Characterization of EV-2, a Virus Isolated from European Eels (Anguilla anguilla) with Stomatopapilloma

Anguilla sp. diseases diagnoses and treatments: The ideal methods at the crossroads of conservation and aquaculture purposes

Anguilla anguilla, A. japonica and A. rostrata are the most fished and consumed eel species. However, these species are Critically Endangered, Endangered and Endangered, respectively. A combination of factors is thought to be responsible for their decline including fisheries, climate change, habitat destruction, barriers to migration, pollution and pathogens. Among them, viruses, bacteria and parasites are causing weakening of wild eels and serious economic losses for fishermen and eel farmers. Early detection of pathogens is essential to provide appropriate responses both for conservation reasons and to limit economic losses. Classic diagnosis approaches are time consuming and invasive and usual treatments, for example, antipathogenic substances are becoming obsolete because of pathogen resistance and environmental impact problems. The need for early and non-invasive diagnostic methods as well as effective and environmentally friendly treatments has increased. Vaccine development and diet supplementation have known a growing interest since their use could allow prevention of diseases. In this review, we summarize the main pathogens-viruses, bacteria and parasites-of the three northern temperate eel species, the methods used to detect these pathogens and the different treatments used. We discussed and highlighted the need for non-invasive, rapid and efficient detection methods, as well as effective and environmentally friendly treatments for both conservation and aquaculture purposes.

Full genome sequence of a novel circo-like virus detected in an adult European eel Anguilla anguilla showing signs of cauliflower disease

An adult European eel Anguilla anguilla, showing typical signs of the so-called cauliflower disease, was subjected to pathological and molecular virological examinations. Samples taken from internal organs and the polypoid proliferative tissue from the mouth were examined by PCR for the detection of several viruses. Positive results were obtained with a nested PCR targeting the rep gene of circoviruses. Analysis of the partial rep sequence indicated the presence of a putative novel circovirus, but attempts to isolate it remained unsuccessful. The missing part of the genome was acquired by an inverse nested PCR with 2 specific primer pairs, designed from the newly determined rep sequence, followed by genome walking. The circular full genome was found to consist of 1378 nt (GenBank accession no. KC469701). Two oppositely oriented open reading frames (ORFs) were present, of which one was unambiguously identified as a circoviral rep gene. However, the predicted product of the other ORF, though it is a clear positional counterpart of the cap genes, showed no obvious homology to any known circoviral capsid proteins. A stem-loop-like element in the intergenic region between the 5' ends of the ORFs was also found. Phylogenetic calculations indicated that the novel virus belongs to the genus Circovirus in the family Circoviridae. The relative amount of the viral DNA in the organ samples was estimated by quantitative real-time PCR. The results suggested that the examined fish was caught in an active viremic state, although the role of this circovirus in the etiology of the cauliflower diseases could not be ascertained.

Viral diseases of wild and farmed European eel Anguilla anguilla with particular reference to the Netherlands

Diseases are an important cause of losses and decreased production rates in freshwater eel farming, and have been suggested to play a contributory role in the worldwide decline in wild freshwater eel stocks. Three commonly detected pathogenic viruses of European eel Anguilla anguilla are the aquabirnavirus eel virus European (EVE), the rhabdovirus eel virus European X (EVEX), and the alloherpesvirus anguillid herpesvirus 1 (AngHV1). In general, all 3 viruses cause a nonspecific haemorrhagic disease with increased mortality rates. This review provides an overview of the current knowledge on the aetiology, prevalence, clinical signs and gross pathology of these 3 viruses. Reported experimental infections showed the temperature dependency and potential pathogenicity of these viruses for eels and other fish species. In addition to the published literature, an overview of the isolation of pathogenic viruses from wild and farmed A. anguilla in the Netherlands during the past 2 decades is given. A total of 249 wild A. anguilla, 39 batches of glass eels intended for farming purposes, and 239 batches of farmed European eels were necropsied and examined virologically. AngHV1 was isolated from wild yellow and silver A. anguilla from the Netherlands from 1998 until the present, while EVEX was only found sporadically, and EVE was never isolated. In farmed A. anguilla AngHV1 was also the most commonly isolated virus, followed by EVE and EVEX.

