Secretions are central to the ability of the salmon louse Lepeophtheirus salmonis to parasitize salmonid hosts by promoting immunomodulation and feeding. Previous characterizations of secretory and excretory products (SEPs) have relied on pooled individuals, preventing assessment of inter-individual variation and responses to host-derived cues. A novel SEP collection method was developed to obtain samples from individual adult female L. salmonis, enabling evaluation of secretory profiles following exposure to conditioned seawater from a susceptible host, Atlantic salmon (Salmo salar), or a resistant host, coho salmon (Oncorhynchus kisutch). SEP concentrations ranged from 328 to 1597 µg mL⁻¹ , with no effects of host species or conditioning treatment on protein yield. Across 16 individuals, a mean of 101.4 proteins were detected per replicate, including 61.2 secretory proteins, indicating substantial inter-individual variation. Conditioning did not alter protein richness, yet conditioned treatments showed clear qualitative differences in composition. S. salar-conditioned lice uniquely secreted 40 proteins, including proteases, protease inhibitors, C-type lectins, gamma crystallins, and labial gland factors, whereas controls yielded a single unique protein. O. kisutch conditioning was further associated with additional proteases, protease inhibitors, epidermal growth factor-like proteins, and other putative virulence factors. Detection of chitin deacetylase-7 and an LY6/uPAR domain protein across multiple conditions highlights previously uncharacterised candidates relevant to louse-host interactions. In total, these findings establish individual-level SEP collection as a robust and sensitive approach for resolving secretomic diversity in L. salmonis and detecting host-associated modulation of parasite secretory activity obscured by pooled analyses.

This study set out to characterise the in vitro development, including musculature, of the microphallid parasite of the barnacle Semibalanus balanoides (Linnaeus, 1767), Maritrema gratiosum Nicoll, 1907 collected in Scotland. An in vitro culture model was developed to obtain ovigerous adults of M. gratiosum and their morphology was observed. Different media were tested and NCTC-109 was chosen as the best medium. The effects of different concentrations of serum upon adult longevity, size and egg production was measured. Survival for 10-days was achieved when flukes were cultured in NCTC-109 plus chicken serum and antibiotics. Forty percent chicken serum seemed to provide better results in terms of survival time and producing flukes with the largest body lengths. Both normal and abnormal eggs were observed from adults cultured in vitro. Confocal microscopy was undertaken to provide details of the development of the parasite's ultrastructure, including musculature, during the course of in vitro culture. While the musculature of M. gratiosum was similar to that of other microphallids, some additional novel structures were observed, most notably a ligament connecting pars prostatica and seminal vesicle and a racket-shaped excretory bladder. This study has provided greater insight into the biology M. gratiosum, and also developed a good in vitro model which might be applied to ecological or medical research in the future.

The study investigates the susceptibility of Atlantic salmon (Salmo salar) and Pacific salmon species (pink salmon, Oncorhynchus gorbuscha; coho salmon, Oncorhynchus kisutch; and chum salmon, Oncorhynchus keta) to the parasitic salmon lice (Lepeophtheirus salmonis). The research had two main objectives: to characterize the morphology of the scaly skin in four salmonid species and to compare the cellular response at the louse attachment site in coho salmon and Atlantic salmon. Three consecutive challenge trials were conducted, with successful louse infestation only achieved across all four species in the third trial using mild anesthesia with tricaine methanesulfonate. Skin and fin samples were collected at 12, 24, 36, 48, 60, and 168 h post-infestation (hpi) for histological, proteomic, and spatial transcriptomic analyses. Results showed that chum salmon had significantly higher mucous cell coverage (30–40

BackgroundSalmonid species have followed markedly divergent evolutionary trajectories in their interactions with sea lice. While sea lice parasitism poses significant economic, environmental, and animal welfare challenges for Atlantic salmon (Salmo salar) aquaculture, coho salmon (Oncorhynchus kisutch) exhibit near-complete resistance to sea lice, achieved through a potent epithelial hyperplasia response leading to rapid louse detachment. The molecular mechanisms underlying these divergent responses to sea lice are unknown.ResultsWe characterized the cellular and molecular responses of Atlantic salmon and coho salmon to sea lice using single-nuclei RNA sequencing. Juvenile fish were exposed to copepodid sea lice (Lepeophtheirus salmonis), and lice-attached pelvic fin and skin samples were collected 12 h, 24 h, 36 h, 48 h, and 60 h after exposure, along with control samples. Comparative analysis of control and treatment samples revealed an immune and wound-healing response that was common to both species, but attenuated in Atlantic salmon, potentially reflecting greater sea louse immunomodulation. Our results revealed unique but complementary roles of three layers of keratinocytes in the epithelial hyperplasia response leading to rapid sea lice rejection in coho salmon. Our results suggest that basal keratinocytes direct the expansion and mobility of intermediate and, especially, superficial keratinocytes, which eventually encapsulate the parasite.ConclusionsOur results highlight the key role of keratinocytes in coho salmon's sea lice resistance and the diverged biological response of the two salmonid host species when interacting with this parasite. This study has identified key pathways and candidate genes that could be manipulated using various biotechnological solutions to improve Atlantic salmon sea lice resistance.

