Characterization of a New Toti-like Virus in Sea Bass, Dicentrarchus labrax

The European sea bass Dicentrarchus labrax is the main species reared in Mediterranean aquaculture. Its larval stage, which is very sensitive and highly affected by sanitary and environmental conditions, is particularly scrutinized in hatcheries. Recently, a Mediterranean sea bass farm had to deal with an abnormal increase in mortality, especially between 20 and 35 days post-hatching (dph). Biological investigations led to the observation of cytopathic effects on three different fish cell lines after almost 3 weeks of culture at 14 °C in contact with homogenized affected larvae, suggesting the presence of a viral agent. High-throughput sequencing revealed a 6818-nucleotide-long RNA genome with six putative ORFs, corresponding to the organization of viruses belonging to the Totiviridae family. This genome clustered with the newly described and suggested Pistolvirus genus, sharing 45.5% to 37.2% nucleotide identity with other piscine toti-like viruses such as Cyclopterus lumpus toti-like virus (CLuTLV) or piscine myocarditis virus (PMCV), respectively. Therefore, we propose to name this new viral agent sea bass toti-like virus (SBTLV). Specific real-time RT-PCR confirmed the presence of the viral genome in the affected larval homogenate from different production batches and the corresponding cell culture supernatant. Experimental infections performed on sea bass fingerlings did not induce mortality, although the virus could be detected in various organs and a specific immune response was developed. Additional studies are needed to understand the exact involvement of this virus in the mortality observed in hatcheries and the potential associated cofactors.

Potential of Marine Strains of Pseudoalteromonas to Improve Resistance of Juvenile Sea Bass to Pathogens and Limit Biofilm Development

The European sea bass (Dicentrarchus labrax), one of the most produced marine fish species in Europe, is acutely vulnerable to multiple infectious hazards. In this study, we investigated the potential probiotic effect of some marine Pseudoalteromonas bacterial strains against two major pathogens of this species, Vibrio harveyi and the nervous necrosis virus (NNV), and examined their antibiofilm effect. Impregnation phase was done by repeated immersion of juvenile's sea bass during 8 to 12 weeks in seawater containing the probiotic candidates at a concentration of 106 CFU/mL. Four candidates were tested: (1) a combination of two strains producing antimicrobial compounds, hCg-42 and hOe-125; (2) strain 3J6, with known antibiofilm properties; (3) strain RA15, from the same genus, but with no identified probiotic effect; and (4) a control group without probiotics. At the end of the impregnation phase, fish underwent an infection challenge with V. harveyi or with a pathogenic strain of NNV and mortality was monitored. For the V. harveyi challenge, improved survival rates of 10 and 25% were obtained for the RA15 and the mix hCg-42 + hOe-125-impregnated groups, respectively. For the NNV challenge, no significant benefic effect of the probiotics on infection kinetics or cumulative mortality was observed. At the end of the impregnation phase, the maximal thickness of biofilm was significantly lower in the 3J6, double strain, and RA15 groups, compared with the non-impregnated control group. This study highlights the interesting probiotic potential of marine bacteria to limit mortalities induced by bacterial pathogens as well as biofilm development.

The relationships between growth rate and mitochondrial metabolism varies over time

Mitochondrial metabolism varies significantly between individuals of the same species and can influence animal performance, such as growth. However, growth rate is usually determined before the mitochondrial assay. The hypothesis that natural variation in mitochondrial metabolic traits is linked to differences in both previous and upcoming growth remains untested. Using biopsies to collect tissue in a non-lethal manner, we tested this hypothesis in a fish model (Dicentrarchus labrax) by monitoring individual growth rate, measuring mitochondrial metabolic traits in the red muscle, and monitoring the growth of the same individuals after the mitochondrial assay. Individual variation in growth rate was consistent before and after the mitochondrial assay; however, the mitochondrial traits that explained growth variation differed between the growth rates determined before and after the mitochondrial assay. While past growth was correlated with the activity of the cytochrome c oxidase, a measure of mitochondrial density, future growth was linked to mitochondrial proton leak respiration. This is the first report of temporal shift in the relationship between growth rate and mitochondrial metabolic traits, suggesting an among-individual variation in temporal changes in mitochondrial traits. Our results emphasize the need to evaluate whether mitochondrial metabolic traits of individuals can change over time.

