Pseudohepatospora borealis n. gen. n. sp. (Microsporidia: Enterocytozoonida): A microsporidian pathogen of the Jonah crab (Cancer borealis).

The microsporidian diversity catalogued so far has resulted in the development of several taxonomic groups, one of which is the Enterocytozoonida - a group of generalist 'ultimate opportunists', which infect many fished and aquacultured animals, as well as a broad suite of host taxa, including humans. In this study, we provide phylogenetic, ultrastructural, developmental, and pathological evidence for the creation of a new genus and species to hold a microsporidian parasite of the Jonah crab, Cancer borealis. Cancer borealis represents a species of commercial interest and has become the target of a recently developed fishery on the USA and Canadian Atlantic coast. This species was found to harbour a microsporidian parasite that develops in the cytoplasm of alpha and beta cells of the hepatopancreas. We retrieved a 937 bp fragment of the parasite SSU region, alongside developmental and ultrastructural data that suggests this species is âˆ¼ 87 % similar to Parahepatospora carcini and develops in a similar manner in direct association with the host cell cytoplasm. The mature spores are ovoid in shape and measure 1.48 ± 0.15 µm (SD) in length and 1.00 ± 0.11 µm (SD) in width. Phylogenetically, the new parasite clades in the Enterocytozoonida on the same branch as P. carcini. We provide a new genus and species to hold the parasite: Pseudohepatospora borealis n. gen. n. sp. (Microsporidia: Enterocytozoonida) and explore the likelihood that this species may fit into the Hepatoporidae family.

Salinity and temperature affect the symbiont profile and host condition of Florida USA blue crabs Callinectes sapidus.

Subtropical Florida blue crabs, Callinectes sapidus, exhibit differing life history traits compared to their temperate counterparts, likely influencing symbiont infection dynamics. Little information exists for Florida C. sapidus symbiont profiles, their distribution among various habitats, and influence on crab condition. Using histopathology, genomics, and transmission electron microscopy, we describe the first symbiont profiles for Florida C. sapidus occupying freshwater to marine habitats. Twelve symbiont groups were identified from 409 crabs including ciliophorans, digenean, microsporidian, Haplosporidia, Hematodinium sp., Nematoda, filamentous bacteria, gregarine, Callinectes sapidus nudivirus, Octolasmis sp., Cambarincola sp., and putative microcell. Overall, 78% of C. sapidus were documented with one or more symbiont groups demonstrating high infection rates in wild populations. Environmental variables water temperature and salinity explained 48% of the variation in symbiont groups among Florida habitats, and salinity was positively correlated with C. sapidus symbiont diversity. This suggests freshwater C. sapidus possess fewer symbionts and represent healthier individuals compared to saltwater populations. Crab condition was examined using the reflex action mortality predictor (RAMP) to determine if reflex impairment could be linked to symbiont prevalence. Symbionts were found positively correlated with crab condition, and impaired crabs were more likely to host symbionts, demonstrating symbiont inclusion may boost predictive ability of the RAMP application. The microsporidian symbiont group had a particularly strong effect on C. sapidus reflex response, and impairment was on average 1.57 times higher compared to all other symbiont groups. Our findings demonstrate the importance of considering full symbiont profiles and their associations with a spatially and temporally variable environment to fully assess C. sapidus population health.

Discovery of the parasite Marteilia cocosarum sp. nov. In common cockle (Cerastoderma edule) fisheries in Wales, UK and its comparison with Marteilia cochillia.

Diseases of bivalve molluscs caused by paramyxid parasites of the genus Marteilia have been linked to mass mortalities and the collapse of commercially important shellfish populations. Until recently, no Marteilia spp. have been detected in common cockle (Cerastoderma edule) populations in the British Isles. Molecular screening of cockles from ten sites on the Welsh coast indicates that a Marteilia parasite is widespread in Welsh C. edule populations, including major fisheries. Phylogenetic analysis of ribosomal DNA (rDNA) gene sequences from this parasite indicates that it is a closely related but different species to Marteilia cochillia, a parasite linked to mass mortality of C. edule fisheries in Spain, and that both are related to Marteilia octospora, for which we provide new rDNA sequence data. Preliminary light and transmission electron microscope (TEM) observations support this conclusion, indicating that the parasite from Wales is located primarily within areas of inflammation in the gills and the connective tissue of the digestive gland, whereas M. cochillia is found mainly within the epithelium of the digestive gland. The impact of infection by the new species, here described as Marteilia cocosarum n. sp., upon Welsh fisheries is currently unknown.