ABSTRACT Background The skin serves as the first line of defence for an organism against the external environment. Despite the global significance of salmon in aquaculture, a critical component of this first line of defence, mesenchymal stromal cells, remains unexplored. These pluripotent cells can differentiate into various tissues, including bone, cartilage, tendon, ligament, adipocytes, dermis, muscle and connective tissue within the skin. These cells are pivotal for preserving the integrity of skin tissue throughout an organism’s lifespan and actively participate in wound healing processes. Results In this study, we characterise mesenchymal stromal cells in detail for the first time in healthy Atlantic salmon tissue and during the wound healing process. Single-nucleus sequencing and spatial transcriptomics revealed the transcriptional dynamics of these cells, elucidating the differentiation pathways leading to osteogenic and fibroblast lineages in the skin of Atlantic salmon. We charted their activity during an in vivo wound healing time course, showing clear evidence of their active role during this process, as they become transcriptionally more active during the remodelling stage of wound healing. Conclusions For the first time, we chart the activity of sub-clusters of differentiating stromal cells during the process of wound healing, revealing different spatial niches of the various MSC subclusters, and setting the stage for investigations into the manipulation of MSCs to improve fish health.

Abstract Background The salmon louse (Lepeophtheirus salmonis) is a parasite of wild and farmed salmonid fish, causing huge economic damage to the commercial farming of Atlantic salmon (Salmo salar) in the northern hemisphere. The avermectin emamectin benzoate (EMB) is widely used for salmon delousing. While resistance to EMB is widespread in Atlantic populations of L. salmonis, the molecular mechanisms of resistance remain to be elucidated. The aim of the present work was to obtain insights into potential EMB resistance mechanisms by identifying genetic and transcriptomic markers associated with EMB resistance. Results Crosses were performed between EMB-susceptible and -resistant L. salmonis, sourced from two parental strains isolated in Scotland, producing fully pedigreed families. The EMB susceptibility of individual parasites was characterised using time-to-response bioassays. Parasites of two families were subjected to double digest restriction site-associated DNA sequencing (ddRAD-seq) for simultaneous discovery of single nucleotide polymorphisms (SNPs) and genotyping. Data analysis revealed that EMB resistance is associated with one quantitative trait locus (QTL) region on L. salmonis chromosome 5. Marker-trait association was confirmed by genotyping assays for 7 SNPs in two additional families. Furthermore, the transcriptome of male parasites of the EMB-susceptible and -resistant L. salmonis parental strains was assessed. Among eighteen sequences showing higher transcript expression in EMB-resistant as compared to drug-susceptible lice, the most strongly up-regulated gene is located in the above QTL region and shows high homology to β spectrin, a cytoskeleton protein that has roles in neuron architecture and function. Further genes differentially regulated in EMB-resistant lice include a glutathione S-transferase (GST), and genes coding for proteins with predicted roles in mitochondrial function, intracellular signalling or transcription. Conclusions Major determinants of EMB resistance in L. salmonis are located on Chromosome 5. Resistance can be predicted using a limited number of genetic markers. Genes transcriptionally up-regulated in EMB resistant parasites include a β spectrin, a cytoskeletal protein with still incompletely understood roles in neuron structure and function, as well as glutathione S-transferase, an enzyme with potential roles in the biochemical defence against toxicants.

•Chromatin extracellular traps (ETs) are released in vitro by rainbow trout neutrophils.•Studies on the antimicrobial actions of ETs released by fish are scarce.•Rainbow trout ETs released in vitro were co-incubated with Vibrio anguillarum.•The ETs decreased bacterial colony-forming units.•The ETs have antimicrobial properties and may function as part of the innate response.