The extensive transgenerational transcriptomic effects of ocean acidification on the olfactory epithelium of a marine fish are associated with a better viral resistance

Background

Progressive CO₂-induced ocean acidification (OA) impacts marine life in ways that are difficult to predict but are likely to become exacerbated over generations. Although marine fishes can balance acid–base homeostasis efficiently, indirect ionic regulation that alter neurosensory systems can result in behavioural abnormalities. In marine invertebrates, OA can also affect immune system function, but whether this is the case in marine fishes is not fully understood. Farmed fish are highly susceptible to disease outbreak, yet strategies for overcoming such threats in the wake of OA are wanting. Here, we exposed two generations of the European sea bass (Dicentrarchus labrax) to end-of-century predicted pH levels (IPCC RCP8.5), with parents (F1) being exposed for four years and their offspring (F2) for 18 months. Our design included a transcriptomic analysis of the olfactory rosette (collected from the F2) and a viral challenge (exposing F2 to betanodavirus) where we assessed survival rates.

Results

We discovered transcriptomic trade-offs in both sensory and immune systems after long-term transgenerational exposure to OA. Specifically, RNA-Seq analysis of the olfactory rosette, the peripheral olfactory organ, from 18-months-old F2 revealed extensive regulation in genes involved in ion transport and neuronal signalling, including GABAergic signalling. We also detected OA-induced up-regulation of genes associated with odour transduction, synaptic plasticity, neuron excitability and wiring and down-regulation of genes involved in energy metabolism. Furthermore, OA-exposure induced up-regulation of genes involved in innate antiviral immunity (pathogen recognition receptors and interferon-stimulated genes) in combination with down-regulation of the protein biosynthetic machinery. Consistently, OA-exposed F2 challenged with betanodavirus, which causes damage to the nervous system of marine fish, had acquired improved resistance.

Conclusion

F2 exposed to long-term transgenerational OA acclimation showed superior viral resistance, though as their metabolic and odour transduction programs were altered, odour-mediated behaviours might be consequently impacted. Although it is difficult to unveil how long-term OA impacts propagated between generations, our results reveal that, across generations, trade-offs in plastic responses is a core feature of the olfactory epithelium transcriptome in OA-exposed F2 offspring, and will have important consequences for how cultured and wild fish interacts with its environment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08647-w.

Characterization of a novel picornavirus isolated from moribund gilthead seabream (Sparus aurata) larvae

Gilthead seabream represents a species of importance in Mediterranean aquaculture. The larval stage is particularly sensitive and frequently impacted in suboptimal environmental or sanitary conditions. In the present study, investigations were carried out in a seabream hatchery following an unusual mortality reaching 70% among 50-day post-hatching. Anorexia, loss of appetite and abnormal swimming behaviour were observed in absence of parasites or pathogenic bacteria. Proliferation of rod-shaped bacteria in the gut lumen was associated with focal degeneration in the intestinal mucosa. Cytopathic effects on an EK-1 cell line after 21 days of culture at 14°C and 20°C in contact with homogenized affected larvae revealed the presence of a viral agent. Molecular characterization by high-throughput sequencing showed a typical picornavirus genome organization with a polyprotein precursor of 2276 amino acids sharing 46.3% identity with that of the Eel Picornavirus-1. A specific real-time PCR confirmed the presence of the viral genome in affected larval homogenate and corresponding cell culture supernatant. We propose the name Potamipivirus daurada for this novel species within the genus Potamipivirus. The etiological role of this virus remains uncertain at this time, and future studies will be necessary to investigate its prevalence in natural and aquaculture-reared populations as well as its ability to cause diseases in gilthead seabream.

Changes in defense capacity to infectious hematopoietic necrosis virus (IHNv) in rainbow trout intergenerationally exposed to glyphosate

Glyphosate, the most widely used herbicide active substance worldwide, has raised many scientific, political and public debates in the context of its recent re-registration in the European Union, highlighting in particular a lack of data concerning its potential generational effects. In this study, we investigated the intergenerational toxicity of this active substance used alone or coformulated in glyphosate-based herbicides (GBHs) on the ability of rainbow trout (Oncorhynchus mykiss) to face a viral challenge. Juvenile trout from parents exposed for eight months to four different chemical exposure conditions (non-exposed control, pure glyphosate, Roundup Innovert®, and Viaglif Jardin® were experimentally infected with the infectious hematopoietic necrosis virus (IHNv). Various enzymatic and hemato-immunological markers were assessed before and after the viral challenge. Chemical contamination with GBHs strongly modulated viral trout susceptibility. Pure glyphosate induced a cumulative mortality of 35.8%, comparable to the control (37.0%), which was significantly reduced with Roundup Innovert® (-9.9%) and increased (+14.8%) with Viaglif Jardin®. No modification was observed for the biomarkers analysed for any conditions. These results demonstrate that the nature of the co-formulants associated to glyphosate in GHBs can modulate the susceptibility of fish to pathogens.

