Non-lethal detection of Eubothrium crassum (Cestoda) in farmed Atlantic salmon, Salmo salar, using anal swabs and real-time PCR.

Detection of intestinal parasites in fish typically requires autopsy, resulting in the sacrifice of the fish. Here, we describe a non-lethal method for detecting the tapeworm Eubothrium crassum in fish using anal swabs and real-time PCR detection. Two assays were developed to detect cytochrome oxidase I (COI) mitochondrial DNA and 18S ribosomal DNA sequences of E. crassum, respectively. The assays were tested on swab samples from confirmed pathogen free Atlantic salmon (Salmo salar L.) and on samples from farmed Atlantic salmon, where the presence and intensity of parasites had been established through autopsy. The COI assay was shown to be specific to E. crassum, while the 18S assay also amplified the closely related E. salvelini, a species infecting Arctic charr (Salvelinus alpinus L.) in freshwater. The COI assay detected E. crassum in all field samples regardless of parasite load while the 18S assay failed to detect the parasite in two samples. The results thus demonstrates that this non-lethal approach can effectively detect E. crassum and can be a valuable tool in assessing the prevalence of infection in farmed salmon, aiding in treatment decisions and evaluating treatment effectiveness.

Evidence for infection influencing survival of the freshwater copepod Salmincola californiensis, a parasite of Pacific salmon and trout.

OBJECTIVE: We explore apparent infection of Salmincola californiensis arising during investigations involving this lernaeopodid copepod parasitic on Pacific salmon and trout Oncorhynchus spp. METHODS: We noted occasional unusual coloration of adult female copepods collected from the wild. These females were bright blue and pink in contrast to the cream white coloration characteristic of the copepod. We also observed that similar color patterns developed under laboratory settings when copepod eggs were held for hatching. In paired egg cases, we found consistent hatching failure of blue and pink eggs and patterns in apparent disease development that would be consistent with both vertical and horizontal transmission. RESULT: Attempts to identify the cause of the apparent infection using genetic methods and transmission electron microscopy were inconclusive. CONCLUSION: Iridovirus infection was initially suspected, but bacterial infection is also plausible. This apparent reduced hatching success of S. californiensis warrants further exploration as it could reduce local abundances. Given the potential importance of a disease impacting this copepod, a parasite that itself affects endangered and commercially important Pacific salmon and trout, future research would benefit from clarification of the apparent infection through additional sequencing, primer development, visualization, and exploration into specificity and transmission.

Spatial and intra-host distribution of myxozoan parasite Tetracapsuloides bryosalmonae among Baltic sea trout (Salmo trutta).

Proliferative kidney disease caused by the myxozoan parasite Tetracapsuloides bryosalmonae has been actively studied in juvenile salmonids for decades. However, very little is known about parasite prevalence and its geographical and intra-host distribution at older life stages. We screened T. bryosalmonae among adult sea trout (Salmo trutta) (n = 295) collected along the Estonian Baltic Sea coastline together with juvenile trout from 33 coastal rivers (n = 1752) to assess spatial infection patterns of the adult and juvenile fish. The parasite was detected among 38.6% of adult sea trout with the prevalence increasing from west to east, and south to north, along the coastline. A similar pattern was observed in juvenile trout. Infected sea trout were also older than uninfected fish and the parasite was detected in sea trout up to the age of 6 years. Analysis of intra-host distribution of the parasite and strontium to calcium ratios from the otoliths revealed that (re)infection through freshwater migration may occur among adult sea trout. The results of this study indicate that T. bryosalmonae can persist in a brackish water environment for several years and that returning sea trout spawners most likely contribute to the parasite life cycle by transmitting infective spores.

First insight in element localisation in different body parts of the acanthocephalan Dentitruncus truttae using TEM and NanoSIMS.