Viruses of lower vertebrates

Viruses of lower vertebrates recently became a field of interest to the public due to increasing epizootics and economic losses of poikilothermic animals. These were reported worldwide from both wildlife and collections of aquatic poikilothermic animals. Several RNA and DNA viruses infecting fish, amphibians and reptiles have been studied intensively during the last 20 years. Many of these viruses induce diseases resulting in important economic losses of lower vertebrates, especially in fish aquaculture. In addition, some of the DNA viruses seem to be emerging pathogens involved in the worldwide decline in wildlife. Irido-, herpes- and polyomavirus infections may be involved in the reduction in the numbers of endangered amphibian and reptile species. In this context the knowledge of several important RNA viruses such as orthomyxo-, paramyxo-, rhabdo-, retro-, corona-, calici-, toga-, picorna-, noda-, reo- and birnaviruses, and DNA viruses such as parvo-, irido-, herpes-, adeno-, polyoma- and poxviruses, is described in this review.

The putative polymerase sequence of infectious salmon anemia virus suggests a new genus within the Orthomyxoviridae

The infectious salmon anemia virus (ISAV) is an orthomyxovirus-like virus infecting teleosts. The disease caused by this virus has had major economic consequences for the Atlantic salmon farming industry in Norway, Canada, and Scotland. In this work, we report the cloning and sequencing of an ISAV-specific cDNA comprising 2,245 bp with an open reading frame coding for a predicted protein with a calculated molecular weight of 80.5 kDa. The putative protein sequence shows the core polymerase motifs characteristic of all viral RNA-dependent RNA polymerases. Comparison of the conserved motifs with the corresponding regions of other segmented negative-stranded RNA viruses shows a closer relationship with members of the Orthomyxoviridae than with viruses in other families. The putative ISAV polymerase protein (PB1) has a length of 708 amino acids, a charge of +22 at neutral pH, and a pI of 9.9, which are consistent with the properties of the PB1 proteins of other members of the family. Calculations of the distances between the different PB1 proteins indicate that the ISAV is distantly related to the other members of the family but more closely related to the influenza viruses than to the Thogoto viruses. Based on these and previously published results, we propose that the ISAV comprises a new, fifth genus in the Orthomyxoviridae.

Characterization of infectious salmon anemia virus, an orthomyxo-like virus isolated from Atlantic salmon (Salmo salar L.)

Infectious salmon anemia (ISA) virus is the cause of infectious salmon anemia in farmed Atlantic salmon. The virus has been shown to contain RNA with structural characteristics similar to those of accepted members of the Orthomyxoviridae. Further biochemical, physiochemical, and morphological characterization of ISA virus was undertaken to clarify its taxonomic position. The virus was found to be sensitive to chloroform, heat, and low pH and agglutinated erythrocytes from fish. Erythrocytes from mammals or birds were not agglutinated. Receptor-destroying enzyme activity was detected, and the nature of this enzyme was suggested to be an acetylesterase. The buoyant density of the virus was 1.18 g/ml in sucrose and CsCl gradients. The maximum rate of virus replication was observed at 15 degrees C, while no virus was produced at 25 degrees C. Actinomycin D inhibited viral replication, and viral antigen was detected in nuclei by immunofluorescence. The addition of trypsin to the culture medium during virus replication had a beneficial effect on virus replication. ISA virus contains four major polypeptides with estimated molecular sizes of 71, 53, 43, and 24 kDa. Electron microscopy revealed structures closely resembling the nucleocapsids of influenza virus. Mushroom-shaped surface projections were a distinctive morphological feature, which differed from the rod-shaped hemagglutinin projections of the influenza viruses. The data reported here support the relationship of ISA virus to the Orthomyxoviridae, although ISA virus differs from influenza viruses in some morphological characteristics and in showing restricted hemagglutination, in different specificity of the receptor-destroying enzyme, in different polypeptide profile, in being unable to replicate at temperatures above 25 degrees C, and in host range.

Genomic characterization of the virus causing infectious salmon anemia in Atlantic salmon (Salmo salar L.): an orthomyxo-like virus in a teleost