High prevalence of Paramarteilia canceri infecting velvet swimming crabs Necora puber in Ireland.

The velvet swimming crab Necora puber has been fished in Ireland since the early 1980s and contributes significant income to smaller fishing vessels. From 2016 onwards, reduced landings have been reported. We undertook a full pathological investigation of crabs from fishing grounds at 3 sites on the west (Galway), southwest (Castletownbere) and east (Howth) coasts of Ireland. Histopathology, transmission electron microscopy and molecular taxonomic and phylogenetic analyses showed high prevalence and infection level of Paramarteilia canceri, previously only reported from the edible crab Cancer pagurus. This study provides the first molecular data for P. canceri, and shows its phylogenetic position in the order Paramyxida (Rhizaria). Other parasites and symbionts detected in the crabs were also noted, including widespread but low co-infection with Hematodinium sp. and a microsporidian consistent with the Ameson and Nadelspora genera. This is the first histological record of Hematodinium sp. in velvet crabs from Ireland. Four N. puber individuals across 2 sites were co-infected by P. canceri and Hematodinium sp. At one site, 3 velvet crabs infected with P. canceri were co-infected with the first microsporidian recorded from this host; the microsporidian 18S sequence was almost identical to Ameson pulvis, known to infect European shore crabs Carcinus maenas. The study provides a comprehensive phylogenetic analysis of this and all other available Ameson and Nadelspora 18S sequences. Together, these findings provide a baseline for further investigations of N. puber populations along the coast of Ireland.

How do abiotic environmental conditions influence shrimp susceptibility to disease? A critical analysis focussed on White Spot Disease.

White Spot Syndrome Virus (WSSV) causes White Spot Disease (WSD) and is historically the most devastating disease in the shrimp industry. Global losses from this disease have previously exceeded $3 bn annually, having a major impact on a global industry worth US$19 bn per annum. Shrimp are cultured predominantly in enclosed ponds that are subject to considerable fluctuations in abiotic conditions and WSD outbreaks are increasingly linked to periods of extreme weather, which may cause major fluctuations in pond culture conditions. Combined with the intensity of production in these systems, the resulting suboptimal physicochemical conditions have a major bearing on the susceptibility of shrimp to infection and disease. Current knowledge indicates that pond temperature and salinity are major factors determining outbreak severity. WSSV appears to be most virulent in water temperatures between 25 and 28 °C and salinities far removed from the isoosmotic point of shrimp. Elevated temperatures (>30 °C) may protect against WSD, depending on the stage of infection, however the mechanisms mediating this effect have not been well established. Other factors relating to water quality that may play key roles in determining outbreak severity include dissolved oxygen concentration, nitrogenous compound concentration, partial pressure of carbon dioxide and pH, but data on their impacts on WSSV susceptibility in cultured shrimps is scarce. This illustrates a major research gap in our understanding of the influence of environmental conditions on disease. For example, it is not clear whether temperature manipulations can be used effectively to prevent or mitigate WSD in cultured shrimp. Therefore, developing our understanding of the impact of environmental conditions on shrimp susceptibility to WSSV may provide insight for WSD mitigation when, even after decades of research, there is no effective practical prophylaxis or treatment.

Patterns of infection in a native and an invasive crayfish across the UK.

Invasive crayfish and the introduction of non-native diseases pose a significant risk for the conservation of endangered, white-clawed crayfish (Austropotamobius pallipes). Continued pollution of waterways is also of concern for native species and may be linked with crayfish disease dynamics. We explore whether crayfish species or environmental quality are predictors of infection presence and prevalence in native A. pallipes and invasive signal crayfish (Pacifastacus leniusculus). We use a seven-year dataset of histology records, and a field survey comparing the presence and prevalence of infectious agents in three isolated A. pallipes populations; three isolated P. leniusculus populations, and three populations where the two species had overlapped in the past. We note a lower diversity of parasites (Simpson's Index) in P. leniusculus ('Pacifastacus leniusculus Bacilliform Virus' - PlBV) (n = 1 parasite) relative to native A. pallipes (n = 4 parasites), which host Thelohania contejeani, 'Austropotamobius pallipes bacilliform virus' (ApBV), Psorospermium haeckeli and Branchiobdella astaci, at the sites studied. The infectious group present in both species was an intranuclear bacilliform virus of the hepatopancreas. The prevalence of A. astaci in A. pallipes populations was higher in more polluted water bodies, which may reflect an effect of water quality, or may be due to increased chance of transmission from nearby P. leniusculus, a species commonly found in poor quality habitats.