Infection with salmonid alphavirus (SAV) was assessed according to the criteria of the Animal Health Law (AHL), in particular the criteria of Article 7 on disease profile and impacts, Article 5 on its eligibility to be listed, Annex IV for its categorisation according to disease prevention and control rules as laid out in Article 9 and Article 8 for listing animal species related to infection with SAV. The assessment was performed following the ad hoc method on data collection and assessment developed by AHAW Panel and already published. The outcome reported is the median of the probability ranges provided by the experts, which indicates whether each criterion is fulfilled (lower bound >= 66%) or not (upper bound <= 33%), or whether there is uncertainty about fulfilment. Reasoning points are reported for criteria with an uncertain outcome. According to the assessment, it was uncertain whether infection with salmonid alphavirus can be considered eligible to be listed for Union intervention according to Article 5 of the AHL (50-80% probability). According to the criteria in Annex IV, for the purpose of categorisation related to the level of prevention and control as in Article 9 of the AHL, the AHAW Panel concluded that infection with salmonid alphavirus does not meet the criteria in Section 1 (Category A; 5-10% probability of meeting the criteria) and it is uncertain whether it meets the criteria in Sections 2, 3, 4 and 5 (Categories B, C, D and E; 50-90%, probability of meeting the criteria). The animal species to be listed for infection with SAV according to Article 8 criteria are provided.

Abstract Infectious pancreatic necrosis (IPN) was assessed according to the criteria of the Animal Health Law (AHL), in particular, the criteria of Article 7 on disease profile and impacts, Article 5 on its eligibility to be listed, Annex IV for its categorisation according to disease prevention and control rules as in Article 9, and Article 8 for listing animal species related to IPN. The assessment was performed following a methodology previously published. The outcome reported is the median of the probability ranges provided by the experts, which indicates whether each criterion is fulfilled (lower bound ≥ 66%) or not (upper bound ≤ 33%), or whether there is uncertainty about fulfilment. Reasoning points are reported for criteria with an uncertain outcome. According to the assessment here performed, it is uncertain whether IPN can be considered eligible to be listed for Union intervention according to Article 5 of the AHL (50–90% probability). According to the criteria in Annex IV, for the purpose of categorisation related to the level of prevention and control as in Article 9 of the AHL, the AHAW Panel concluded that IPN does not meet the criteria in Section 1 (Category A; 0–1% probability of meeting the criteria) and it is uncertain whether it meets the criteria in Sections 2, 3, 4 and 5 (Categories B, C, D and E; 33–66%, 33–66%, 50–90% and 50–99% probability of meeting the criteria, respectively). The animal species to be listed for IPN according to Article 8 criteria are provided.

Spring Viraemia of Carp (SVC) was assessed according to the criteria of the Animal Health Law (AHL), in particular the criteria of Article 7 on disease profile and impacts, Article 5 on its eligibility to be listed, Annex IV for its categorisation according to disease prevention and control rules as in Article 9 and Article 8 for listing animal species related to SVC. The assessment was performed following the ad hoc method for data collection and assessment previously developed by the AHAW panel and already published. The outcome reported is the median of the probability ranges provided by the experts, which indicates whether each criterion is fulfilled (lower bound >= 66%) or not (upper bound <= 33%), or whether there is uncertainty about fulfilment. Reasoning points are reported for criteria with an uncertain outcome. According to the assessment performed here, it is uncertain whether SVC can be considered eligible to be listed for Union intervention according to Article 5 of the AHL (45-90% probability). According to the criteria in Annex IV, for the purpose of categorisation related to the level of prevention and control as in Article 9 of the AHL, the AHAW Panel concluded that SVC does not meet the criteria in Section 1 (Category A; 5-33% probability of meeting the criteria) and it is uncertain whether it meets the criteria in Sections 2, 3, 4 and 5 (Categories B, C, D and E; 33-66%, 10-66%, 45-90% and 45-90% probability of meeting the criteria, respectively). The animal species to be listed for SVC according to Article 8 criteria are provided.