Acanthocephalans, intestinal parasites of vertebrates, are characterised by orders of magnitude higher metal accumulation than free-living organisms, but the mechanism of such effective metal accumulation is still unknown. The aim of our study was to gain new insights into the high-resolution localization of elements in the bodies of acanthocephalans, thus taking an initial step towards elucidating metal uptake and accumulation in organisms under real environmental conditions. For the first time, nanoscale secondary ion mass spectrometry (NanoSIMS) was used for high-resolution mapping of 12 elements (C, Ca, Cu, Fe, N, Na, O, P, Pb, S, Se, and Tl) in three selected body parts (trunk spines, inner part of the proboscis receptacle and inner surface of the tegument) of Dentitruncus truttae, a parasite of brown trout (Salmo trutta) from the Krka River in Croatia. In addition, the same body parts were examined using transmission electron microscopy (TEM) and correlated with NanoSIMS images. Metal concentrations determined using HR ICP-MS confirmed higher accumulation in D. truttae than in the fish intestine. The chemical composition of the acanthocephalan body showed the highest density of C, Ca, N, Na, O, S, as important and constitutive elements in living cells in all studied structures, while Fe was predominant among trace elements. In general, higher element density was found in trunk spines and tegument, as body structures responsible for substance absorption in parasites. The results obtained with NanoSIMS and TEM-NanoSIMS correlative imaging represent pilot data for mapping of elements at nanoscale resolution in the ultrastructure of various body parts of acanthocephalans and generally provide a contribution for further application of this technique in all parasite species.

Tetracapsuloides bryosalmonae and proliferative kidney disease in Icelandic salmonids - Comparative data from two different time periods.

Tetracapsuloides bryosalmonae is a myxozoan parasite and the causative agent of proliferative kidney disease (PKD), a serious, temperature-dependent and emerging disease affecting salmonid fish. It was first identified in Iceland in 2008, from Arctic charr inhabiting a shallow lowland lake. The aim of this study was to investigate the distribution and prevalence of macroscopic and subclinical T. bryosalmonae infections in Icelandic salmonids and compare different time periods, in context with depths, volumes, altitudes and areas of the lakes and fish age. Arctic charr (Salvelinus alpinus) and brown trout (Salmo trutta) from 34 lakes, sampled between 1994-1998 and 2009-2017, were examined for macroscopic signs of PKD (n = 2,151) and the presence of T. bryosalmonae infections (n = 1,424). In the earlier period, 43% of lakes (10/23) harboured T. bryosalmonae -infected fish. The mean prevalence in those lakes was 62.1%, being most common in shallow lowland lakes whilst deeper lakes at high altitudes were all free from infection. Only a single fish from one lake showed macroscopic signs of PKD, a shallow lowland lake in southwestern Iceland. In the latter period, T. bryosalmonae was found in 16/18 lakes studied (89%), with a mean prevalence of 78-79% (excluding T.b. free lakes), being most common in the smaller, shallower lakes at lower alttudes. Macroscopic signs of PKD were observed in 11 of 18 of the lakes studied (61%) with prevalences up to 67%, most common in younger fish inhabiting small shallow lowland lakes. The results indicate that the distribution of T. bryosalmonae and the presence of PKD in Iceland have increased over the last few decades. The disease was almost non-existent in the 1990s but has become very common during the last decade or two. With further water temperature increases, as predicted by climate models, PKD is likely to increasingly affect wild salmonid populations in Iceland.

Gastrointestinal helminths in brown trout (Salmo trutta Linnaeus, 1758) captured in Galician rivers (NW Spain).

Specimens of Salmo trutta (n = 613) captured by local anglers in different rivers in Galicia (NW Spain) during the 2015 fishing season (15 March-15 August) were examined. In total 1479 adult helminths were recovered from the gastrointestinal tracts of 221 fish. Moreover, the microscopic observation of the sediments obtained, previous diphasic concentration, revealed the presence of helminth eggs in 485 trout specimens. The following species were identified by morphological and molecular analysis: Crepidostomum metoecus (8.97%) (Trematoda); Salmonema ephemeridarum (16.97%), Raphidascaris acus (9.46%) and Pseudocapillaria sp. (2.12%) (Nematoda); and Echinorhynchus truttae (8.48%) (Acanthocephala). The prevalence, mean intensity and mean abundance of each helminth species were determined in relation to size/age of the fish. The helminth infracommunity comprised a maximum of four species and the species richness was S = 5. The biological cycles of most of the helminth species recovered are dependent on benthic macroinvertebrate fauna, which, in turn, is influenced by the water quality. Therefore, any changes that take place in the aquatic ecosystem (due to anthropogenic activities or climate change) may be reflected in the helminth composition.