Generational effects of a chronic exposure to a low environmentally relevant concentration of glyphosate on rainbow trout, Oncorhynchus mykiss

In the past few decades, glyphosate became the most used herbicide substance worldwide. As a result, the substance is ubiquitous in surface waters. Concerns have been raised about its ecotoxicological impact, but little is known about its generational toxicity. In this study, we investigate the impact of an environmentally relevant concentration of glyphosate and its co-formulants on an F2 generation issued from exposed generations F0 and F1. Trans, inter and multigenerational toxicity of 1 μgL-1 of the active substance was evaluated on early stages of development and juvenile rainbow trout (Oncorhynchus mykiss) using different molecular, biochemical, immuno-hematologic, and biometric parameters, behavior analysis, and a viral challenge. Reproductive parameters of generation F1 were not affected. However, developmental toxicity in generation F2 due to glyphosate alone or co-formulated was observed with head size changes (e.g. head surface up to +10%), and metabolic disruptions (e.g. 35% reduction in cytochrome-c-oxidase). Moreover, larvae exposed transgenerationally to Viaglif and intergenerationally to glyphosate and Roundup presented a reduced response to light, potentially indicating altered escape behavior. Overall methylation was, however, not altered and further experiments using gene-specific DNA metylation analyses are required. After several months, biochemical parameters measured in juvenile fish were no longer impacted, only intergenerational exposure to glyphosate drastically increased the susceptibility of rainbow trout to hematopoietic necrosis virus. This result might be due to a lower antibody response in exposed fish. In conclusion, our results show that generational exposure to glyphosate induces developmental toxicity and increases viral susceptibility. Co-formulants present in glyphosate-based herbicides can modulate the toxicity of the active substance. Further investigations are required to study the specific mechanisms of transmission but our results suggest that both non-genetic mechanisms and exposure during germinal stage could be involved.

Immunological and metabolic effects of acute sublethal exposure to glyphosate or glyphosate-based herbicides on juvenile rainbow trout, Oncorhynchus mykiss

Glyphosate is a commonly used agrochemical active substance co-formulated in glyphosate-based herbicides (GBHs) whose environmental safety is still a subject of debate in the European Union. We evaluated the effects of acute sublethal exposure to glyphosate on rainbow trout by measuring changes in their metabolic and hemato-immunologic functions and their ability to survive a viral challenge. Juvenile fish were exposed for 96 h to 500 μg L-1 of glyphosate through the active substance alone or two GHBs, Roundup Innovert® and Viaglif Jardin®, and fish were then infected with the infectious hematopoietic necrosis virus. Red and white blood cell counts (RBCC and WBCC), as well as several enzymatic activities (citrate synthase, CS; cytochrome-c oxidase, CCO; lactate dehydrogenase, LDH; glucose-6-phosphate dehydrogenase, G6PDH; acetylcholinesterase, AChE), were measured 96 h after chemical contamination (S1), and 96 h post-viral infection (S2). Mortality rates were monitored, and virus titers at the mortality peaks and seropositivity of the survivors were analyzed at 60 days post-viral infection (S3). Cumulative mortalities, viral titers, and seropositivity induced by virus infection were similar among conditions. Hematological analysis revealed significant increases of 30% for RBCC for Roundup at S1, and of 22% for WBCC at S2. No changes were observed in metabolic enzyme activities at S1. At S2, CCO and G6PDH activities were significantly higher than controls in all the chemically contaminated groups (+61 to 62% and +65 to 138%, respectively). LDH and AChE activities were increased for the Viaglif (p = 0.07; +55%) and for glyphosate and Roundup conditions (p < 0.05, +62 to 79%), respectively. Rainbow trout acutely exposed to glyphosate or GBHs presented no major physiological changes. Viral infection revealed disruptions, potentially modulated by co-formulants, of hematological and metabolic parameters, showing that it is essential to consider the stressful natural environment of fish in the chemical assessment.

The Viral Hemorrhagic Septicemia Virus (VHSV) Markers of Virulence in Rainbow Trout (Oncorhynchus mykiss)