Development and application of a duplex PCR assay for detection of Crangon crangon bacilliform virus in populations of European brown shrimp (Crangon crangon).

Crangon crangon bacilliform virus (CcBV) was first discovered in 2004 in European brown shrimp (Crangon crangon) caught along the English coast. This study describes a duplex PCR assay developed for the detection of CcBV, based on amplification of the lef-8 gene (211 bp) of CcBV and the E75 gene (105 bp) of C. crangon as an internal amplification control. The lef-8 and E75 primer pairs were designed based on preliminary genome sequencing information of the virus and transcriptomic data available for C. crangon, respectively. Sequencing of the resulting amplicons confirmed the specificity of this PCR assay and sequence analysis of the lef-8 fragment revealed amino acid identity percentages ranging between 31 and 42% with members of the Nudiviridae, proposing that CcBV may reside within this family. Finally, the duplex PCR assay was applied to samples of C. crangon hepatopancreas tissue collected along the Belgian coast to screen for the presence of CcBV. The prevalence of CcBV averaged 87%, which is comparable to previous reports of high prevalence, based upon histological analysis, in shrimp collected along the English coast. Development of a specific and sensitive PCR assay to detect CcBV will provide a useful tool for future aquaculture and research programs involving C. crangon.

Green crab Carcinus maenas symbiont profiles along a North Atlantic invasion route.

The green crab Carcinus maenas is an invader on the Atlantic coast of Canada and the USA. In these locations, crab populations have facilitated the development of a legal fishery in which C. maenas is caught and sold, mainly for use as bait to capture economically important crustaceans such as American lobster Homarus americanus. The paucity of knowledge on the symbionts of invasive C. maenas in Canada and their potential for transfer to lobsters poses a potential risk of unintended transmission. We carried out a histological survey for symbionts of C. maenas from their native range in Northern Europe (in the UK and Faroe Islands), and invasive range in Atlantic Canada. In total, 19 separate symbiotic associations were identified from C. maenas collected from 27 sites. These included metazoan parasites (nematodes, Profilicollis botulus, Sacculina carcini, Microphallidae, ectoparasitic crustaceans), microbial eukaryotes (ciliates, Hematodinium sp., Haplosporidium littoralis, Ameson pulvis, Parahepatospora carcini, gregarines, amoebae), bacteria (Rickettsia-like organism, milky disease), and viral pathogens (parvo-like virus, herpes-like virus, iridovirus, Carcinus maenas bacilliform virus and a haemocyte-infecting rod-shaped virus). Hematodinium sp. were not observed in the Canadian population; however, parasites such as Trematoda and Acanthocephala were present in all countries despite their complex, multi-species lifecycles. Some pathogens may pose a risk of transmission to other decapods and native fauna via the use of this host in the bait industry, such as the discovery of a virus resembling the previously described white spot syndrome virus (WSSV), B-virus and 'rod-shaped virus' (RV-CM) and amoebae, which have previously been found to cause disease in aquaculture (e.g. Salmo salar) and fisheries species (e.g. H. americanus).

Decay of the glycolytic pathway and adaptation to intranuclear parasitism within Enterocytozoonidae microsporidia.

Glycolysis and oxidative phosphorylation are the fundamental pathways of ATP generation in eukaryotes. Yet in microsporidia, endoparasitic fungi living at the limits of cellular streamlining, oxidative phosphorylation has been lost: energy is obtained directly from the host or, during the dispersive spore stage, via glycolysis. It was therefore surprising when the first sequenced genome from the Enterocytozoonidae - a major family of human and animal-infecting microsporidians - appeared to have lost genes for glycolysis. Here, we sequence and analyse genomes from additional members of this family, shedding new light on their unusual biology. Our survey includes the genome of Enterocytozoon hepatopenaei, a major aquacultural parasite currently causing substantial economic losses in shrimp farming, and Enterospora canceri, a pathogen that lives exclusively inside epithelial cell nuclei of its crab host. Our analysis of gene content across the clade suggests that Ent. canceri's adaptation to intranuclear life is underpinned by the expansion of transporter families. We demonstrate that this entire lineage of pathogens has lost glycolysis and, uniquely amongst eukaryotes, lacks any obvious intrinsic means of generating energy. Our study provides an important resource for the investigation of host-pathogen interactions and reductive evolution in one of the most medically and economically important microsporidian lineages.

