. Non-native invasive parasites, in particular, may have large effects on the endemic hosts, given that the hosts lack evolved specific defences against such parasites. The swim bladder nematode Anguillicola crassus, an invasive parasite originating from Asia, is found in the European eel (Anguilla anguilla, L. 1758), since the 1980s. We investigated whether A. crassus affected several indicators related to health of the European eel (spleen- and liver size, body fat content and relative condition). Our results indicate that during the continental residency of the eels, infection by A. crassus had no major negative impacts on the investigated health indicators at the generally low infection intensities present in this study (median 2-3 visible parasites). Given that many of the adult eels were found to have swim bladder damage, concerns about their spawning migration through deeper oceanic environments can still be raised. To allow further investigations, we suggest that quantification of swim bladder damage should be implemented in eel-monitoring programs. Compared to other parasite pressure parameters, swim bladder damage provides additional information about past infections and future problems.

A qPCR-based method to detect the eel parasitic nematode Anguillicola crassus in intermediate and final hosts

Being able to systematically detect parasitic infection, even when no visual signs of infection are present, is crucial to the establishment of accurate conservation policies. The nematode Anguillicola crassus infects the swimbladder of anguillid species and is a potential threat for eel populations. In North America, naïve hosts such as the American eel Anguilla rostrata are affected by this infection. The accidental introduction of A. crassus following restocking programs may contribute to the actual decline of the American eel in Canada. We present a quantitative real time PCR-based method to detect A. crassus infection in final and intermediate hosts. We tested two protocols on samples from different geographical origins in Canada: 1) a general detection of A. crassus DNA in pools of young final hosts (glass eels) or crustacean intermediate hosts 2) a detection at the individual scale by analyzing swim bladders from elvers, or from adult yellow and silver eels. The DNA of A. crassus was detected in one pool of zooplankton (intermediate host) from the Richelieu River (Montérégie-Québec), as well as in individual swim bladders of 13 elvers from Grande and Petite Trinité rivers (Côte-Nord-Québec). We suggest that our qPCR approach could be used in a quantitative way to estimate the parasitic burden in individual swim bladders of elvers. Our method, which goes beyond most of previous developed protocols that restricted the diagnosis of A. crassus to the moment when it was fully established in its final host, should help to detect early A. crassus infection in nature.

Monitoring for Anguillicoloides crassus, Anguillid herpesvirus 1, aquabirnavirus EVE and rhabdovirus EVEX in the European eel population of southern Spain

European eel is critically endangered in Europe. Among other stressors, pathogens are well-known to harm eels' fitness. One hundred and eighty-two eels were captured in three Eel Management Units in Andalucía (SE Spain) and analysed for Anguillicoloides crassus, Anguillid herpesvirus 1 (AngHV1), the rhabdovirus Eel Virus European X (EVEX) and the aquabirnavirus Eel Virus European (EVE). A. crassus adults and preadults were isolated and morphometrically identified, and the eel swimbladders were artificially digested to count A. crassus larvae. Also, eel tissues were examined by PCRs for the presence of viruses. EVEX and EVE were not detected in any of the eels. The estimated prevalence (95% confidence limits) was 71 (64-78)% for A. crassus and 35 (28-42)% for AngHV-1, varying these prevalences significantly between and within EMUs. Moreover, A. crassus prevalence was highest in smaller eels, in sites closest to the sea and eels sampled in the autumn. By contrast, AngHV-1 prevalence was highest in biggest eels, in sites far from the sea and sampled in the summer or winter. However, in mixed effects logistic models including site as a random variable, the risk of infection was associated with distance to the sea in both A. crassus and AngHV-1 infections and also to winter sampling in the case of AngHV-1 and not to other variables. These results are evidence that both pathogens are highly endemic in eels from Andalusian habitats. Further studies are needed to better understand the risk factors associated with these pathogens on eel populations.

Parasites and the neuroendocrine control of fish intestinal function: an ancient struggle between pathogens and host

Most individual fish in wild and farmed populations can be infected with parasites. Fish intestines can harbour protozoans, myxozoans and helminths, which include several species of digeneans, cestodes, nematodes and acanthocephalans. Enteric parasites often induce inflammation of the intestine; the pathogen provokes changes in the host physiology, which will be genetically selected for if they benefit the parasite. The host response to intestinal parasites involves neural, endocrine and immune systems and interaction among these systems is coordinated by hormones, chemokines, cytokines and neurotransmitters including peptides. Intestinal fish parasites have effects on the components of the enteric nervous and endocrine systems; mechanical/chemical changes impair the activity of these systems, including gut motility and digestion. Investigations on the role of the neuroendocrine system in response to fish intestinal parasites are very few. This paper provides immunohistochemical and ultrastructural data on effects of parasites on the enteric nervous system and the enteric endocrine system in several fish–parasite systems. Emphasis is on the occurrence of 21 molecules including cholecystokinin-8, neuropeptide Y, enkephalins, galanin, vasoactive intestinal peptide and serotonin in infected tissues.

A complete check-up of European eel after eight years of restocking in an upland river: Trends in growth, lipid content, sex ratio and health status

By combining field research and careful laboratory analysis of samples over the course of an eight-year study, we met the challenge of assessing the life history traits and health status of eels restocked in freshwater ecosystems. We found that restocked eels exhibited good growth performance; moreover, the stocks were female-dominated, showed a good Fulton's condition factor (K) and lipid stores and had high survival probability estimated using the best model of Jolly-Seber stock assessment method for open populations. A necropsy revealed the absence of internal lesions. A swim bladder examination revealed the absence of the parasite Anguillicola crassus. Polymerase chain reaction (PCR) analyses revealed an increase of Anguillid herpesvirus-1 (AngHV-1) prevalence throughout the study. Most positive subjects expressed viral loads compatible with a latent infection and correlated positively with K. All restocked eels were contaminated by at least one of the organic pollutant congeners studied, but the pollution loads corresponded to the lowest range of pollutant concentrations reported in the available literature for European eels and did not exceed the maximum residue and contaminant limits in food and feed of several national and international regulations. Pollutant loads were negatively correlated with K, lipid content and eel density for polychlorinated biphenyls PCB 138, 153 and 180 and K for pesticides p.p'-DDE, p.p'-DDD, p.p'-DDT and PBDE47. This study highlights the potential role played by upland aquatic ecosystems in enhancing riverine silver eel production from the perspective of species conservation. To be successful, restocking must be accompanied by improved ecosystem quality and migration routes for eels in inland freshwaters. We also provide some recommendations for future research to improve the management of restocking programmes.

Anguillicola (Anguillicoides) crassus: Morphometric characteristics and pathogenicity in eels (Anguilla anguilla) in Greece

Anguillicola (Anguillicoides) crassus is a swimbladder nematode parasite of Anguilla anguilla (Linnaeus, 1758). The present study investigates the morphology and pathogenicity of A. crassus in European eel, as well as, the effects of different aquatic environments on the infection of A. crassus, in Greece. A total of 268 nematodes were collected from four estuarine systems in Greece. In all collected parasites, measurements carried out to define the morphological characteristics of parasites: parasite length and width, oesophagus max length and max width. Τhe mean parasite length was 23.50 mm (95% CI: 22.42-24.58) for females, 12.95 mm (95% CI: 12.25-13.64) for the males and 6.39 mm (95% CI: 5.27-7.50) for the juveniles. The mean parasite width was 1.99 mm (95% CI: 1.88-2.10) for females, 0.93 mm (95% CI: 0.86-1.00) for males and 0.51 mm (95% CI: 0.39-0.64) for juveniles. In total, the mean intensity was found 3.15 (95% CI: 2.53-3.78), while the highest mean intensity per eel was 8.00 (95% CI: 0.00-29.51) in Tholi Lagoon and the lowest was 2.20 (95% CI: 0.36-4.04) in Amvrakikos Gulf. Longitudinal and transverse histological sections of two adults A. crassus and two swimbladders were carried out. Multiple granules were observed, as a tissue reaction of the organism to swath around the 4th stage larvae (L4) that have entered the swimbladder. Molecular analysis was performed on three female adults A. crassus derived from eel specimens coming from the Vistonis estuarine system. The high reproductive capacity of the parasite reveals that A. anguilla has low effective defense mechanisms against the parasite. Also, the morphometric variations of the A. crassus seems to have a plastic feature which is being differently expressed when exposed to various environments.