Salmincola markewitschi or S. carpionis (Copepoda: Lernaeopodidae)? A requirement for taxonomic revision due to their high morphological variations.

Salmincola markewitschi Shedko et Shedko, 2002 (Copepoda: Lernaeopodidae) is an ectoparasitic copepod mainly infecting the buccal cavities of white-spotted charr Salvelinus leucomaenis (Pallas) (Salmonidae). This species has only been recorded from Northeast Asia, where a morphologically similar congener Salmincola carpionis (Krøyer, 1837) is also distributed, using the same host species. These copepods are hard to distinguish from each other because of their similarities. We thus examined the newly collected specimens morphologically and genetically from five populations of white-spotted charr in Japan. Most of the specimens were morphologically consistent with S. markewitschi but showed great variations in the numbers of spines on the exopods of the antennae, shape of the maxilliped myxal palps, and the bulla diameter. Consequently, some specimens shared characteristics with S. carpionis. In addition to the mophological continuities, genetic analyses of 28S rDNA and COI mitochondrial DNA confirmed that all specimens belong to a single species. Further taxonomic revisions are required to draw conclusions of whether S. markewitschi is a valid species different from S. carpionis, by collecting samples from across their wide distributional ranges, such as Europe, North America, and Northeast Asia. A key to identification of species of Salmincola Wilson, 1915 occurring in Japan is also provided.

Identification of in vivo induced antigens of the malacosporean parasite Tetracapsuloides bryosalmonae (Cnidaria) using in vivo induced antigen technology.

Tetracapsuloides bryosalmonae is a malacosporean endoparasite that causes proliferative kidney disease (PKD) in wild and farmed salmonids in Europe and North America. The life cycle of T. bryosalmonae completes between invertebrate bryozoan and vertebrate fish hosts. Inside the fish, virulence factors of T. bryosalmonae are induced during infection or interactions with host cells. T. bryosalmonae genes expressed in vivo are likely to be important in fish pathogenesis. Herein, we identify in vivo induced antigens of T. bryosalmonae during infection in brown trout (Salmo trutta) using in vivo induced antigen technology (IVIAT). Brown trout were exposed to the spores of T. bryosalmonae and were sampled at different time points. The pooled sera were first pre-adsorbed with antigens to remove false positive results. Subsequently, adsorbed sera were used to screen a T. bryosalmonae cDNA phage expression library. Immunoscreening analysis revealed 136 immunogenic T. bryosalmonae proteins induced in brown trout during parasite development. They are involved in signal transduction, transport, metabolism, ion-protein binding, protein folding, and also include hypothetical proteins, of so far unknown functions. The identified in vivo induced antigens will be useful in the understanding of T. bryosalmonae pathogenesis during infection in susceptible hosts. Some of the antigens found may have significant implications for the discovery of candidate molecules for the development of potential therapies and preventive measures against T. bryosalmonae in salmonids.

Genome-wide alternative splicing profile in the posterior kidney of brown trout (Salmo trutta) during proliferative kidney disease.