Viral hemorrhagic septicemia virus (VHSV) is a highly contagious virus leading to high mortality in a large panel of freshwater and marine fish species. VHSV isolates originating from marine fish show low pathogenicity in rainbow trout. The analysis of several nearly complete genome sequences from marine and freshwater isolates displaying varying levels of virulence in rainbow trout suggested that only a limited number of amino acid residues might be involved in regulating the level of virulence. Based on a recent analysis of 55 VHSV strains, which were entirely sequenced and phenotyped in vivo in rainbow trout, several amino acid changes putatively involved in virulence were identified. In the present study, these amino acid changes were introduced, alone or in combination, in a highly-virulent VHSV 23–75 genome backbone by reverse genetics. A total of 35 recombinant VHSV variants were recovered and characterized for virulence in trout by bath immersion. Results confirmed the important role of the NV protein (R116S) and highlighted a major contribution of the nucleoprotein N (K46G and A241E) in regulating virulence. Single amino acid changes in these two proteins drastically affect virus pathogenicity in rainbow trout. This is particularly intriguing for the N variant (K46G) which is unable to establish an active infection in the fins of infected trout, the main portal of entry of VHSV in this species, allowing further spread in its host. In addition, salmonid cell lines were selected to assess the kinetics of replication and cytopathic effect of recombinant VHSV and discriminate virulent and avirulent variants. In conclusion, three major virulence markers were identified in the NV and N proteins. These markers explain almost all phenotypes (92.7%) observed in trout for the 55 VHSV strains analyzed in the present study and herein used for the backward validation of virulence markers. The identification of VHSV specific virulence markers in this species is of importance both to predict the in vivo phenotype of viral isolates with targeted diagnostic tests and to improve prophylactic methods such as the development of safer live-attenuated vaccines.

VHSV Single Amino Acid Polymorphisms (SAPs) Associated With Virulence in Rainbow Trout

The Viral Hemorrhagic Septicemia Virus (VHSV) is an OIE notifiable pathogen widespread in the Northern Hemisphere that encompasses four genotypes and nine subtypes. In Europe, subtype Ia impairs predominantly the rainbow trout industry causing severe rates of mortality, while other VHSV genotypes and subtypes affect a number of marine and freshwater species, both farmed and wild. VHSV has repeatedly proved to be able to jump to rainbow trout from the marine reservoir, causing mortality episodes. The molecular mechanisms regulating VHSV virulence and host tropism are not fully understood, mainly due to the scarce availability of complete genome sequences and information on the virulence phenotype. With the scope of identifying in silico molecular markers for VHSV virulence, we generated an extensive dataset of 55 viral genomes and related mortality data obtained from rainbow trout experimental challenges. Using statistical association analyses that combined genetic and mortality data, we found 38 single amino acid polymorphisms scattered throughout the complete coding regions of the viral genome that were putatively involved in virulence of VHSV in trout. Specific amino acid signatures were recognized as being associated with either low or high virulence phenotypes. The phylogenetic analysis of VHSV coding regions supported the evolution toward greater virulence in rainbow trout within subtype Ia, and identified several other subtypes which may be prone to be virulent for this species. This study sheds light on the molecular basis for VHSV virulence, and provides an extensive list of putative virulence markers for their subsequent validation.

Susceptibility of pike Esox lucius to VHSV and IHNV and potential transmission to rainbow trout Oncorhynchus mykiss

Determining the origin of recurrent outbreaks of fish diseases occurring on fish farms is essential for disease prevention and control measures. In this study, we investigated the potential reservoir role of wild fish species living near salmonid farms which were regularly found to be positive for viral hemorrhagic septicemia virus (VHSV). In addition to VHSV, infectious hematopoietic necrosis virus (IHNV) was also isolated from several pike Esox lucius samples collected from a pond near the salmonid farms of interest. All isolates of VHSV and IHNV analyzed had 100% identical partial glycoprotein gene sequences. VHSV pike strain OO128-25 belonged to the Ia genotype and shared 99.1 to 99.5% nucleotide identity with strains recently isolated from the farms. IHNV pike strain OO121-8, European genotype, appeared to be different from strains from France characterized since the first isolation in 1987. Isolates representative of both viral species were highly virulent in rainbow trout Oncorhynchus mykiss. OO128-25 induced 65% mortality in pike fingerlings, whereas only weak mortality was observed with OO121-8, despite characteristic symptoms in infected fish. High levels of specific antibodies to VHSV and IHNV were detected in adult pike in the absence of clinical signs. Infection of rainbow trout in contact with experimentally VHSV- or IHNV-infected pike fingerlings indicates possible horizontal transmission. These results suggest that pike could act as a reservoir for VHSV and IHNV in the wild, providing additional evidence to explain viral persistence and resurgence in certain areas.

Validation Study for the Determination of Tetracycline Residues in Animal Tissues

An interlaboratory study of the liquid chromatographic (LC) determination of tetracyclines—oxytetracycline (OTC), tetracycline (TTC), and chlortetracycline (CTC)—in animal tissues was conducted. Isolation was performed with oxalic buffer followed by dechelation and deproteination with oxalic acid–acetonitrile. The extract was cleaned with a styrene–divinylbenzene cartridge. LC analysis was performed with a PLRP-S column and 0.01 M oxalic acid–acetonitrile (75 + 25, v/v) as mobile phase. Participants analyzed 2 control and 10 fortified porcine muscle and kidney samples. Additionally, porcine muscle samples containing incurred residues of tetracyclines were analyzed. Mean recoveries of fortified residues from porcine tissue ranged from 76.00 to 86.89%. Repeatabilities varied from 2.05% for OTC to 3.61 % for TTC for muscle samples and from 6.75% for CTC to 8.74% for OTC for kidney samples. Reproducibilities ranged from 2.05 to 4.30% for muscle samples and from 15.77 to 18.81 % for kidney samples.