The impact of Anguillicoloides crassus (Nematoda) on European eel swimbladder: histopathology and relationship between neuroendocrine and immune cells

The swimbladder functions as a hydrostatic organ in most bony fishes, including the European eel, Anguilla anguilla. Infection by the nematode Anguillicoloides crassus impairs swimbladder function, significantly compromising the success of the eel spawning migration. Swimbladders from 32 yellow eels taken from Lake Trasimeno (Central Italy) were analysed by histopathology- and electron microscopy-based techniques. Sixteen eels (50%) harboured A. crassus in their swimbladders and intensity of infection ranged from 2 to 17 adult nematodes per organ (6.9 ± 1.6, mean ± s.e.). Gross observations of heavily infected swimbladders showed opacity and histological analysis found a papillose aspect to the mucosa and hyperplasia of the lamina propria, muscularis mucosae and submucosa. Inflammation, haemorrhages, dilation of blood vessels and epithelial erosion were common in infected swimbladders. In the epithelium of parasitized swimbladders, many empty spaces and lack of apical junctional complexes were frequent among the gas gland cells. In heavily infected swimbladders, we observed hyperplasia, cellular swelling and abundant vacuolization in the apical portion of the gas gland cells. Numerous mast cells and several macrophage aggregates were noticed in the mucosal layer of infected swimbladders. We found more nervous and endocrine elements immunoreactive to a panel of six rabbit polyclonal antibodies in infected swimbladders compared to uninfected.

Mutual adaptations between hosts and parasites determine stress levels in eels

Invasive parasites may severely affect their new hosts. Two invasive parasites occurring in the European eel (Anguilla anguilla) are the Asian swim bladder nematode Anguillicola crassus and the Ponto-caspian acanthocephalan Pomphorhynchus sp., which were introduced to the river Rhine in the early 1980/90s. The Japanese eel (Anguilla japonica), as the native host of A. crassus, developed mutual adaptations to the swim bladder parasite, which are lacking in the European eel. Therefore, after its spread to Europe, infestations of European eels with A. crassus were found to be more severe and caused massive swim bladder wall damages mainly due to the feeding activity of the adult nematodes. A suppression of the immune system also appears to be likely, which allows secondary infections e.g. by bacteria or other parasites in European eels. Acanthocephalans of the genus Pomphorhynchus have not been described so far in Japanese eels, in contrast to European eels, which regularly show infestations with Pomphorhynchus sp. By using these differentially adapted host-parasite associations for experimental studies, host stress responses were examined in the present study in relation to the degree of mutual adaptations between eel hosts and parasites.

Under laboratory conditions, Japanese and European eels were each inoculated with A. crassus and Pomphorhynchus sp., respectively, to investigate their stress responses against differently adapted parasites. The stress response was determined by analyzing plasma levels of cortisol, which is the main corticosteroid hormone during stress response of fish. The results show a strong cortisol release in European eels after infestation with A. crassus whereas Japanese eels only react against Pomphorhynchus sp. infestations. These results are consistent with the initial hypothesis that a low degree of host-parasite adaptations lead to stronger host stress responses against the parasite.

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We measured cortisol as the main corticosteroid hormone in fish.

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Japanese and European eels were inoculated with A. crassus and Pomphorhynchus sp.

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Lower mutual adaptations lead to a higher stress response in host-parasite-systems.

Swimming under elevated hydrostatic pressure increases glycolytic activity in gas gland cells of the European eel

In spite of many decades of research, the spawning migration of the European eel Anguilla anguilla from the European coast to the Sargasso Sea remains a mystery. In particular, the role of the swimbladder as a buoyancy regulating structure is not yet understood. In this study, we exercised silver eels in a swim tunnel under elevated hydrostatic pressure. The transcriptome of gas gland tissue of these exercised eels was then compared to the known transcriptome of not exercised (control) silver eel gas gland cells. Due to the high infection rate of the eel population with the swimbladder parasite Anguillicola crassus, the comparison also included an exercised group of silver eels with a heavily damaged swimbladder, and we compared the previously published transcriptome of not exercised silver eels with a highly damaged swimbladder with the exercised group of silver eels with a heavily damaged swimbladder. The comparisons of unexercised (control) silver eels with exercised silver eels with functional swimbladder (EF), as well as with exercised silver eels with damaged swimbladder (ED), both showed a significant elevation in transcripts related to glycolytic enzymes. This could also be observed within the comparison of unexercised silver eels with a highly infected swimbladder with exercised eels with a damaged swimbladder (DED). In contrast to EF, in ED a significant elevation in transcript numbers of mitochondrial NADH dehydrogenase was observed. While in EF the transcriptional changes suggested that acid production and secretion was enhanced, in ED these changes appeared to be related to thickened tissue and thus elevated diffusion distances. The remarkable number of differentially expressed transcripts coding for proteins connected to cAMP-dependent signaling pathways indicated that metabolic control in gas gland cells includes cAMP-dependent pathways. In contrast to ED, in EF significant transcriptional changes could be related to the reconstruction of the extracellular matrix, while in ED tissue repair and inflammation was more pronounced. Surprisingly, in exercised eels hypoxia inducible transcription factor expression was elevated. In EF, a large number of genes related to the circadian clock were transcriptionally modified, which may be connected to the circadian vertical migrations observed during the spawning migration.

A mechanical approach to understanding the impact of the nematode Anguillicoloides crassus on the European eel swimbladder

One of the most detrimental factors in the drastic decline of the critically endangered European eel (Anguilla anguilla) was the inadvertent introduction of the invasive nematode Anguillicoloides crassus Infection primarily affects the swimbladder, a gas-filled organ that enables the eel to control its depth in the water. A reduction in swimbladder function may be fatal for eel undergoing their spawning migration to the Sargasso Sea, a journey of over 5000 km. Although the physiological damage caused by this invasive parasite is well studied through the use of quantifiable gross pathological indices, providing a good measure of the swimbladder health status, they cannot separate the role of mechanical and morphological damage. Our study examined the appropriateness of three commonly used indices as a measure of mechanical damage by performing uniaxial tensile tests on swimbladder specimens obtained from an infected eel population. When the test results were compared with the gross pathological indices it was found that thickness correlated most strongly with mechanical damage, both confirming and, more importantly, explaining the counterintuitive findings of earlier work. In a damaged swimbladder, the immune response leads to a trade-off; increasing wall thickness raises the pressure required for organ rupture but decreases strength. The results indicate that for moderate infection the mechanical integrity of the swimbladder can be maintained. For severe infection, however, a reduction in mechanical integrity may reach a tipping point, thereby affecting the successful completion of their oceanic migration.