BACKGROUND: The cnidarian myxozoan parasite Tetracapsuloides bryosalmonae causes chronic proliferative kidney disease (PKD) in salmonids. This parasite is a serious threat to wild and cultured salmonids. T. bryosalmonae undergoes intra-luminal sporogonic development in the kidney of brown trout (Salmo trutta) and the viable spores are released via urine. We investigated the alternative splicing pattern in the posterior kidney of brown trout during PKD. RESULTS: RNA-seq data were generated from the posterior kidney of brown trout collected at 12 weeks post-exposure to T. bryosalmonae. Subsequently, this data was mapped to the brown trout genome. About 153 significant differently expressed alternatively spliced (DEAS) genes, (delta PSI = 5%, FDR P-value < 0.05) were identified from 19,722 alternatively spliced events. Among the DEAS genes, the least and most abundant alternative splicing types were alternative 5' splice site (5.23%) and exon skipping (70.59%), respectively. The DEAS genes were significantly enriched for sodium-potassium transporter activity and ion homeostasis (ahcyl1, atp1a3a, atp1a1a.1, and atp1a1a.5). The protein-protein interaction network analysis enriched two local network clusters namely cation transporting ATPase C-terminus and Sodium/potassium ATPase beta chain cluster, and mixed inclusion of Ion homeostasis and EF-hand domain cluster. Furthermore, the human disease-related salmonella infection pathway was significantly enriched in the protein-protein interaction network. CONCLUSION: This study provides the first baseline information about alternative splicing in brown trout during PKD. The generated data lay a foundation for further functional molecular studies in PKD - brown trout infection model. The information generated from the present study can help to develop therapeutic strategies for PKD in the future.

Mitigating human impacts including climate change on proliferative kidney disease in salmonids of running waters.

Over the last two decades, an increasing number of reports have identified a decline in salmonid populations, possibly linked to infection with the parasite Tetracapsuloides bryosalmonae and the corresponding disease, that is, proliferative kidney disease (PKD). The life cycle of this myxozoan parasite includes sessile bryozoan species as invertebrate host, which facilitates the distribution of the parasite in running waters. As the disease outcome is temperature dependent, the impact of the disease on salmonid populations is increasing with global warming due to climate change. The goal of this review is to provide a detailed overview of measures to mitigate the effects of PKD on salmonid populations. It first summarizes the parasite life cycle, temperature-driven disease dynamics and new immunological and molecular research into disease resistance and, based on this, discusses management possibilities. Sophisticated management actions focusing on local adaptation of salmonid populations, restoration of the riverine ecosystem and keeping water temperatures cool are necessary to reduce the negative effects of PKD. Such actions include temporary stocking with PKD-resistant salmonids, as this may assist in conserving current populations that fail to reproduce.

Prevalence of rhabdochoniasis in snow trout of river Swat and river Panjkora, Khyber Pakhtunkhwa province, Pakistan / Prevalência de rabdoconíase em trutas das neves do rio Swat e do rio Panjkora, província de Khyber Pakhtunkhwa, Paquistão

This research was aimed to explore the helminth parasitic diseases in Schizothorax plagiostomus (the snow trout) from river Swat and river Panjkora, Pakistan. Collection of 360 fish specimens have done from the lower, middle and upper reaches of both the rivers through gill nets, cast nets, dragon nets and hooks. All the samples were examined in the University of Malakand, Zoology Department for helminth parasites during the months from January 2015 to December 2016. Of the total examined fish samples 21.9% (n=79) were infected with Rhabdochona spp including 17.7% (n=32/180) in river Swat and 26.6% (n=47/180) in river Panjkora. Highest month-wise prevalence (p=0.09878,p0.05) prevalence (28.8%) than males (16.6%). Fishes of the lower reaches had highest (p=0.0029, P>0.05) prevalence (31.7%) followed by middle reaches (16.5%) while the lowest prevalence was observed in samples of fish collected from upper reaches (9.87%). Present study address that Rhabdochona spp in the intestine of snow trout has a long term relationship and call as a natural infection in cyprinids and zoonotic risk to human.

Esta pesquisa teve como objetivo explorar as doenças parasitárias por helmintos em Schizothorax plagiostomus (a truta das neves) do rio Swat e do rio Panjkora, Paquistão. A coleta de 360 espécimes de peixes foi feita nos trechos inferior, médio e superior de ambos os rios por meio de redes de emalhar, de lançamento, de dragão e anzóis. Todas as amostras foram examinadas na Universidade de Malakand, Departamento de Zoologia, para helmintos parasitas durante os meses de janeiro de 2015 a dezembro de 2016. Do total de amostras de peixes examinadas, 21,9% (n = 79) estavam infectados com Rhabdochona spp, incluindo 17,7% (n = 32/180) no rio Swat e 26,6% (n = 47/180) no rio Panjkora. A maior prevalência no mês (p = 0,09878, p 0,05) prevalência (28,8%) do que o masculino (16,6%). Os peixes do curso inferior tiveram maior (p = 0,0029, P> 0,05) prevalência (31,7%) seguida do curso médio (16,5%), enquanto a menor prevalência foi observada em amostras de peixes coletados do curso superior (9,87%). O presente estudo aborda que Rhabdochona spp no intestino da truta das neves tem uma relação de longo prazo e pode ser considerada uma infecção natural em ciprinídeos e risco zoonótico para humanos.