Reprint of 'Diseases in marine invertebrates associated with mariculture and commercial fisheries'.

Diseases in marine invertebrates are increasing in both frequency and intensity around the globe. Diseases in individuals which offer some commercial value are often well documented and subsequently well studied in comparison to those wild groups offering little commercial gain. This is particularly the case with those associated with mariculture or the commercial fisheries. Specifically, these include many Holothuroidea, and numerous crustacea and mollusca species. Pathogens/parasites consisting of both prokaryotes and eukaryotes from all groups have been associated with diseases from such organisms, including bacteria, viruses, fungi and protozoa. Viral pathogens in particular, appear to be an increasingly important group and research into this group will likely highlight a larger number of diseases and pathogens being described in the near future. Interestingly, although there are countless examples of the spread of disease usually associated with transportation of specific infected hosts for development of aquaculture practices, this process appears to be continuing with no real sign of effective management and mitigation strategies being implicated. Notably, even in well developed countries such as the UK and the US, even though live animal trade may be well managed, the transport of frozen food appears to be less well so and as evidence suggests, even these to have the potential to transmit pathogens when used as a food source for example.

De novo assembly of the Carcinus maenas transcriptome and characterization of innate immune system pathways.

BACKGROUND: The European shore crab, Carcinus maenas, is used widely in biomonitoring, ecotoxicology and for studies into host-pathogen interactions. It is also an important invasive species in numerous global locations. However, the genomic resources for this organism are still sparse, limiting research progress in these fields. To address this resource shortfall we produced a C. maenas transcriptome, enabled by the progress in next-generation sequencing technologies, and applied this to assemble information on the innate immune system in this species. RESULTS: We isolated and pooled RNA for twelve different tissues and organs from C. maenas individuals and sequenced the RNA using next generation sequencing on an Illumina HiSeq 2500 platform. After de novo assembly a transcriptome was generated encompassing 212,427 transcripts (153,699 loci). The transcripts were filtered, annotated and characterised using a variety of tools (including BLAST, MEGAN and RSEM) and databases (including NCBI, Gene Ontology and KEGG). There were differential patterns of expression for between 1,223 and 2,741 transcripts across tissues and organs with over-represented Gene Ontology terms relating to their specific function. Based on sequence homology to immune system components in other organisms, we show both the presence of transcripts for a series of known pathogen recognition receptors and response proteins that form part of the innate immune system, and transcripts representing the RNAi, Toll-like receptor signalling, IMD and JAK/STAT pathways. CONCLUSIONS: We have produced an assembled transcriptome for C. maenas that provides a significant molecular resource for wide ranging studies in this species. Analysis of the transcriptome has revealed the presence of a series of known targets and functional pathways that form part of their innate immune system and illustrate tissue specific differences in their expression patterns.

Diseases in marine invertebrates associated with mariculture and commercial fisheries.

Diseases in marine invertebrates are increasing in both frequency and intensity around the globe. Diseases in individuals which offer some commercial value are often well documented and subsequently well studied in comparison to those wild groups offering little commercial gain. This is particularly the case with those associated with mariculture or the commercial fisheries. Specifically, these include many Holothuroidea, and numerous crustacea and mollusca species. Pathogens/parasites consisting of both prokaryotes and eukaryotes from all groups have been associated with diseases from such organisms, including bacteria, viruses, fungi and protozoa. Viral pathogens in particular, appear to be an increasingly important group and research into this group will likely highlight a larger number of diseases and pathogens being described in the near future. Interestingly, although there are countless examples of the spread of disease usually associated with transportation of specific infected hosts for development of aquaculture practices, this process appears to be continuing with no real sign of effective management and mitigation strategies being implicated. Notably, even in well developed countries such as the UK and the US, even though live animal trade may be well managed, the transport of frozen food appears to be less well so and as evidence suggests, even these to have the potential to transmit pathogens when used as a food source for example.

Isolation and culture of Sphaerothecum destruens from Sunbleak (Leucaspius delineatus) in the UK and pathogenicity experiments in Atlantic salmon (Salmo salar).