Cellular, humoral and molecular responses in rainbow trout (Oncorhynchus mykiss) exposed to a herbicide and subsequently infected with infectious hematopoietic necrosis virus

Aquatic ecosystems are now chronically polluted by a cocktail of many chemical substances. There is now clear evidence of associations between exposure to pollutants and greater susceptibility to pathogens. The aim of the present study was to characterize the defense capacities of rainbow trout (Oncorhynchus mykiss), chronically exposed to pendimethalin (PD), to subsequent experimental challenge with the infectious hematopoietic necrosis virus (IHNV). Immunological responses were examined at different organizational levels, from individuals to gene expression. No negative effects of PD were noted on the Fulton index nor on the liver or spleen somatic indices (LSI; SSI) before viral infection, but the infectious stress seems to generate a weak but significant decrease in Fulton and LSI values, which could be associated with consumption of energy reserves. During the viral challenges, the distribution of cumulative mortality was slightly different between infected groups. The impact of the virus on fish previously contaminated by PD started earlier and lasted longer than controls. The proportion of seropositive fish was lower in the fish group exposed to PD than in the control group, with similar quantities of anti-IHNV antibodies secreted in positive fish, regardless of the treatment. While no significant differences in C3-1 expression levels were detected throughout the experiment, TNF1&2, TLR3, Il-1β and IFN expression levels were increased in all infected fish, but the difference was more significant in fish groups previously exposed to herbicide. On the other hand, β-def expression was decreased in the pendimethalin-IHNV group compared to that in fish only infected by the virus (control-IHNV group).

Comparative biomarker responses in Japanese medaka (Oryzias latipes) exposed to benzo[a]pyrene and challenged with betanodavirus at three different life stages

It is now well documented that several contaminants can modulate the fish immune system, leading to disrupted host resistance against pathogens and increased incidence of disease. Since fish are usually co-exposed to chemicals and pathogens in the natural environment, analysis of the immunotoxic effects of pollutants is particularly relevant. The authorities in the European Union have recommended the development of toxicity assays on cell cultures and embryos, as an alternative to testing in vertebrates. This is why in our study, a fish immune challenge assay was developed for the early life stages of Japanese medaka to evaluate and compare the relevance of new biomarkers. Fish were exposed to benzo[a]pyrene (BaP), a model pollutant, for 8days at the embryonic stage, or for 48h at the larvae and juvenile stages, and fish were infected with betanodavirus by bath-challenge of 10⁶TCID₅₀/mL. Biometric changes and induction of malformations were observed after embryonic exposure. DNA damage and induction of EROD activity were recorded at the end of all chemical exposures. Viral infection increased the mortality rate significantly and disturbed the behavior of fish after light stimulation. While BaP exposure increased swimming speed, betanodavirus infection slowed swimming activity. In larvae co-exposed to BaP and the virus, the viral titer in the whole body was higher than in fish infected only with the virus. This study highlighted the sensitivity and usefulness of the immune challenge assay on the early life stages of Japanese medaka to evaluate the toxic effects of pollutants.

Genome-wide association and genomic prediction of resistance to viral nervous necrosis in European sea bass (Dicentrarchus labrax) using RAD sequencing

BACKGROUND

European sea bass (Dicentrarchus labrax) is one of the most important species for European aquaculture. Viral nervous necrosis (VNN), commonly caused by the redspotted grouper nervous necrosis virus (RGNNV), can result in high levels of morbidity and mortality, mainly during the larval and juvenile stages of cultured sea bass. In the absence of efficient therapeutic treatments, selective breeding for host resistance offers a promising strategy to control this disease. Our study aimed at investigating genetic resistance to VNN and genomic-based approaches to improve disease resistance by selective breeding. A population of 1538 sea bass juveniles from a factorial cross between 48 sires and 17 dams was challenged with RGNNV with mortalities and survivors being recorded and sampled for genotyping by the RAD sequencing approach.