Morphological and molecular characterization of a novel Myxobolus species from the gastrointestinal tract of brown trout (Salmo trutta) in Spain.

The genus Myxobolus Bütschli, 1882 is the largest group within the class Myxosporea and includes 905 nominal species, 18 of which have been found to infect fish belonging to the family Salmonidae. In the present study, microscopic analysis enabled detection of myxospores in 43 of 613 (7.0%) gastrointestinal tracts from brown trout (Salmo trutta) captured in several rivers in the northwest of the Iberian Peninsula. Measurement of the whole myxospores, polar capsules and other morphological characteristics, together with identification of the site of infection, has led us to propose a novel salmonid-myxobolid species, Myxobolus compostellanus n. sp. Molecular analysis of the small subunit ribosomal RNA (SSU-rRNA) gene yielded the same consensus sequence of 2039 bp in 14 fish specimens. A BLAST search indicated 97.6% similarity to Myxobolus neurobius. Phylogenetic analysis revealed that M. compostellanus n. sp. is clustered with other salmonid-infecting myxobolids. The present findings contribute to the existing knowledge about the genus Myxobolus, providing both morphological and molecular data on a novel species of Myxobolus found to infect the gastrointestinal tract of salmonids, M. compostellanus n. sp. in the brown trout (S. trutta).

Higher mortality of the less suitable brown trout host compared to the principal Atlantic salmon host when infested with freshwater pearl mussel (Margaritifera margaritifera) glochidia.

The freshwater pearl mussel (Margaritifera margaritifera) is a highly host-specific parasite, with an obligate parasitic stage on salmonid fish. Atlantic salmon (Salmo salar) and brown trout (Salmo trutta f. trutta and Salmo trutta f. fario) are the only hosts in their European distribution. Some M. margaritifera populations exclusively infest either Atlantic salmon or brown trout, while others infest both hosts with one salmonid species typically being the principal host and the other a less suitable host. Glochidial abundance, prevalence and growth are often used as parameters to measure host suitability, with the most suitable host species displaying the highest parameters. However, it is not known if the degree of host specialisation will negatively influence host fitness (virulence) among different host species. In this study we examined the hypothesis that glochidial infestation would result in differential virulence in two salmonid host species and that lower virulence would be observed on the most suitable host. Atlantic salmon and brown trout were infested with glochidia from two M. margaritifera populations that use Atlantic salmon as their principal host, and the difference in host mortality among infested and control (sham infested) fish was examined. Higher mortality was observed in infested brown trout (the less suitable host) groups, compared to the other test groups. Genetic assignment was used to identify offspring from individual mother mussels. We found that glochidia from individual mothers can infest both the salmonid hosts; however, some mothers displayed a bias towards either salmon or trout. We believe that the differences in host-dependent virulence and the host bias displayed by individual mothers were a result of genotype × genotype interactions between the glochidia and their hosts, indicating that there is an underlying genetic component for this parasite-host interaction.

Know your enemy - transcriptome of myxozoan Tetracapsuloides bryosalmonae reveals potential drug targets against proliferative kidney disease in salmonids.