Gene expression response to a nematode parasite in novel and native eel hosts

Invasive parasites are involved in population declines of new host species worldwide. The high susceptibilities observed in many novel hosts have been attributed to the lack of protective immunity to the parasites which native hosts acquired during their shared evolution. We experimentally infected Japanese eels (Anguilla japonica) and European eels (Anguilla anguilla) with Anguillicola crassus, a nematode parasite that is native to the Japanese eel and invasive in the European eel. We inferred gene expression changes in head kidney tissue from both species, using RNA-seq data to determine the responses at two time points during the early stages of infection (3 and 23 days postinfection). At both time points, the novel host modified the expression of a larger and functionally more diverse set of genes than the native host. Strikingly, the native host regulated immune gene expression only at the earlier time point and to a small extent while the novel host regulated these genes at both time points. A low number of differentially expressed immune genes, especially in the native host, suggest that a systemic immune response was of minor importance during the early stages of infection. Transcript abundance of genes involved in cell respiration was reduced in the novel host which may affect its ability to cope with harsh conditions and energetically demanding activities. The observed gene expression changes in response to a novel parasite that we observed in a fish follow a general pattern observed in amphibians and mammals, and suggest that the disruption of physiological processes, rather than the absence of an immediate immune response, is responsible for the higher susceptibility of the novel host.

Transcript levels of members of the SLC2 and SLC5 families of glucose transport proteins in eel swimbladder tissue: the influence of silvering and the influence of a nematode infection

The rate of glucose metabolism has been shown to be correlated to glucose uptake in swimbladder gas gland cells. Therefore, it is assumed that in the European eel silvering, i.e., the preparation of the eel for the spawning migration to the Sargasso Sea, coincides with an enhanced capacity for glucose uptake. To test this hypothesis expression of all known glucose transport proteins has been assessed at the transcript level in yellow and in silver eels, and we also included Anguillicola crassus infected swimbladders. Glucose uptake by rete mirabile endothelial cells could be crucial for the countercurrent exchange capacity of the rete. Therefore, this tissue was also included in our analysis. The results revealed expression of ten different members of the slc2 family of glucose transporters, of four slc5 family members, and of kiaa1919 in gas gland tissue. Glucose transporters of the slc2 family were expressed at very high level, and slc2a1b made up about 80% of all slc2 family members, irrespective of the developmental state or the infection status of the eel. Overall, the slc5 family contributed to only about 8% of all detected glucose transport transcripts in gas gland tissue, and the slc2 family to more than 85%. In rete capillaries, the contribution of sodium-dependent glucose transporters was significantly higher, leaving only 66% for the slc2 family of glucose transporters. Neither silvering nor the infection status had a significant effect on the expression of glucose transporters in swimbladder gas gland tissue, suggesting that glucose metabolism of eel gas gland cells may not be related to transcriptional changes of glucose transport proteins.

Anthropogenic spreading of anguillid herpesvirus 1 by stocking of infected farmed European eels, Anguilla anguilla (L.), in the Schlei fjord in northern Germany

The Schlei fjord in northern Germany is the recipient water of a comprehensive eel, Anguilla anguilla (L.), stocking programme. Since 2015, stocked eels become alizarin red S marked, but to date no control mechanism is implemented in this stock enhancement measure to prevent anthropogenic spreading of diseases. Consequentially, it was possible that farmed stocking cohorts of 2015 and 2016 (in total ca. 1040 kg) were subsequently tested positive for anguillid herpesvirus 1 (AngHV 1). For this study, 100 eels [total length (TL) 24.3-72.9 cm, age ca. 1-6 years] were caught in 2016 and investigated with regard to AngHV 1 infection, parasite load (Anguillicoloides crassus) and body conditions. 68% of the eels were found to be virus positive while larger specimens were more often infected. In addition, a fitted generalized linear model (area under the curve = 0.741) demonstrated that an increase in individual TL is accompanied with an increased risk of clinically relevant virus loads. Anguillicoloides crassus turned out to be an important stressor for eels, because parasite and virus load revealed a significant positive correlation. The results of this study evidently show the urgent need of a disease containment strategy for eel stocking programmes.

Anguillicola crassus infection affects mRNA expression levels in gas gland tissue of European yellow and silver eel

Using Illumina sequencing, we investigated transcriptional changes caused by the nematode Anguillicola crassus within yellow and silver eels by comparing swimbladder samples of uninfected yellow with infected yellow eels, and uninfected silver with infected silver eels, respectively. In yellow eel gas gland, the infection caused a modification of steady state mRNA levels of 1675 genes, most of them being upregulated. Functional annotation analysis based on GO terms was used to categorize identified genes with regard to swimbladder metabolism or response to the infection. In yellow eels, the most prominent category was 'immune response', including various inflammatory components, complement proteins, and immunoglobulins. The elevated expression of several glucose and monocarboxylate transporters indicated an attempt to maintain the level of glucose metabolism, even in due to the infection thickened swimbladder tissue. In silver eel swimbladder tissue, on the contrary, the mRNA levels of only 291 genes were affected. Genes in the categories 'glucose metabolism' and 'ROS metabolism' barely responded to the infection and even the reaction of the immune system was much less pronounced compared to infected yellow eels. However, in the category 'extracellular matrix', the mRNA levels of several mucin genes were strongly elevated, suggesting increased mucus production as a defense reaction against the parasite. The present study revealed a strong reaction to an Anguillicola crassus infection on mRNA expression levels in swimbladder tissue of yellow eels, whereas in silver eels the changes ware almost negligible. A possible explanation for this difference is that the silvering process requires so much energy that there is not much scope to cope with the additional challenge of a nematode infection. Another possible explanation could be that gas-secreting activity of the silver eel swimbladder was largely reduced, which could coincide with a reduced responsiveness to other challenges, like a nematode infection.

Parasites as drivers of key processes in aquatic ecosystems: Facts and future directions

Despite the advances in our understanding of the ecological importance of parasites that we have made in recent years, we are still far away from having a complete picture of the ecological implications connected to parasitism. In the present paper we highlight key issues that illustrate (1) important contributions of parasites to biodiversity, (2) their integral role in ecosystems, (3) as well as their ecological effects as keystone species (4) and in biological invasion processes. By using selected examples from aquatic ecosystems we want to provide an insight and generate interest into the topic, and want to show directions for future research in the field of ecological parasitology. This may help to convince more parasitologists and ecologists contributing and advancing our understanding of the complex and fascinating interplay of parasites, hosts and ecosystems.

Anguillicola crassus impairs the silvering-related enhancements of the ROS defense capacity in swimbladder tissue of the European eel (Anguilla anguilla)

In a process called silvering, European eels prepare for their long-distance migration from European freshwater systems to the Sargasso Sea for reproduction. During this journey, eels perform extended diel vertical migrations, and the concomitant changes in hydrostatic pressure significantly affect the swimbladder, functioning as a buoyancy organ. As the swimbladder is primarily filled with oxygen, the tissue has to cope with extreme hyperoxic conditions, which typically are accompanied by the generation of reactive oxygen species (ROS) and oxidative stress. In addition, since the introduction of the parasitic nematode Anguillicola crassus in the early 1980s, swimbladder function of most of the European eels is impaired by the infection with this parasite. However, the exact pathways to detoxify ROS and how these pathways are affected by silvering or the infection are still unknown. In swimbladder and muscle tissue from uninfected and infected yellow, and from uninfected and infected silver eels, we measured the level of lipid peroxidation, which increases with ROS stress. To assess the capacity of the ROS defense systems, we analyzed the activities of superoxide dismutase (SOD), catalase, glutathione peroxidase (GPx) and glutathione reductase (GR), and determined the concentration of the antioxidant glutathione (GSH + GSSG). In swimbladder tissue, we found increased concentrations of GSH + GSSG as well as higher activities of SOD, GPx and GR, suggesting that SOD and the glutathione cycle are important for ROS detoxification. Comparing swimbladder tissue of uninfected yellow with uninfected silver eels, the concentration of GSH + GSSG and the activity of SOD were higher after silvering, corresponding with lower levels of lipid peroxidation. Whereas in yellow eels the infection with A. crassus had no effect, in silver eels the capacity to cope with ROS was significantly impaired. In muscle tissue, silvering or the infection only affected the activity of SOD but in exactly the same way as in swimbladder tissue.