RESULTS

We used genome-wide genotype data from 9195 single nucleotide polymorphisms (SNPs) for downstream analysis. Estimates of heritability of survival on the underlying scale for the pedigree and genomic relationship matrices were 0.27 (HPD interval 95%: 0.14-0.40) and 0.43 (0.29-0.57), respectively. Classical genome-wide association analysis detected genome-wide significant quantitative trait loci (QTL) for resistance to VNN on chromosomes (unassigned scaffolds in the case of 'chromosome' 25) 3, 20 and 25 (P < 1e06). Weighted genomic best linear unbiased predictor provided additional support for the QTL on chromosome 3 and suggested that it explained 4% of the additive genetic variation. Genomic prediction approaches were tested to investigate the potential of using genome-wide SNP data to estimate breeding values for resistance to VNN and showed that genomic prediction resulted in a 13% increase in successful classification of resistant and susceptible animals compared to pedigree-based methods, with Bayes A and Bayes B giving the highest predictive ability.

CONCLUSIONS

Genome-wide significant QTL were identified but each with relatively small effects on the trait. Tests of genomic prediction suggested that incorporating genome-wide SNP data is likely to result in higher accuracy of estimated breeding values for resistance to VNN. RAD sequencing is an effective method for generating such genome-wide SNPs, and our findings highlight the potential of genomic selection to breed farmed European sea bass with improved resistance to VNN.

Molecular evolution and phylogeography of infectious hematopoietic necrosis virus with a focus on its presence in France over the last 30 years

Infectious hematopoietic necrosis virus (IHNV) is among the most important pathogens affecting the salmonid industry. Here, we investigated the molecular evolution and circulation of isolates from 11 countries or regions all over the world, with a special focus on the epidemiological situation in France. The phylogeography, time to the most recent common ancestor (TMRCA) and nucleotide substitution rate were studied using 118 full-length glycoprotein gene sequences isolated from 9 countries (5 genogroups) over a period of 47 years. The TMRCA dates back to 1943, with the L genogroup identified as the likely root (67 %), which is consistent with the first report of this pathogen in the USA. A Bayesian inference approach was applied to the partial glycoprotein gene sequences of 88 representative strains isolated in France over the period 1987-2015. The genetic diversity of these 88 sequences showed mean nucleotide and amino-acid identities of 97.1 and 97.8 %, respectively, and a d _N/d _S ratio (non-synonymous to synonymous mutations) of 0.25, indicating purifying selection. The French viral populations are divided into eight sub-clades and four individual isolates, with a clear spatial differentiation, suggesting the predominant role of local reservoirs in contamination. The atypical 'signatures' of some isolates underlined the usefulness of molecular phylogeny for epidemiological investigations that track the spread of IHNV.

A single amino acid change in the non-structural NV protein impacts the virulence phenotype of Viral hemorrhagic septicemia virus in trout

Novirhabdoviruses like the Viral hemorrhagic septicemia virus (VHSV) are rhabdoviruses infecting fish. In the current study, RNA genomes of different VHSV field isolates classified as high, medium or low virulent phenotypes have been sequenced by next-generation sequencing and compared. Various amino acid changes, depending on the VHSV phenotype, have been identified in all the VHSV proteins. As a starting point, we focused our study on the non-virion (NV) non-structural protein in which an arginine residue (R116) is present in all the virulent isolates and replaced by a serine/asparagine residue S/N116 in the attenuated isolates. A recombinant virus derived from a virulent VHSV strain in which the NV R116 residue has been replaced by a serine, rVHSVNVR116S, was generated by reverse genetics and used to infect juvenile trout. We showed that rVHSVNVR116S was highly attenuated and that surviving fish were almost completely protected from a challenge with the wild-type VHSV.

Validation of a serum neutralization test for detection of antibodies specific to cyprinid herpesvirus 3 in infected common and koi carp (Cyprinus carpio)

Cyprinid herpesvirus 3 (CyHV-3) is the aetiological agent of a serious infective, notifiable disease affecting common carp and varieties. In survivors, infection is generally characterized by a subclinical latency phase with restricted viral replication. The CyHV-3 genome is difficult to detect in such carrier fish that represent a potential source of dissemination if viral reactivation occurs. In this study, the analytical and diagnostic performance of an alternative serum neutralization (SN) method based on the detection of CyHV-3-specific antibodies was assessed using 151 serum or plasma samples from healthy and naturally or experimentally CyHV-3-infected carp. French CyHV-3 isolate 07/108b was neutralized efficiently by sera from carp infected with European, American and Taiwanese CyHV-3 isolates, but no neutralization was observed using sera specific to other aquatic herpesviruses. Diagnostic sensitivity, diagnostic specificity and repeatability of 95.9%, 99.0% and 99.3%, respectively, were obtained, as well as a compliance rate of 89.9% in reproducibility testing. Neutralizing antibodies were steadily detected in infected carp subjected to restrictive or permissive temperature variations over more than 25 months post-infection. The results suggest that this non-lethal diagnostic test could be used in the future to improve the epidemiological surveillance and control of CyHV-3 disease.