The myxozoan Tetracapsuloides bryosalmonae is a widely spread endoparasite that causes proliferative kidney disease (PKD) in salmonid fish. We developed an in silico pipeline to separate transcripts of T. bryosalmonae from the kidney tissue of its natural vertebrate host, brown trout (Salmo trutta). After stringent filtering, we constructed a partial transcriptome assembly T. bryosalmonae, comprising 3427 transcripts. Based on homology-restricted searches of the assembled parasite transcriptome and Atlantic salmon (Salmo salar) proteome, we identified four protein targets (Endoglycoceramidase, Legumain-like protease, Carbonic anhydrase 2, Pancreatic lipase-related protein 2) for the development of anti-parasitic drugs against T. bryosalmonae. Earlier work of these proteins on parasitic protists and helminths suggests that the identified anti-parasitic drug targets represent promising chemotherapeutic candidates also against T. bryosalmonae, and strengthen the view that the known inhibitors can be effective in evolutionarily distant organisms. In addition, we identified differentially expressed T. bryosalmonae genes between moderately and severely infected fish, indicating an increased abundance of T. bryosalmonae sporogonic stages in fish with low parasite load. In conclusion, this study paves the way for future genomic research in T. bryosalmonae and represents an important step towards the development of effective drugs against PKD.

Modulation of local and systemic immune responses in brown trout (Salmo trutta) following exposure to Myxobolus cerebralis.

Myxobolus cerebralis, the etiological agent of Whirling Disease (WD), is a freshwater myxozoan parasite with considerable economic and ecological relevance for salmonids. There are differences in disease susceptibility between species and strains of salmonids. Recently, we have reported that the suppressor of cytokine signaling SOCS1 and SOCS3 are key in modulating rainbow trout (Oncorhynchus mykiss) immune responses and that resistant fish apparently exhibit effective Th17 cell response after exposure to M. cerebralis. It is unclear whether such molecules and pathways are also involved in the immune response of M. cerebralis infected brown trout (Salmo trutta). Hence, this study aimed to explore their role during immune modulation in infected brown trout, which is considered resistant to this parasite. Fish were exposed to the triactinomyxon (TAM) stages of M. cerebralis and quantitative real-time PCR (RT-qPCR) was carried out to examine local (caudal fin) and systemic (head kidney, spleen) immune transcriptional changes associated with WD over time in infected and control fish. All of the immune genes in the three tissues studied were differentially expressed in infected fish at multiple time points. Brown trout reduced the parasite load and demonstrated effective immune responses, likely by keeping pro-inflammatory and anti-inflammatory cytokines in balance whilst stimulating efficient Th17-mediated immunity. This study increases knowledge on the brown trout immune response to M. cerebralis and helps us to understand the underlying mechanisms of WD resistance.

Field study indicating susceptibility differences between salmonid species and their lineages to proliferative kidney disease.

Tetracapsuloides bryosalmonae (Myxozoa: Malacosporea) is the causative agent of proliferative kidney disease (PKD), which affects both wild and farmed salmonid fish. The objective of this study was to outline differences in susceptibility to PKD in different salmonid species, hybrids and breeding lineages. Susceptibility to T. bryosalmonae infection was established based on cumulative mortality, pathological findings and detection of T. bryosalmonae in the kidney using immunohistochemistry and molecular methods. Determination of pure and hybrid individuals of different species in the genus Salvelinus, and dissimilarity of rainbow trout lineages, was performed using traditional polymerase chain reaction (PCR) and microsatellite analyses. Rainbow trout displayed higher disease severity compared with brook trout and Alsatian charr. Moreover, the results indicated differences in infection susceptibility, not only among different salmonid species but also among different lineages of charr and rainbow trout. Our study indicated that some salmonid species and even different lineages of the same species are more suitable for farming under PKD pressure.

On the potential role of Mergus merganser as transport hosts for Tetracapsuloides bryosalmonae.

Transmission paths in the distribution of proliferative kidney disease (PKD) of salmonids are still largely unknown. In this study, the role of goosander (Mergus merganser) as possible transport host for Tetracapsuloides bryosalmonae through faeces was examined. Goosander fledglings were fed exclusively with diseased brown trout (Salmo trutta fario). In all trout used for feeding, intratubular sporogonic stage of the parasite was confirmed histologically. Between one to 10 hours post-feeding, the goosander faeces were sampled and tested for T. bryosalmonae DNA. In qPCR, only DNA fragments were found, and in conventional PCR, no amplification was confirmed. Therefore, we hypothesize that the role of goosander as transport hosts for T. bryosalmonae via their faeces can be neglected.