Anguillicola crassus Infection Significantly Affects the Silvering Related Modifications in Steady State mRNA Levels in Gas Gland Tissue of the European Eel

Using Illumina sequencing, transcriptional changes occurring during silvering in swimbladder tissue of the European eel have been analyzed by comparison of yellow and silver eel tissue samples. Functional annotation analysis based on GO terms revealed significant expression changes in a number of genes related to the extracellular matrix, important for the control of gas permeability of the swimbladder, and to reactive oxygen species (ROS) defense, important to cope with ROS generated under hyperbaric oxygen partial pressures. Focusing on swimbladder tissue metabolism, levels of several mRNA species encoding glucose transport proteins were several-fold higher in silver eels, while enzymes of the glycolytic pathway were not affected. The significantly higher steady state level of a transcript encoding for membrane bound carbonic anhydrase, however, suggested that CO₂ production in the pentose phosphate shunt and diffusion of CO₂ was of particular importance in silver eel swimbladder. In addition, the mRNA level of a large number of genes related to immune response and to sexual maturation was significantly modified in the silver eel swimbladder. The modification of several processes related to protein metabolism and transport, cell cycle, and apoptosis suggested that these changes in swimbladder metabolism and permeability were achieved by increasing cell turn-over. The impact of an infection of the swimbladder with the nematode Anguillicola crassus has been assessed by comparing these expression changes with expression changes observed between uninfected yellow eel swimbladder tissue and infected silver eel swimbladder tissue. In contrast to uninfected silver eel swimbladder tissue, in infected tissue the mRNA level of several glycolytic enzymes was significantly elevated, and with respect to extracellular matrix, several mucin genes were many-fold higher in their mRNA level. Modification of many immune related genes and of the functional categories “response to DNA damage stimulus” and “cellular response to stress” illustrated the damaging effect of the nematode infection. This study has identified a range of cellular processes in the swimbladder of silver eels that appear to be altered by nematode infection. These altered cellular processes could contribute to detrimental changes in swimbladder function that, in turn, may lead to impairment of spawning migration.

The Hsp70 response of Anguillicola species to host-specific stressors

The present study is based on infection experiments of two different swim bladder parasite species, Anguillicola crassus Kuwahara et al., 1974 and Anguillicola novaezelandiae Moravec and Taraschewski, 1988, which were experimentally transferred to the two eel species Anguilla anguilla Linnaeus, 1758 and Anguilla japonica Temmink and Schlegel, 1846, respectively. The host-parasite groups were selected due to their different grades of mutual adaptation. The main aim of this study was to analyze the stress responses within the parasites, which were confronted with different hosts, i.e. with different stressors related to the respective host. For this purpose, mean intensities, recovery rates, larvae output, and levels of synthesized heat shock proteins (Hsp70) were determined in nematodes of each infection group. Increased stress responses were detected in the endemic system of A. crassus parasitizing A. japonica and A. crassus in its recently acquired host A. anguilla, which seems to be associated with the immune response of the particular host species and the expenditure of energy on producing larvae. A. novaezelandiae showed overall weak activities in its unknown host species A. japonica, with the lowest recovery rate of all examined groups neither featuring elevated Hsp responses, nor a high mean intensity, nor any reproductive output. On the contrary, in A. anguilla, the parasite reached higher recovery rates, mean intensities, and reproductive output, but no increased Hsp70 levels could be detected. The four considered factors proved partially interdependent, whereas few results did not follow a clear pattern.

Influence of adult Anguillicoloides crassus load in European eels swimbladder on macrophage response

Anguillicoloides crassus has become one of the most important threats to the European eel (Anguilla anguilla). Adult parasites colonize the swimbladder leading to an impaired functioning of this organ. The infection is also responsible for an increased in the stress level of infected eels, that could produce an altered immune response as well. Differences in parasite loads and effects in the European and Japanese eel (Anguilla japonica) have been described. We have studied the influence of the number of adult parasites present in the swimbladder of wild eels on the macrophage response (phagocytosis and respiratory burst) as part of the first immune response to pathogens. Our results show an increased phagocytozed bacterial survival 24 h post-infection in macrophages of eels infected with more than ten adult parasites compared to macrophages from eels infected with less than those ten adult parasites. Respiratory burst results also showed a less efficient response in macrophages from eels infected with more than ten adult parasites, although in this case results were not found to be significant.

Competing invaders: Performance of two Anguillicola species in Lake Bracciano

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Two Anguillicola spp. occurring as nonindigenous species are compared in the review.

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Both species differ in their life cycles.

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Anguillicola crassus replaced Anguillicola novaezelandiae in Lake Bracciano.

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Possible reasons for the disappearance of A. novaezelandiae are discussed.

Anguillicola crassus is one of the most successful parasitic invasive species as it has spread from its original habitat in East Asia throughout the world and has acquired five new eel host species in the course of its invasion within the last three decades. Records from an Italian lake indicate that this species has even displaced an established population of its close relative A. novaezelandiae originating from New Zealand. In order to analyze the reasons for its high invasive potential, this review highlights recent studies, which substantiate the selective advantages of A. crassus over A. novaezelandiae.

Laboratory infection experiments revealed that A. crassus features a less synchronized development compared to A. novaezelandiae in the European eel, which enables this species to emit eggs over a longer period of time. Differences in the time period required for first egg output and in the maturation process of second stage larvae in intermediate hosts could also be detected, which may lead to differences in infection potential. Finally, microsatellite analyses have shown that hybridization processes are possible, but might only occur between A. crassus males and A. novaezelandiae females. Taken as a whole, the sum of minor selective advantages and differences in life cycle traits could have considerably contributed to a replacement of one species by the other.

Swimbladder function and the spawning migration of the European eel Anguilla anguilla

The spawning migration of the European eel is an extensive journey over 5000 to 7000 km from the European coast to the Sargasso Sea. Eels do not feed during this journey and on-board fuels must be sufficient to support the journey of 3.5 to 6 month, as well as sexual maturation and the spawning activity. Swimming of eels appears to be quite energy efficient compared to other fish species, and elevated hydrostatic pressure has been shown to even reduce the costs of transport. Recent studies revealed, however, that during traveling eels perform extensive diurnal migrations and swim at a depth of about 100-300 m at night time, but go down to 600-1000 m at day time. At a depth of 200 m eels are exposed to a hydrostatic pressure of 21 atmospheres (2.13 MPa), while at 800 m hydrostatic pressure increases to 81 atmospheres (8.21 MPa). Accordingly, without any compensation at a depth of 800 m swimbladder volume will be reduced to about 25% of the volume established with neutral buoyancy at 200 m. Consequently, these diurnal changes in depth must be taken into consideration for a calculation of the energy requirements of the spawning migration. Without compensation a compression of the swimbladder will result in a status of negative buoyancy, which makes swimming more costly. Trying to keep the status of neutral buoyancy during descent by gas secretion into the swimbladder in turn requires metabolic activity to enhance swimbladder perfusion and for acid production of the gas gland cells to stimulate gas secretion. During ascent gas is passively removed from the swimbladder in the resorbing section and in the blood transported to the gills, where it is lost into the water. Accordingly, the swimbladder appears to be a crucial organ for the spawning migration. It can be assumed that an impairment of swimbladder function for example due to an infection with the nematode Anguillicola crassus significantly threatens the success of the spawning migration.