The genome of infectious salmon anemia virus (ISAV), which infects farmed Atlantic salmon (Salmo salar L.), is characterized here. The virus has an RNA genome, as shown by using specific DNA virus metabolic inhibitors and radioactive in vivo labeling of ISAV nucleic acid. Electrophoresis of [14C]uridine-labeled ISAV RNA revealed that the ISAV genome is segmented. The genome consists of eight segments that range from 1.0 to 2.3 kb, with a total molecular size of approximately 14.5 kb. One ISAV-specific molecular clone, corresponding to the smallest genome segment, was obtained by cDNA cloning of mRNA from an ISAV-infected cell culture. This clone gave a positive hybridization signal on Northern blots of pelleted ISAV. Pretreatment of the ISAV pellet with RNase A resulted in the disappearance of the positive hybridization signal, demonstrating that the genome is single stranded. Reverse transcriptase PCR with primers corresponding to sequences from the molecular clone and target RNA from ISAV-infected and noninfected fish tissues gave specific positive reactions. Alignments of the nucleotide sequence of the molecular clone did not reveal significant homology with any other available sequence in databases. However, the data presented here, together with morphological and replicational properties previously described, indicate that ISAV has a strong resemblance to members of the Orthomyxoviridae family. This is the first thoroughly characterized orthomyxo-like virus isolated from a teleost.

A double-stranded RNA-inducible fish gene homologous to the murine influenza virus resistance gene Mx

We cloned and sequenced a 2.35-kilobase EcoRI fragment of genomic DNA from a local freshwater fish (Perca fluviatilis) that strongly hybridized to probes derived from the murine influenza virus resistance gene Mx. The cloned fish DNA contained blocks of sequences related to Mx gene exons 3 to 8, which appeared to represent exons of a bona fide fish gene because they were separated by intron sequences flanked by consensus splice acceptor and donor sites. Injection of double-stranded RNA into the peritoneal cavity of trouts resulted in 5- to 10-fold elevated levels of two liver mRNAs of about 2.0 to 2.5 kilobases in length that hybridized to the cloned genomic DNA. High sequence similarity between this fish gene and the murine Mx gene, identical exon lengths, and similar inducibilities in vivo by double-stranded RNA indicate that we isolated a fragment of a fish Mx gene.

A double-stranded RNA-inducible fish gene homologous to the murine influenza virus resistance gene Mx

We cloned and sequenced a 2.35-kilobase EcoRI fragment of genomic DNA from a local freshwater fish (Perca fluviatilis) that strongly hybridized to probes derived from the murine influenza virus resistance gene Mx. The cloned fish DNA contained blocks of sequences related to Mx gene exons 3 to 8, which appeared to represent exons of a bona fide fish gene because they were separated by intron sequences flanked by consensus splice acceptor and donor sites. Injection of double-stranded RNA into the peritoneal cavity of trouts resulted in 5- to 10-fold elevated levels of two liver mRNAs of about 2.0 to 2.5 kilobases in length that hybridized to the cloned genomic DNA. High sequence similarity between this fish gene and the murine Mx gene, identical exon lengths, and similar inducibilities in vivo by double-stranded RNA indicate that we isolated a fragment of a fish Mx gene.

Morphogenesis and fine structure of eel virus (Berlin), a member of the proposed Birnavirus group

Eel Virus (Berlin) is associated with the occurrence of skin tumors in European eels. The genome of the virus consists of two segments of double-stranded (ds) RNA. The agent is assembled exclusively in the cytoplasm. Isometric particles with a diameter of 61 nm and in addition tubular structures and smaller particles were observed. The virion has a single shell: its capsid is composed of 132 interconnected morphological units with T = 13 dextro symmetry. According to particle size and bipartite nature of the genome, this virus has to be assigned to the tentatively proposed group of bisegmented ds RNA animal viruses.

Elevated sister chromatid exchange rate in lymphocytes of the European eel (Anguilla anguilla) with cauliflower tumor

Sister chromatid exchange (SCE) frequency and chromosomal aberration were studied in cultured lymphocytes from the European eel (Anguilla anguilla) with cauliflower tumors. SCE was significantly increased in the affected fishes compared with normal controls, whereas chromosomal aberration did not. In comparison to normal cells, the lymphocytes responding to phytohemagglutinin M (PHA-M) in cultures from tumorous eels showed no evidence of cell cycle retardation. A possible mechanism for elevation of SCE and the significance of this finding relation to the etiology were discussed.

Poikilotherm vertebrate cell lines and viruses: a current listing for fishes

A survey of the literature and of work being done in vertebrate cell culture shows that there are currently in existence and available to investigators some 61 cell lines representing 17 families and 36 species of fish. The literature of fish virology shows that at least 17 fish viruses have been isolated and that at least 15 others have been visualized by electron microscopy. A minimum of four major virus groups--rhabdovirus, orthomyxovirus, iridovirus and herpesvirus--are known from fish. Original references, key reviews and sources of cell lines are given.