Infection with Gyrodactylus salaris was assessed according to the criteria of the Animal Health Law (AHL), in particular, the criteria of Article 7 on disease profile and impacts, Article 5 on its eligibility to be listed, Annex IV for its categorisation according to disease prevention and control rules as laid down in Article 9 and Article 8 for listing animal species related to infection with G. salaris. The assessment was performed following the ad hoc method for data collection and assessment previously developed by AHAW panel and already published. The outcome reported is the median of the probability ranges provided by the experts, which indicates whether each criterion is fulfilled (lower bound >= 66%) or not (upper bound <= 33%), or whether there is uncertainty about fulfilment. Reasoning points are reported for criteria with an uncertain outcome. According to the assessment here performed, it is uncertain whether infection with G. salaris can be considered eligible to be listed for Union intervention according to Article 5 of the AHL (33-70% probability). According to the criteria in Annex IV, for the purpose of categorisation related to the level of prevention and control as in Article 9 of the AHL, the AHAW Panel concluded that Infection with G. salaris does not meet the criteria in Section 1 and 3 (Category A and C; 1-5% and 10-33% probability of fulfilling the criteria, respectively) and it is uncertain whether it meets the criteria in Sections 2, 4 and 5 (Categories B, D and E; 33-80%, 33-66% and 33-80% probability of meeting the criteria, respectively). The animal species to be listed for infection with G. salaris according to Article 8 criteria are provided.

Bacterial kidney disease (BKD) was assessed according to the criteria of the Animal Health Law (AHL), in particular the criteria of Article 7 on disease profile and impacts, Article 5 on its eligibility to be listed, Annex IV for its categorisation according to disease prevention and control rules as laid out in Article 9 and Article 8 for listing animal species related to BKD. The assessment was performed following the ad hoc method on data collection and assessment developed by AHAW Panel and already published. The outcome reported is the median of the probability ranges provided by the experts, which indicates whether each criterion is fulfilled (lower bound <= 66%) or not (upper bound >= 33%), or whether there is uncertainty about fulfilment. Reasoning points are reported for criteria with an uncertain outcome. According to this assessment, BKD can be considered eligible to be listed for Union intervention according to Article 5 of the AHL (66-90% probability). According to the criteria in Annex IV, for the purpose of categorisation related to the level of prevention and control as in Article 9 of the AHL, the AHAW Panel concluded that BKD does not meet the criteria in Sections 1, 2 and 3 (Categories A, B and C; 1-5%, 33-66% and 33-66% probability of meeting the criteria, respectively) but meets the criteria in Sections 4 and 5 (Categories D and E; 66-90% and 66-90% probability of meeting the criteria, respectively). The animal species to be listed for BKD according to Article 8 criteria are provided.

Current treatment strategies for relevant infectious diseases in Atlantic salmon (Salmo salar L.) include the use of low salinity or freshwater bathing. However, often availability is restricted, and hydrogen peroxide (H2O2) is used as an alternative. The potential impacts of H2O2 on fish mucosal tissues, especially the gills therefore need to be considered. In this study the mucosal and immunological effects of H2O2 treatment on the gills of healthy Atlantic salmon were examined by gene expression (qPCR) and immunohistochemistry (IHC) investigating T-cell, B-cell, and mucin activity. Healthy fish were treated with H2O2 and sampled at different times: 4 h, 24 h and 14 days post-H2O2 treatment (dpt) (total n = 18) to investigate the effect of holding time and H2O2 treatment. Treatment with H2O2 resulted in up-regulation of markers for T-cell activity and anti-inflammatory response and down-regulation of mucin expression in the gills at 14 dpt compared to fish sampled prior to treatment (0h; n = 5 fish). These findings were supported by IHC analysis, which despite being highly variable between samples, showed an increase in the number of CD3(+) T cells at 14 dpt in 50% of treated fish compared to pre-treatment fish. The results from this study suggest that H2O2 treatment does not immune compromise healthy Atlantic salmon after 14 dpt (i.e., post-recovery) but modulates gill immune activity and disrupts the mucus covering of the gills. However, further studies are required to determine whether the effects observed are related to H2O2 treatment in isolation or other variables such as holding time or environmental factors.

The microsporidian Desmozoon lepeophtherii Freeman and Sommerville, 2009 is considered significant in the pathogenesis of gill disease in Atlantic salmon (Salmo salar Linnaeus, 1758). Due to the difficulty in detecting D. lepeophtherii in tissue sections, infections are normally diagnosed by molecular methods, routine haematoxylin and eosin (H&E) stained gill tissue sections and the use of other histochemical stains and labels to confirm the presence of spores. An in situ hybridization (ISH) protocol specific for D. lepeophtherii was developed using DIG-labelled oligonucleotide probes. Diseased Atlantic salmon gills were analysed by ISH, calcofluor white (CW) and H&E. All methods showed high levels of specificity (100%) in their ability to detect D. lepeophtherii, but the sensitivity was higher with ISH (92%), compared with CW (64%) and the presence of microvesicles on H&E stained sections (52%). High levels of D. lepeophtherii spores were significantly associated (p < .05) with the development of D. lepeophtherii-associated pathology in the gills, with Ct values below 19 and over 100 microsporidia/10 mm(2) of gill tissue (from the ISH counts) seemingly necessary for the development of microvesicles. The ISH method has the advantage over other histological techniques in that it allows all life stages of the microsporidian to be detected in infected salmon gill tissue sections.