Comparison of infection success, development and swim bladder pathogenicity of two congeneric Anguillicola species in experimentally infected Anguilla anguilla and A. japonica

Two closely related parasites, Anguillicola crassus and Anguillicola novaezelandiae, originally parasitizing swim bladders of the Japanese eel (Anguilla japonica and the Short-finned eel (Anguilla australis), respectively, were used for analyzing the infection success of each parasite species on either long-known, recently acquired or new definitive host species and the associated effects on the eels' swim bladders. On that account, European eels (Anguilla anguilla) and Japanese eels were experimentally infected with both Anguillicola species in the laboratory. Susceptibility of the two eel species to both parasite species was determined by analyses of infection data. Subsequently, histopathological effects of the nematodes on the hosts' swim bladders were characterized according to already established indices.The present study revealed significant differences between the four different host-parasite systems regarding recovery rates, infrapopulations, and damage levels. Both nematode species achieved significantly lower recovery rates in Japanese eels than in European eels, since the examined swim bladders of Japanese eels contained a high amount of dead encapsulated larvae, whereas those of European eels contained only living nematodes. Encapsulation of larvae in Japanese eels was associated with a distinct thickening of the swim bladder walls. The swim bladders of uninfected Japanese eels turned out to be generally thicker than those of European eels. Infection with both Anguillicola species resulted in a further thickening process of the swim bladder walls in Japanese eels, whereas those of European eels showed only minor changes. The two established classification systems turned out to be inapplicable, since the measurements and the macroscopic evaluations of the swim bladders of the two infected eel species did not entirely correspond to the underlying criteria.

The aetiology, histopathology, and ultrastructural features of perianal erythema (red anus syndrome) in the European eel (Anguilla anguilla)

The European eel (Anguilla anguilla) is a critically endangered species. Red anus syndrome (RAS) is known to be associated with parasitic infections of the eel, particularly with Anguillicola crassus, but the full range of causative pathogenic organisms has not been systematically investigated. Here we examined the infective organisms and histopathological and ultrastructural features of seventy eels with RAS. In total, nine different pathogens were detected in association with RAS: Pseudomonas aeruginosa were present in twelve specimens (17%), the metacercaria of Euclinostromum heterostomum in three cases (4%), Gastrostome (Bucephalidae family) in seven cases (10%), A. crassus in forty-five cases (64%), Bothriocephalus in seventeen cases (24%), and Proteocephalus in twenty-three cases (32%). Yeast, amoeba, and myxobolus-like pathogens were seen in the anal skin in all cases when examined in combination with electron microscopy. Histopathologically, the lesions appeared as anoproctitis of varying severity from mild anusitis to severe haemorrhagic anoproctitis, with severe perianal oedema, haemorrhage, and proctoptosis. Gut inflammation ranged from mild catarrhal enteritis to severe haemorrhagic enteritis with mucosal sloughing. RAS is associated with a range of parasitic infections, not only A. crassus, some of which we describe here for the first time. Since RAS is not associated with direct invasion by parasites, it is likely that RAS is a secondary phenomenon caused by superadded infection on a background of generalised immunosuppression, or indirect local toxic effects. RAS may be used as a non-invasive indicator of underlying parasitic infection, but further investigations are required to establish the causative organisms for effective fishery management.

Invasive swimbladder parasite Anguillicoloides crassus: infection status 15 years after discovery in wild populations of American eel Anguilla rostrata

A year-round survey of American eels Anguilla rostrata was performed at 5 localities in South Carolina (SC), USA, 15 yr after the first infection by the nematode Anguillicoloides crassus was reported from Winyah Bay, SC. Infections by adult stages of A. crassus in the swimbladder lumen occurred with a prevalence of 45% (n = 479), a mean intensity (± SE) of 2.3 ± 0.2 worms per infected eel (range = 1-22), and a mean abundance of 2.0 ± 0.1 among all eels. Infections by larval stages of A. crassus in the swimbladder wall occurred with a prevalence, intensity, and abundance of 29%, 2.4 ± 0.3 (range = 1-15), and 0.7 ± 0.1, respectively (n = 471). Overall prevalence of the parasite (any stage) was 58%, with a mean intensity ± SE of 3.0 ± 0.2 and a mean abundance of 1.8 ± 0.2. Biomass of the adult parasite stage varied significantly with eel body length, but the direction of the effect depended on salinity. Prevalence and intensity of infection by adult nematodes varied by locality but not by eel total length, salinity, or season. Larval prevalence was significantly greater in the winter and spring and also differed among localities. The lack of seasonal effects on infection by the adult worm stage contrasts with studies from higher latitudes in North America and Europe and may be due to the warmer winter temperatures at southern latitudes. Significant variation in infection among localities reflects possible differences in abundance of intermediate and/or paratenic hosts. Overall, infection levels were higher than previous reports for eels in SC but comparable to more recent reports from other areas in North America.

Evolutionary divergence of the swim bladder nematode Anguillicola crassus after colonization of a novel host, Anguilla anguilla

Background

Anguillicola crassus, a swim bladder nematode naturally parasitizing the Japanese eel, was introduced about 30 years ago from East Asia into Europe where it colonized almost all populations of the European eel. We conducted a common garden experiment under a reciprocal transfer design infecting both European and Japanese eels with populations of A. crassus from Germany, Poland and Taiwan. We tested, whether differences in infectivity, developmental dynamics and reproductive output between the European and Asian parasite populations occur while harboured in the specimens of native and colonized eel host, and if these differences are genetically based or are plastic responses to the new environment.

Results

Under common garden conditions an evolutionary change in the both European parasite populations of A. crassus compared with their Taiwanese conspecifics was observed for infectivity and developmental dynamics, but not for reproductive output. When infecting the European eel, current European populations of the parasite were less infective and developed faster than their Taiwanese conspecifics. In the reciprocally infected Japanese eel the genetically induced differences between the parasite strains were less apparent than in the European eel but higher infectivity, faster development and higher larval mortality of the European parasite populations could be inferred.

Conclusions

The differences in infectivity and developmental dynamics between European and Taiwanese populations of A. crassus found in our study suggest rapid genetic divergence of this parasite after a successful host switch in Europe.