First generic one step real-time Taqman RT-PCR targeting the RNA1 of betanodaviruses

The detection of betanodavirus genomic components is a major issue for diagnostics and control of viral nervous necrosis (VNN), a devastating disease affecting fish worldwide. Despite a number of published molecular-based tests, most of them targeting the RNA2 molecule of the virus, diagnostics is still a challenge due to the high genetic diversity within this genus. In the present study, a new one-step real-time RT-PCR (rRT-PCR), targeting RNA1 of most genotypes of betanodaviruses, was proposed and validated. The test detected successfully various isolates of betanodavirus representatives of the four species RGNNV, SJNNV, TPNNV and BFNNV, either produced on cell culture or from clinical samples. It was specific as shown by the absence of signal on samples from healthy sea bass or from field samples of six other fish species without clinical signs of VNN. The assay detected reliably 50-100 copies of plasmids containing the targeted cloned RNA1 region, as well as an infectious dose of virus of 10(2.5)-10(2.85) TCID50/ml. A set of samples was tested by two different laboratories, with similar results, demonstrating the robustness of the test. This is the first one step generic rRT-PCR method for betanodaviruses. It is simple to perform and may be used for first intention diagnostics as well as for confirmation in case of doubtful results obtained with other published tests targeting RNA2.

First isolation of a rhabdovirus from perch Perca fluviatilis in Switzerland

Perca fluviatilis is a fish species of increasing interest to the Swiss fish farming industry. In recent years, recirculation systems have been specifically set up to increase production. In one of these farms, abnormal spiral swimming associated with elevated mortalities occurred in repeated batches of imported perch shortly after stocking on several occasions. No bacterial or parasitic etiology was detected, but a virus grown in bluegill fry (BF-2) cells was identified as perch rhabdovirus. Subsequent investigations of other samples suggested a viral tropism for the central nervous system (CNS). Phylogenetic analysis of the partial N and entire G gene sequences positioned this isolate in genogroup C of the species Perch rhabdovirus, with high nucleotide and amino acid (aa) sequence identities with the DK5533 strain isolated in Denmark in 1989. Comparative studies using other closely related isolates allowed the distinction of 2 serological patterns among perch rhabdoviruses and the identification of a proline substitution by a serine in position 147 of the glycoprotein potentially involved in antigenic differentiation. Even if perch imported onto the farm tested negative by virus isolation prior to transport, they may have been the origin of this outbreak since CNS tissue was not included in the samples that were analyzed. Another possibility might be a sub-clinical infection with a viral load in resident fish too low to be detected. This study reports the first isolation of a perch rhabdovirus in Switzerland, and emphasizes the necessity of optimizing diagnostic tools that facilitate better control of the risks associated with fish translocation.

Effect of chronic exposure to pendimethalin on the susceptibility of rainbow trout, Oncorhynchus mykiss L., to viral hemorrhagic septicemia virus (VHSV)

In this study, the in vivo effects of chronic pollution by the active substance (AS) pendimethalin, a dinitroaniline herbicide, on the susceptibility of rainbow trout, Oncorhynchus mykiss L., to an experimental challenge with viral hemorrhagic septicemia virus (VHSV) were assessed. After four weeks of exposure to fresh water (C group) or 500 ng L(-1) of AS (P500 group), the fish were challenged by immersion in water containing 10(4) TCID(50) mL(-1) of VHSV. While exposure to pendimethalin was maintained throughout the experiment, mortalities were recorded during the 40 days post-infection (dpi) and organs were collected from dead fish for virological examination. At the end of the experiment, anti-VHSV antibodies and the classical pathway of complement activity were assessed in trout plasma. Exposure to pendimethalin significantly affected the distribution of cumulative mortality accelerating death in fish infected by VHSV. Pendimethalin appeared to decrease the Mean Time to Death (MTD) after virus treatment from 14.9 days (C-VHSV) to 10.2 days (P500-VHSV). Nevertheless, by the end of the experiment, differences in cumulative mortality were no longer observed between the two groups, which had reached the same stage (50 percent). Furthermore, a higher concentration of the virus was recovered from the pools of organs from the P500-VHSV group than the C-VHSV group. Moreover, at 40 dpi, although no significant difference was observed in the immune response between the two groups, more fish in the P500-VHSV group had set up an immune response by secreting antibodies than in the control viral group (C-VSHV).