The pyrethroid deltamethrin (DTM) is used to treat Atlantic salmon ( Salmo salar ) against salmon louse ( Lepeophtheirus salmonis ) infestations. However, DTM resistance has evolved in L. salmonis and is currently common in the North Atlantic. This study aimed to re-assess the association between DTM resistance and mitochondrial (mtDNA) mutations demonstrated in previous reports. Among 218 L. salmonis collected in Scotland in 2018–2019, 89.4% showed DTM resistance in bioassays, while 93.6% expressed at least one of four mtDNA single nucleotide polymorphisms (SNPs) previously shown to be resistance associated. Genotyping at further 14 SNP loci allowed to define three resistance-associated mtDNA haplotypes, named 2, 3 and 4, occurring in 72.0%, 14.2% and 7.3% of samples, respectively. L. salmonis strains IoA-02 (haplotype 2) and IoA-10 (haplotype 3) both showed high levels (~ 100-fold) of DTM resistance, which was inherited maternally in crossing experiments. MtDNA haplotypes 2 and 3 differed in genotype for 17 of 18 studied SNPs, but shared one mutation that causes an amino acid change (Leu107Ser) in the cytochrome c oxidase subunit 1 (COX1) and was present in all DTM resistant while lacking in all susceptible parasites. We conclude that Leu107Ser (COX1) is a main genetic determinant of DTM resistance in L. salmonis.

Complex gill disorder (CGD) is an important condition in Atlantic salmon aquaculture, but the roles of the putative aetiological agents in the pathogenesis are uncertain. A longitudinal study was undertaken on two salmon farms in Scotland to determine the variations in loads of CGD-associated pathogens (Desmozoon lepeophtherii, Candidatus Branchiomonas cysticola, salmon gill pox virus (SGPV) and Neoparamoeba perurans) estimated by quantitative PCR. In freshwater, Ca. B. cysticola and SGPV were detected in both populations, but all four pathogens were detected on both farms during the marine stage. Candidatus B. cysticola and D. lepeophtherii were detected frequently, with SGPV detected sporadically. In the marine phase, increased N. perurans loads associated significantly (p < 0.05) with increases in semi-quantitative histological gill-score (HGS). Increased Ca. B. cysticola load associated significantly (p < 0.05) with increased HGS when only Farm B was analysed. Higher loads of D. lepeophtherii were associated significantly (p < 0.05) with increased HGS on Farm B despite the absence of D. lepeophtherii-type microvesicles. Variations in SGPV were not associated significantly (p > 0.05) with changes in HSG. This study also showed that water temperature (season) and certain management factors were associated with higher HGS. This increase in histological gill lesions will have a deleterious impact on fish health and welfare, and production performance.

Amyloodiniosis is a disease resulting from infestation by the ectoparasitic dinoflagellate Amyloodinium ocellatum (AO) and is a threat for fish species such as European sea bass (ESB, Dicentrarchus labrax), which are farmed in lagoon and land-based rearing sites. During the summer, when temperatures are highest, mortality rates can reach 100%, with serious impacts for the aquaculture industry. As no effective licensed therapies currently exist, this study was undertaken to improve knowledge of the biology of AO and of the host-parasite relationship between the protozoan and ESB, in order to formulate better prophylactic/therapeutic treatments targeting AO. To achieve this, a multi-modal study was performed involving a broad range of analytical modalities, including conventional histology (HIS), immunohistochemistry (IHC) and confocal laser scanning microscopy (CLSM). Gills and the oro-pharyngeal cavity were the primary sites of amyloodiniosis, with hyperplasia and cell degeneration more evident in severe infestations (HIS). Plasmacells and macrophages were localised by IHC and correlated with the parasite burden in a time-course experimental challenge. CLSM allowed reconstruction of the 3D morphology of infecting trophonts and suggested a protein composition for its anchoring and feeding structures. These findings provide a potential starting point for the development of new prophylactic/therapeutic controls.