Prevalence of Anguillicoloides crassus and growth variation in migrant yellow-phase American eels of the upper Potomac River drainage

Prevalence of the non-native swim bladder nematode Anguillicoloides crassus has recently increased in American eels from estuaries of the North American Atlantic coast, but little is known about parasite prevalence or conditions of previous infection in upstream migrant eels within upper watersheds. This study is the first to confirm presence of A. crassus in the upper Potomac River watershed. We estimated A. crassus prevalence during 3 time periods: September to October 2006 (5/143 eels, 3.5%), August to October 2007 (0/49 eels), and June 2008 (0/50 eels). All eels were sampled from the Millville Dam eel ladder on the lower Shenandoah River, a Potomac River tributary located approximately 285 km upstream of Chesapeake Bay, USA. Of the 5 infected eels, parasite intensity was 1 for each eel, and mean intensity was also 1.0. A swim bladder degenerative index (SDI) was calculated for the 50 eels from the final sampling period, and 38% of those eels (19 of 50) showed signs of previous infection by A. crassus. We also aged 42 of the 50 eels (mean ± SE = 6.7 ± 0.29 yr, range 4 to 11 yr) from the final sampling period. Based on the range of possible SDI scores (0 to 6), severity of previously infected swim bladders was moderate (SDI = 1 or 2). Previously infected eels, however, had a lower length-at-age than that of uninfected eels. Female yellow-phase eels in upper watersheds develop into large highly fecund silver-phase adults; hence, a parasite-induced effect on growth of yellow-phase eels could ultimately reduce reproductive potential.

An invasion record for the swimbladder nematode Anguillicoloides crassus in European eel Anguilla anguilla in a deep warm-monomictic [corrected] lake, from invasion to steady state

This study is the first account of the establishment and development of the neozoic nematode parasite Anguillicoloides crassus in its host, the European eel Anguilla anguilla, in a deep, warm-monomictic [corrected] lake. A 21 year study of A. crassus took place in Upper Lake Constance (ULC), Europe's second largest pre-alpine lake. The study included two extensive surveys, one in 1991 during the initial parasite invasion phase and the second in 2006 when the infection was well established. The subtropical swimbladder nematode A. crassus was first recorded in A. anguilla in ULC in 1989. Prevalence reached 60% in 1992 and remained at this level until 2007. In 2008, prevalence decreased to 48%. Infection intensity peaked in 1993 at a mean value of 16 adult parasites per host fish. Around 90% of all A. anguilla examined displayed swimbladder lesions, with a significant trend to increasing severity over time. Moreover, heavy swimbladder lesions were seen in c. 10% of A. anguilla ready to migrate to their spawning habitat. Both ruffe Gymnocephalus cernuus and sunfish Lepomis gibbosus serve as paratenic hosts for A. crassus in ULC. Gymnocephalus cernuus seems to be the main vector, and infection is especially frequent in spring possibly caused by reduced immune system efficacy of G. cernuus during winter. In 1991, hypochromic anaemia was prevalent in ULC A. anguilla acutely infected with A. crassus, whereas in 2006 blood values were indicative of chronic infection. The growth and survival rates of A. anguilla during their continental phase were not noticeably altered in infected fish, but damage to the swimbladder probably impairs migration potential and thus the subsequent breeding success of the oceanic phase.

Early marine migration of European silver eel Anguilla anguilla in northern Norway

To study the migratory behaviour in wild northern European silver eel Anguilla anguilla during sea entry and early marine migration, 32 individuals were tagged with acoustic transmitters and registered at four automatic listening station arrays from the mouth of the north Norwegian River Alta and throughout the Alta Fjord. The A. anguilla entered the fjord during all parts of the tidal cycle and did not seem to utilize the outgoing tidal currents. They migrated mainly during the night, in both the river mouth and the fjord. On average, they spent 2·7 days travelling from the river mouth to the outermost array, 31 km from the river mouth, corresponding to an average migratory speed of 0·5 km h(-1) . The A. anguilla generally migrated in the central part of the fjord and in the uppermost 10-25% of the water column, but with frequent dives to greater depths. Already 4 km after sea entry, A. anguilla were observed diving deeper than 130 m within 20-30 min periods. Hence, this study demonstrated that A. anguilla may perform an active diving behaviour during the early marine migration. The study took place in a pristine area with a minimum of anthropogenic interventions and by individuals from a population still uninfected by the introduced parasite Anguillicoloides crassus. The results may therefore be used as a baseline for future studies of the A. anguilla early marine migration.

An evaluation of indices of gross pathology associated with the nematode Anguillicoloides crassus in eels

This study compares two alternative indices for quantifying the gross pathology of the swimbladder of eels, Anguilla anguilla (L.), infected with the nematode Anguillicoloides crassus. Two observers recorded twice the scores obtained by the two indices on the same set of 71 wild caught eels (from elver to silver eels, French Mediterranean lagoons). The Length Ratio Index (LRI), performed better than the Swimbladder Degenerative Index (SDI), in three of four predefined criteria of decision. First, the LRI better correlated with an estimate of the swimbladder volume reduction, a functional consequence of the infection (representativeness). Also, the LRI was less prone to subjectivity (inter-observer variability) and more precise (intra-observer variability), although less easy to generate (time needed for measurement/assessment). Using a sub-sample of 32 unaffected eels (showing minor if any swimbladder damage and no living worms at autopsy), we ascertained a linear relationship between the swimbladder length and the total body length, a prerequisite of isometric growth, to definitively accept the new ratio index as a valid alternative to the SDI. Also, because the LRI can be recorded on live specimens with radio-imagery (non-invasive method), we recommend its use, and provide a graph of correspondence between the SDI scores, the LRI scores and the estimated proportion of gas loss in the swimbladder.

The European eel--the swim bladder-nematode system provides a new view of the invasion paradox

It is widely assumed that the likelihood of invasion decreases with increased species richness in the recipient community. However, the invasion paradox supports a negative and a positive relationship between native biodiversity and the success of an invader. Here, we show that for a host-parasite system (Anguilla anguilla as host and Anguillicoloides crassus as parasitic invader), invasion increases with native micro- and macroparasitic species richness. In fact, about 30% of the A. crassus intensity in eels could be explained by the number of both micro- and macroparasite species. This pattern could be due to the fact that A. crassus exploits a niche (the swim bladder) that is unoccupied by native parasite species and by the Th1/Th2 trade-off between native microparasites and the invader. We conclude that the host-parasite system resistance to invasion may depend on both niche availability and the Th1/Th2 trade-off. As well, we encourage researchers to incorporate native parasite richness as a risk factor in epidemiological models of A. crassus.

Massive encapsulation of larval Anguillicoloides crassus in the intestinal wall of Japanese eels

Background

Within the last 25 years, after the introduction of the swimbladder nematode Anguillicoloides crassus from East-Asia to Europe, a body of work has aggregated on the host parasite interactions in the acquired host Anguilla anguilla. Despite the emerging evolutionary interest there is still a lack of knowledge about host parasite relations of A. crassus in its natural host Anguilla japonica. We examined the Anguillicoloides infections of wild-caught Japanese eels as well as from aquacultured specimens in Taiwan with respect to the fate of migratory L3 larvae and performed infection experiments with Japanese eels.

Results

Inside the intestinal wall of cultured eels, where the infective pressure was higher than among wild eels, we found large numbers of granuloma-like cysts. In a few eels these cysts contained nematodes still recognizable as L3 larvae of A. crassus, while in most cases the content of these capsules was degraded to amorphous matter. Occurrence of these objects was correlated with the number of encapsulated larvae in the swimbladder wall. We were able to show, that the cysts contained disintegrated L3 larvae by amplification and subsequent sequencing of large subunit ribosomal rRNA. Furthermore we identified repeated infections with high doses of larvae as prerequisites for the processes of encapsulation in infection experiments.

Conclusion

Under high infective pressure a large percentage of L3 larvae of A. crassus coming from the gut lumen are eliminated by the natural host within its intestinal tissue. It is possible to reproduce this condition in infection experiments. We provide a fast, easy and reliable PCR-based method for identification of encapsulated swimbladder parasites.