The sunbleak (Leucaspius delineatus), a cyprinid fish native to continental Europe and now established in the UK, is experiencing population decline which appears to be linked to the spread of the invasive Asian cyprinid (Pseudorasbora parva). A population of sunbleak in the UK has previously been identified as infected with S. destruens at low prevalence. Because Sphaerothaecum destruens has, on occasion, caused severe disease in cultured and wild salmonids the aim of this work was to establish laboratory cultures of S. destruens from sunbleak in the UK and use these cultures in challenge experiments to determine if the UK isolate of S. destruens from cyprinid species is a potential threat to Atlantic salmon (Salmo salar). The first isolation and culture of S. destruens in the UK and from a cyprinid species is described. Cultured S. destruens spores from sunbleak are infective to EPC, CHSE and FHM cells, replicating most rapidly in FHM and EPC cells. Spores can be induced to zoosporulate in water forming motile, uni-flagellated zoospores. Challenge experiments indicated the spores are able to replicate and disperse in Atlantic salmon and are associated with increased mortality (up to 90%) when injected intraperitonealy.

Parasites and pathogens of the endosymbiotic pea crab (Pinnotheres pisum) from blue mussels (Mytilus edulis) in England.

A histopathological survey of the commensal pea crab (Pinnotheres pisum) from the mantle cavities of blue mussels (Mytilus edulis) has been conducted. A total of 266 pea crabs from eight sites around the English coastline were examined. Of these, 82 were negative for any visible infections by histology. The remaining pea crabs were infected with an intranuclear bacilliform virus designated as P. pisum bacilliform virus (PpBV) in the hepatopancreatic epithelial cells, peritrichous ciliates on the gills, an intracytoplasmic microsporidian infection of the hepatopancreatocytes, a myophilic microsporidian infection, the gregarine Cephaloidophora fossor in the hepatopancreas, the entoniscid isopod Pinnotherion vermiforme, a low level nematode infection and an acanthocephalan cystacanth. Host reactions to infections were generally subdued. Results are discussed in relation to the endocommensal habitat of the pea crabs.

Histopathological survey of pathogens and commensals of white-clawed crayfish (Austropotamobius pallipes) in England and Wales.

A histopathological survey of white-clawed crayfish (Austropotamobius pallipes) from six rivers in England and Wales was conducted between November 2007 and September 2011. A. pallipes bacilliform virus (ApBV) was present in four samples; infected crayfish showed pathological responses ranging from mild hypertrophy of infected nuclei and emargination of chromatin through to loss of architecture of the hepatopancreatic cells. Crayfish were found to be hosts to two different ciliate species, mites, nematodes, digeneans and the microsporidian Thelohania contejeani. The variation in disease presence between populations highlights the importance of conducting appropriate surveys of native crayfish prior to movement of animals to refugia sites for relocation, restocking or breeding programmes.

Resistance to white spot syndrome virus in the European shore crab is associated with suppressed virion trafficking and heightened immune responses.

Introduction: All decapod crustaceans are considered potentially susceptible to White Spot Syndrome Virus (WSSV) infection, but the degree of White Spot Disease (WSD) susceptibility varies widely between species. The European shore crab Carcinus maenas can be infected with the virus for long periods of time without signs of disease. Given the high mortality rate of susceptible species, the differential susceptibility of these resistant hosts offers an opportunity to investigate mechanisms of disease resistance. Methods: Here, the temporal transcriptional responses (mRNA and miRNA) of C. maenas following WSSV injection were analysed and compared to a previously published dataset for the highly WSSV susceptible Penaeus vannamei to identify key genes, processes and pathways contributing to increased WSD resistance. Results: We show that, in contrast to P. vannamei, the transcriptional response during the first 2 days following WSSV injection in C. maenas is limited. During the later time points (7 days onwards), two groups of crabs were identified, a recalcitrant group where no replication of the virus occurred, and a group where significant viral replication occurred, with the transcriptional profiles of the latter group resembling those of WSSV-susceptible species. We identify key differences in the molecular responses of these groups to WSSV injection. Discussion: We propose that increased WSD resistance in C. maenas may result from impaired WSSV endocytosis due to the inhibition of internal vesicle budding by dynamin-1, and a delay in movement to the nucleus caused by the downregulation of cytoskeletal transcripts required for WSSV cytoskeleton docking, during early stages of the infection. This response allows resistant hosts greater time to fine-tune immune responses associated with miRNA expression, apoptosis and the melanisation cascade to defend against, and clear, invading WSSV. These findings suggest that the initial stages of infection are key to resistance to WSSV in the crab and highlight possible pathways that could be targeted in farmed crustacean to enhance resistance to WSD.