Disease and parasitism cause major welfare, environmental and economic concerns for global aquaculture. In this review, we examine the status and potential of technologies that exploit genetic variation in host resistance to tackle this problem. We argue that there is an urgent need to improve understanding of the genetic mechanisms involved, leading to the development of tools that can be applied to boost host resistance and reduce the disease burden. We draw on two pressing global disease problems as case studies-sea lice infestations in salmonids and white spot syndrome in shrimp. We review how the latest genetic technologies can be capitalised upon to determine the mechanisms underlying inter- and intra-species variation in pathogen/parasite resistance, and how the derived knowledge could be applied to boost disease resistance using selective breeding, gene editing and/or with targeted feed treatments and vaccines. Gene editing brings novel opportunities, but also implementation and dissemination challenges, and necessitates new protocols to integrate the technology into aquaculture breeding programmes. There is also an ongoing need to minimise risks of disease agents evolving to overcome genetic improvements to host resistance, and insights from epidemiological and evolutionary models of pathogen infestation in wild and cultured host populations are explored. Ethical issues around the different approaches for achieving genetic resistance are discussed. Application of genetic technologies and approaches has potential to improve fundamental knowledge of mechanisms affecting genetic resistance and provide effective pathways for implementation that could lead to more resistant aquaculture stocks, transforming global aquaculture.

The relative rarity of the planktonic larval stages of salmon lice in comparison to other animals captured in a zooplankton assemblage is an obstacle to estimating their abundance and distribution. Due to the labour intensiveness of standard plankton sorting approaches, the planktonic stages of salmon lice remain understudied and unmonitored despite their importance to the spread of the parasite between salmon farms and to wild salmonids. Alternative methods of identification have been investigated and in a previous study a fluorescence signal was identified. Using filters to target that signal with fluorescence microscopy (excitation/emission wavelengths of 470/525 nm), the salmon louse has a fluorescence intensity 2.4 times greater than non-target animals, which distinguishes it from the zooplankton assemblage and enables rapid enumeration. Here, we present a novel method for the enumeration of planktonic salmon lice larvae, nauplius and copepodid stages, in a mixed zooplankton sample using fluorescence-aided microscopy. Performance of the method was evaluated with a blind trial which found a median accuracy of 81.8% and a mean sample processing time of 31 min. Compared with previously published findings, the novel method provides satisfactory accuracy and enumeration that is more than 20 times faster than traditional light microscopy approaches. Factors influencing the performance of the method are identified and recommendations are made for targeted sampling and automated enumeration.

Gyrodactylus salaris Malmberg, 1957 is an OIE (Office International des Epizooties)-listed parasitic pathogen and had until the current study been reported from 19 countries across Europe, although many of these records require confirmation. The last comprehensive evaluation regarding the distribution of G. salaris, however, was made in 2007, although some of the states identified as being G. salaris-positive were ascribed this status based on misidentifications, on partial data resulting from either morphological or molecular tests, or from records that have not been revisited since their early reporting. It is thus important to go through the reports on G. salaris to obtain a status for each country. To provide a revised update of the G. salaris distribution, a literature review was necessary. This literature, however, was not always readily accessible and, in certain cases, the article only made superficial reference to the parasite without providing details or data to support the identification. In most cases, the original specimens were not deposited in a national collection. Additional Gyrodactylus material for the current study was obtained from selected salmonid populations with the aim to contribute to current understanding regarding the distribution of G. salaris. Additional parasite material collected for this study was processed following standard procedures for species identification in Gyrodactylus [1]. From the work conducted in the current study, G. salaris is reported from a further three regions in Italy, alongside three other species, and appears to occur extensively throughout central Italy without causing significant mortalities to its rainbow trout, Oncorhynchus mykiss (Walbaum), host. The analysis of archive material from G. salaris-positive farms would suggest that G. salaris has been in this country since at least 2000. Material obtained from rainbow trout from Finland and Germany are confirmed as G. salaris, supporting existing data for these countries. No specimens of G. salaris, however, were found in the additional Gyrodactylus material obtained from rainbow trout reared in Portugal and Spain. A morphologically similar species, Gyrodactylus teuchis Lautraite, Blanc, Thiery, Daniel et Vigneulle, 1999, however, was found. Following the present review, Gyrodactylus salaris is reported from 23 out of 50 recognised states throughout Europe; only records from 14 of these states have been confirmed by either morphology and/or by an appropriate molecular test and are considered valid, while only nine of these records have been confirmed by a combination of both methods.