Element concentrations in the swimbladder parasite Anguillicola crassus (nematoda) and its host the European eel, Anguilla anguilla from Asi River (Hatay-Turkey)

The European eel's swimbladder nematode, Anguillicola crassus, sampled from the Asi River (Orontes River) in Antakya (Hatay, Turkey) in May 2006 were analysed by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) for their some heavy metal (Cd, Cr, Cu, Fe, Hg, Mn, Pb and Zn) levels. The metal concentrations of the parasites were compared to different organs (swimbladder, liver, muscle and skin) of the fish hosts. The parasite contained statistically highly significantly amounts of Fe (P < 0.05). The iron level of nematode was up to 25.52 times than the muscle of its host, Anguilla anguilla. However, bioconcentration of Cd, Cr, Cu, Hg, Mn, Pb, Zn were detected in the A. crassus and it contained no statistically differences with the other tissues of its host, the eel (P > 0.05). Furthermore, no significant differences were detected in the heavy metal accumulations between the parasitized and un-parasitized fish tissues. The analysed metals (Cd, Cr, Cu, Fe, Mn, Pb and Zn) were found in fish muscle at mean concentrations under the permissible limits proposed by FAO.

Parasite communities in eels of the Island of Reunion (Indian Ocean): a lesson in parasite introduction

Eel populations from the small rivers on the Island of Reunion (French Overseas Department in the Indian Ocean) were investigated with respect to the occurrence and abundance of helminths during the autumn of 2005. The native species Anguilla marmorata (n = 80), Anguilla bicolor (n = 23), and Anguilla mossambica (n = 15) were studied. Six species of helminths were identified, four of them having a definitely nonnative status. Furthermore, unidentified intra-intestinal juvenile cestodes and extra-intestinal encapsulated anisakid nematode larvae were present in a few eels. We found that the invasive swim bladder nematode Anguillicoloides (Anguillicola) crassus had been introduced into the island. Six specimens were collected, four from A. marmorata, one from A. bicolor and one from A. mossambica. The maximum intensity of infection was two worms. The other helminths also showed a low abundance. These species were the monogenean gill worms Pseudodactylogyrus anguillae and Pseudodactylogyrus bini and the intestinal parasites Bothriocephalus claviceps (Cestodes), Paraquimperia africana (Nematodes), and the acanthocephalan Acanthocephalus reunionensis Warner, Sasal, and Taraschewski, 2007. The latter species, found as intra-intestinal immatures, is thought to utilize amphibians as required hosts; its status, introduced or native, could not be determined. P. africana was described from A. mossambica in South Africa and has not been recorded outside Africa. The other species are known from populations of European and American eels. However, A. crassus and the two Pseudodactylogyrus species originate from East Asia, where they are indigenous parasites of Anguilla japonica. Both an assignment test based on seven specific microsatellite loci and subsequent sequencing of mitochondrial haplotypes of a partial fragment of cytochrome c oxidase 1 strongly suggest that the A. crassus may originated around the Baltic Sea. According to the results presented here, populations of the indigenous eel species from Reunion can be considered to harbor extremely isolationist alien parasite communities. Our findings support the hypothesis that during the present time of global biological change, invasion by a nonnative species into a target island is more likely to reflect the political affiliation of the colonized environment and the pathways of trade and tourism than geographic proximity between donor and recipient areas or other natural circumstances.

The pathogenic helminth parasites of eels

Although 63 and 55 species of helminths have been reported from each species of Atlantic eel and from 29 to 19 for each species of Pacific eel only the monogeneans Pseudodactylogyrus bini and P. anguillae and the nematode Anguillicola crassus, originally specific to species of Pacific eels, can be considered serious pathogens. None of the three are normally pathogenic to their preferred natural eel host species in the wild. Pseudodactylogyrus spp. only cause serious local gill damage when present on a host in large numbers under optimal conditions that facilitate transmission. This is the case in eel aquaculture, where infections can be controlled by drugs. Anguillicola crassus is only pathogenic to Anguilla anguilla and A. rostrata when Atlantic eels are introduced to the far east or when the parasites have been introduced to Europe. Here the parasite life cycle differs in that A. crassus can infect a wide range of intermediate hosts, employ paratenic hosts and survive as larvae for months in the swimbladder wall. This makes it an excellent colonizer. Its major pathogenic effects on eels result from haemorrhaging in, and thickening of, the swimbladder wall. It reduces the oxygen concentration in the swimbladder, reducing its ability to function as a hydrostatic organ, and increases the stress response of eels. In shallow lakes at warm temperatures this can result in mass mortalities. It is also feared that the parasite affects the ability of eels to migrate to the Sargasso Sea and so contributes to the decline in eel populations. Control by drug treatment is possible in culture, but not in the wild.

The swimbladder nematodeAnguillicola crassusin the European eelAnguilla anguillaand the Japanese eelAnguilla japonica: differences in susceptibility and immunity between a recently colonized host and the original host

The swimbladder nematodeAnguillicola crassusoriginates from Asia where it is a parasite of the Japanese eelAnguilla japonica. After its introduction to Europe about 25 years ago, the parasite spread rapidly within the indigenous populations of the European eelAnguilla anguillaand subsequently the prevalence and mean intensity appeared to stabilize. Under experimental and aquaculture conditions the naïve new host appears to be more susceptible toA. crassuscompared to the original host. Both eel species develop a immune response againstA. crassus. The antibody response is well characterized for the European eel, but poorly characterized for the Japanese eel. It remains unclear if antibodies have any protective function againstA. crassus. Encapsulation of larvae ofA. crassuscan be observed in naturally infected European eels. However, encapsulation of larvae following experimental infection has not been detected in European eels, but only in Japanese eels. Reinfection experiments and intraperitoneal injection ofA. crassushomogenates failed to demonstrate the development of acquired immunity in European eels. Immunization with irradiated third stage larvae provided preliminary evidence for acquired immunity againstA. crassusin the Japanese eel, but not in the European eel.

Hosts and parasites as aliens

Over the past decades, various free-living animals (hosts) and their parasites have invaded recipient areas in which they had not previously occurred, thus gaining the status of aliens or exotics. In general this happened to a low extent for hundreds of years. With variable frequency, invasions have been followed by the dispersal and establishment of non-indigenous species, whether host or parasite. In the literature thus far, colonizations by both hosts and parasites have not been treated and reviewed together, although both are usually interwoven in various ways. As to those factors permitting invasive success and colonization strength, various hypotheses have been put forward depending on the scientific background of respective authors and on the conspicuousness of certain invasions. Researchers who have tried to analyse characteristic developmental patterns, the speed of dispersal or the degree of genetic divergence in populations of alien species have come to different conclusions. Among parasitologists, the applied aspects of parasite invasions, such as the negative effects on economically important hosts, have long been at the centre of interest. In this contribution, invasions by hosts as well as parasites are considered comparatively, revealing many similarities and a few differences. Two helminths, the liver fluke,Fasciola hepatica,of cattle and sheep and the swimbladder nematode,Anguillicola crassus,of eels are shown to be useful as model parasites for the study of animal invasions and environmental global change. Introductions ofF. hepaticahave been associated with imports of cattle or other grazing animals. In various target areas, susceptible lymnaeid snails serving as intermediate hosts were either naturally present and/or were introduced from the donor continent of the parasite (Europe) and/or from other regions which were not within the original range of the parasite, partly reflecting progressive stages of a global biota change. In several introduced areas,F. hepaticaco-occurs with native or exotic populations of the congenericF. gigantica, with thus far unknown implications. Over the fluke's extended range, in addition to domestic stock animals, wild native or naturalized mammals can also serve as final hosts. Indigenous and displaced populations ofF. hepatica, however, have not yet been studied comparatively from an evolutionary perspective.A. crassus, from the Far East, has invaded three continents, without the previous naturalization of its natural hostAnguilla japonica, by switching to the respective indigenous eel species. Local entomostrac crustaceans serve as susceptible intermediate hosts. The novel final hosts turned out to be naive in respect to the introduced nematode with far reaching consequences for the parasite's morphology (size), abundance and pathogenicity. Comparative infection experiments with Japanese and European eels yielded many differences in the hosts' immune defence, mirroring coevolution versus an abrupt host switch associated with the introduction of the helminth. In other associations of native hosts and invasive parasites, the elevated pathogenicity of the parasite seems to result from other deficiencies such as a lack of anti-parasitic behaviour of the naïve host compared to the donor host which displays distinct behavioural patterns, keeping the abundance of the parasite low. From the small amount of available literature, it can be concluded that the adaptation of certain populations of the novel host to the alien parasite takes several decades to a century or more. Summarizing all we know about hosts and parasites as aliens, tentative patterns and principles can be figured out, but individual case studies teach us that generalizations should be avoided.