Global mRNA and miRNA Analysis Reveal Key Processes in the Initial Response to Infection with WSSV in the Pacific Whiteleg Shrimp.

White Spot Disease (WSD) presents a major barrier to penaeid shrimp production. Mechanisms underlying White Spot Syndrome Virus (WSSV) susceptibility in penaeids are poorly understood due to limited information related to early infection. We investigated mRNA and miRNA transcription in Penaeus vannamei over 36 h following infection. Over this time course, 6192 transcripts and 27 miRNAs were differentially expressed-with limited differential expression from 3-12 h post injection (hpi) and a more significant transcriptional response associated with the onset of disease symptoms (24 hpi). During early infection, regulated processes included cytoskeletal remodelling and alterations in phagocytic activity that may assist WSSV entry and translocation, novel miRNA-induced metabolic shifts, and the downregulation of ATP-dependent proton transporter subunits that may impair cellular recycling. During later infection, uncoupling of the electron transport chain may drive cellular dysfunction and lead to high mortalities in infected penaeids. We propose that post-transcriptional silencing of the immune priming gene Dscam (downregulated following infections) by a novel shrimp miRNA (Pva-pmiR-78; upregulated) as a potential mechanism preventing future recognition of WSSV that may be suppressed in surviving shrimp. Our findings improve our understanding of WSD pathogenesis in P. vannamei and provide potential avenues for future development of prophylactics and treatments.

Identification and Full Characterisation of Two Novel Crustacean Infecting Members of the Family Nudiviridae Provides Support for Two Subfamilies.

Multiple enveloped viruses with rod-shaped nucleocapsids have been described, infecting the epithelial cell nuclei within the hepatopancreas tubules of crustaceans. These bacilliform viruses share the ultrastructural characteristics of nudiviruses, a specific clade of viruses infecting arthropods. Using histology, electron microscopy and high throughput sequencing, we characterise two further bacilliform viruses from aquatic hosts, the brown shrimp (Crangon crangon) and the European shore crab (Carcinus maenas). We assembled the full double stranded, circular DNA genome sequences of these viruses (~113 and 132 kbp, respectively). Comparative genomics and phylogenetic analyses confirm that both belong within the family Nudiviridae but in separate clades representing nudiviruses found in freshwater and marine environments. We show that the three thymidine kinase (tk) genes present in all sequenced nudivirus genomes, thus far, were absent in the Crangon crangon nudivirus, suggesting there are twenty-eight core genes shared by all nudiviruses. Furthermore, the phylogenetic data no longer support the subdivision of the family Nudiviridae into four genera (Alphanudivirus to Deltanudivirus), as recently adopted by the International Committee on Taxonomy of Viruses (ICTV), but rather shows two main branches of the family that are further subdivided. Our data support a recent proposal to create two subfamilies within the family Nudiviridae, each subdivided into several genera.

Isolation of a Chinook Salmon Bafinivirus (CSBV) in Imported Goldfish Carassius auratus L. in the United Kingdom and Evaluation of Its Virulence in Resident Fish Species.

This is the first record of a fish nidovirus isolated from a consignment of goldfish at the United Kingdom (UK) border. The full-length viral genome was 25,985 nt, sharing a 97.9% nucleotide identity with the Chinook salmon bafinivirus (CSBV) NIDO with two deletions of 537 and 480 nt on the ORF Ia protein. To assess the potential impact on UK fish species, Atlantic salmon, common carp and goldfish were exposed to the virus via an intraperitoneal (IP) injection and bath challenge. Moribundity was recorded in only 8% of IP-injected goldfish. A high viral load, ≈107 of the CSBV PpIa gene, was measured in the kidney of moribund goldfish. Mild histopathological changes were observed in the kidneys of challenged carps. Ultrastructural observations in renal tubule epithelial cells of goldfish showed cylindrical tubes (≈15 nm in diameter) and tubular structures budding spherical virions (≈200 nm in diameter) with external spike-like structures. Negative staining showed both circular and bacilliform virions. Seroconversion was measured in common carp and goldfish but not in Atlantic salmon. This study reinforces the potential risk of novel and emerging pathogens being introduced to recipient countries via the international ornamental fish trade and the importance of regular full health screens at the border inspection posts to reduce this risk.