Morphological and molecular data show no evidence of the proposed replacement of endemic Pomphorhynchus tereticollis by invasive P. laevis in salmonids in southern Germany.

Changes in parasite communities might result in new host-parasite dynamics and may threaten local fish populations. This phenomenon has been suggested for acanthocephalan parasites in the river Rhine and Danube where the species Pomphorhynchus tereticollis is becoming replaced by the Ponto-Caspian P. laevis. Developing knowledge on morphologic, genetic and behavioural differences between such species is important to follow such changes. However, disagreements on the current phylogeny of these two acanthocephalan species are producing conflicts that is affecting their correct identification. This study is offering a clearer morphological and genetic distinction between these two species. As P. tereticollis is found in rhithral tributaries of the Rhine, it was questioned whether the local salmonid populations were hosts for this species and whether P. laevis was expanding into the Rhine watershed as well. In order to test for this, brown trout, Salmo trutta, and grayling, Thymallus thymallus from South-Western Germany watersheds have been samples and screened for the occurrence of acanthocephalan parasites. For the first time, both species were confirmed to be hosts for P. tereticollis in continental Europe. P. tereticollis was found to be common, whereas P. leavis was found only at a single location in the Danube. This pattern suggest either that the expansion of P. laevis through salmonid hosts into rhithral rivers has not yet occurred, or that not yet ascertained biotic or abiotic features of rhithral rivers hinder P. laevis to spread into these areas.

Identification and Expression Profiling of Toll-Like Receptors of Brown Trout (Salmo trutta) during Proliferative Kidney Disease.

Proliferative kidney disease is an emerging disease among salmonids in Europe and North America caused by the myxozoan parasite Tetracapsuloides bryosalmonae. The decline of endemic brown trout (Salmo trutta) in the Alpine streams of Europe is fostered by T. bryosalmonae infection. Toll-like receptors (TLRs) are a family of pattern recognition receptors that acts as sentinels of the immune system against the invading pathogens. However, little is known about the TLRs' response in salmonids against the myxozoan infection. In the present study, we identified and evaluated TLR1, TLR19, and TLR13-like genes of brown trout using data-mining and phylogenetic analysis. The expression pattern of TLRs was examined in the posterior kidney of brown trout infected with T. bryosalmonae at various time points. Typical Toll/interleukin-1 receptor protein domain was found in all tested TLRs. However, TLR13-like chr2 had a short amino acid sequence with no LRR domain. Phylogenetic analysis illustrated that TLR orthologs are conserved across vertebrates. Similarly, a conserved synteny gene block arrangement was observed in the case of TLR1 and TLR19 across fish species. Interestingly, all tested TLRs showed their maximal relative expression from 6 to 10 weeks post-exposure to the parasite. Our results suggest that these TLRs may play an important role in the innate defense mechanism of brown trout against the invading T. bryosalmonae.

Negative associations between parasite avoidance, resistance and tolerance predict host health in salmonid fish populations.

Genetic variation in defence against parasite infections is fundamental for host-parasite evolution. The overall level of defence of a host individual or population includes mechanisms that reduce parasite exposure (avoidance), establishment (resistance) or pathogenicity (tolerance). However, how these traits operate and evolve in concert is not well understood. Here, we investigated genetic variation in and associations between avoidance, resistance and tolerance in a natural host-parasite system. Replicated populations of Atlantic salmon (Salmo salar) and sea trout (an anadromous form of brown trout, Salmo trutta) were raised under common garden conditions and infected with the eye fluke Diplostomum pseudospathaceum. We demonstrate significant genetic variation in the defence traits across host populations and negative associations between the traits, with the most resistant populations showing the weakest avoidance and the lowest infection tolerance. These results are suggestive of trade-offs between different components of defence and possibly underlie the genetic variation in defence traits observed in the wild. Because the three defence mechanisms affect host-parasite evolution in profoundly different ways, we emphasize the importance of studying these traits in concert.