Interaction between the severity of the infection by the nematode Anguillicola crassus and the tolerance to hypoxia in the European eel Anguilla anguilla

An experiment was conducted to test the effect of the infection by the swimbladder nematode Anguillicola crassus on the survival of the European eel Anguilla anguilla when exposed to hypoxic conditions. Forty-four wild caught and naturally infected eels were placed in an aquarium filled with water from the fishing site (Vaccarès Lagoon, French Mediterranean coast). In this confined environment (271), under decreasing oxygen resources, the first eel death occurred after 45 h (O2 = 0.98 mg l−1) and the last one after 96 h (O2 = 0.48 mg l−1). After dissection and parasite examination, analyses revealed significant negative correlations between the time to death and various parameters of parasite pressure (e.g., number of lumen worms, parasite mass, health state of the infected organ). It was shown that the severity of damage to the swimbladder rather than the count of living parasites was the major contributing factor in explaining the variation in time to death. These semi-experimental data demonstrating an increased eel mortality rate under severe oxygen stress are discussed for their relevance under field conditions, especially during hot summer months.

Histopathological Examination of Wild American Eels Infected with Anguillicola crassus

We describe for the first time the histopathology caused in wild American eels Anguilla rostrata by the introduced nematode Anguillicola crassus. All of the American eels examined showed signs of previous or ongoing infections with A. crassus. Gross observations included opacity of the normally translucent swim bladder and dilation of blood vessels. The swim bladders of infected American eels showed focal, multifocal, and diffuse histological changes. Consistently observed pathologies included abnormal papillose appearance of the mucosa; hyperplasia of the lamina propria, muscularis mucosa, and submucosa; edema of the mucosa and muscularis mucosa; dilation of the blood vessels; and damage in the submucosa caused by migrating A. crassus L3 and L4 larvae. Less-common pathologies included fibrosis and lymphocytic aggregates around L3 and L4 larvae in the submucosa; destruction of the mucosa, which in some cases completely exposed the mucosal blood vessels; L2 larval penetration of the tissues of the swim bladder; bacterial infections in the submucosa and muscularis mucosa; and migration of an L4 larva through the rete mirabile.

Effects of infection with Anguillicola crassus and simultaneous exposure with Cd and 3,3′,4,4′,5-pentachlorobiphenyl (PCB 126) on the levels of cortisol and glucose in European eel ( Anguilla anguilla )

To investigate whether the stress response of European eels infected with Anguillicola crassus is influenced by environmental pollutants, experimentally infected eels were exposed to Cd and/or to 3,3', 4,4', 5-pentachlorobiphenyl (PCB 126). Serum cortisol and glucose concentrations of these eels were monitored over a period of 103 days and were compared with data from infected, unexposed eels as well as with data from uninfected eels. Additionally, the levels of cortisol were correlated with concentrations of Anguillicola-specific antibodies. All eels showed an initial increase of the cortisol levels until day 63. This general elevation of plasma cortisol is most likely due to handling stress, as all eels were repeatedly netted and afterwards inoculated with a feeding tube. At the end of the exposure period eels which were infected and those which were infected and simultaneously exposed to Cd and PCB showed significantly higher levels than the controls. The general course of serum glucose levels in eels resembled that of cortisol. Accordingly, Spearman correlation analysis revealed that an increase in serum cortisol concentrations is correlated with rising levels of glucose. With respect to immune-endocrine interactions a significant negative correlation between cortisol and anti-A. crassus antibodies was found. Our data show that A. crassus is the most potent stressor for European eels among the treatments tested within this study. This is important in terms of ecotoxicological studies as the main effects are caused by parasites rather than chemicals. Accordingly, effects of parasites on the physiological homeostasis of organisms must be considered in ecotoxicology. From the parasitological point of view our results suggest that probably as part of an unbalanced host-parasite interaction A. crassus evokes a strong cortisol response in A. anguilla, thereby suppressing the immune response which in turn enables the parasite to establish. The parasite-induced stress response in the newly adopted European eel might be one of the factors which contributes to the extremely effective colonizing strategy of A. crassus.

European eels, Anguilla anguilla (L.), infected with Anguillicola crassus exhibit a more pronounced stress response to severe hypoxia than uninfected eels

The parasite, Anguillicola crassus is a non-native species that infects naive European eels, Anguilla anguilla, and causes pathological damage to the swimbladder, potentially compromising their ability to cope with hypoxic conditions. This study aimed to elucidate whether anguillicolosis exacerbates the stress responses to exposure to hypoxic water, conditions that have been implicated in mass mortalities of wild infected European eels. Blood parameters in infected and uninfected eels were measured during exposure to severe hypoxia over an 8-h period. Infected fish showed significantly higher levels of plasma cortisol compared with uninfected eels after 4 h of hypoxia. Uninfected fish showed an almost twofold increase in plasma glucose after 8-h exposure to hypoxia but infected fish showed no significant change, so that the plasma glucose concentration was significantly higher in uninfected eels than in infected eels. Both groups showed similar elevations in blood haematocrit, suggesting a similar catecholamine response in infected and uninfected eels. The lack of a hyperglycaemic response in infected eels, despite indirect evidence of a catecholamine response to hypoxia, may reflect an increase in glucose turnover. The data suggest that anguillicolosis results in a significantly greater corticosteroid stress response to hypoxia accompanied by a higher metabolic cost.

Ultrastructure of the body wall of female Philometra obturans (Nematoda: Dracunculoidea)

The somatic cuticle of adult female Philometra obturans (Prenant, 1886) (Nematoda: Dracunculoidea) consists of five zones and an overlying prominent fuzzy epicuticle, closely adherent to the gill arterial wall of the host. The cuticle is fenestrated and infiltrated with electron-dense substances. It protrudes into the hypodermis in the form of numerous protuberances. The plasma membrane of the hypodermis forms prominent infoldings within the subcuticular region. Numerous endosomes bud off from the cuticle into the hypodermis. Glycogen deposits are present within the hypodermis and muscle cells. The muscle cells are well developed, polymyarian in number (up to 30 in each quadrant) and coelomyarian in shape. The body wall of adult female P. obturans reveals several ultrastructural features which are common to the members of Filarioidea.