Outbreak of betanodavirus infection in tilapia, Oreochromis niloticus (L.), in fresh water

A betanodavirus associated with a massive mortality was isolated from larvae of tilapia, Oreochromis niloticus, maintained in fresh water at 30 degrees C. Histopathology revealed vacuolation of the nervous system, suggesting an infection by a betanodavirus. The virus was identified by indirect fluorescent antibody test in the SSN1 cell line and further characterized by sequencing of a PCR product. Sequencing of the T4 region of the coat protein gene indicated a phylogenetic clustering of this isolate within the red-spotted grouper nervous necrosis virus type. However, the tilapia isolate formed a unique branch distinct from other betanodavirus isolates. The disease was experimentally reproduced by bath infection of young tilapia at 30 degrees C. The reservoir of virus at the origin of the outbreak remains unidentified. To our knowledge, this is the first report of natural nodavirus infection in tilapia reared in fresh water.

Differentiation between Cyprinid herpesvirus type-3 lineages using duplex PCR

To date, all the isolates of Cyprinid herpesvirus type-3 (CyHV3) responsible for serious outbreaks in carps Cyprinus carpio have been found to be very similar or identical on the basis of DNA sequences of a few reference genes. However, two genetic lineages (U/I and J) are distinguished by full-length genome sequencing. Two molecular markers presenting genetic variations were targeted for developing a duplex PCR assay able to distinguish CyHV3-U/I from CyHV3-J while avoiding DNA sequencing. The method was validated on a series of 42 samples of infected carps from France, The Netherlands and Poland collected from 2001 to 2008. Among these samples, both the U/I and J genotypes were identified, but also a third genotype representing a genetic intermediate between U/I and J for one of the two molecular markers. A classification of CyHV3 genotypes, based on the alleles of the two molecular markers, is proposed. The assay is easy to perform and provides a genotype information with samples moderately or highly concentrated. This tool should improve our knowledge regarding the present distribution and future diversification of this emerging virus.

Study of the viral interference between infectious pancreatic necrosis virus (IPNV) and infectious haematopoietic necrosis virus (IHNV) in rainbow trout (Oncorhynchus mykiss)

The resistance of rainbow trout (Oncorhynchus mykiss) to an infectious haematopoietic necrosis virus (IHNV) challenge following a preceding non-lethal infection with infectious pancreatic necrosis virus (IPNV) was investigated through experimental dual infections. Trout initially infected with IPNV were inoculated 14 days later with IHNV. Single infections of trout with 1 of the 2 viruses or with cell culture supernatant were also carried out and constituted control groups. No mortality was noted in fish after a single infection with IPNV. This virus had no influence on the head kidney leucocyte phagocytic activity and plasma haemolytic complement activity. IHNV induced a high mortality (72%) and reduced the macrophage phagocytic activity and complement haemolytic activity. It also induced a late production of anti-IHNV antibodies which occurred after clearance of the virus in the fish. In trout co-infected with both viruses, a mortality rate of 2% occurred and the immune parameters were similar to those observed in the fish infected with IPNV only, demonstrating that in co-infected trout IPNV inhibits the effects of IHNV. The studied parameters did not allow us to define the mechanism of interference occurring between these 2 viruses, but some hypothesis are put forward to explain the interference between the 2 viruses.

Induction of a protective immune response against viral nervous necrosis in the European sea bass Dicentrarchus labrax by using betanodavirus virus-like particles

Betanodaviruses are causative agents of viral nervous necrosis (VNN), a devastating disease of cultured marine fish worldwide. Virus particles contain a single type of coat protein that spontaneously assembles into virus-like particles (VLPs) when expressed in a baculovirus expression system. In the present study, the immunogenicity of betanodavirus VLPs and the protection they confer against VNN in the European sea bass Dicentrarchus labrax were investigated. Enzyme-linked immunosorbent assay and seroneutralization tests performed on plasma from fish vaccinated intramuscularly with doses as low as 0.1 microg of VLPs indicated that the VLPs elicited the synthesis of specific antibetanodavirus antibodies with neutralizing activity. Moreover, fish vaccinated with VLPs were protected from challenge with live virus. Both the immune response and the protective effect against viral challenge were dose dependent. Reverse transcription-PCR data indicated that higher doses of vaccine also reduced the number of fish containing detectable quantities of betanodavirus RNA on day 30 after challenge. Taken together these data strongly support the hypothesis that VLPs obtained in the baculovirus expression system may represent an effective vaccine against VNN.

Experimental transmission of sleeping disease in one-year-old rainbow trout, Oncorhynchus mykiss (Walbaum), induced by sleeping disease virus

Sleeping disease (SD) is a serious disease of rainbow trout, Oncorhynchus mykiss, reared in fresh water caused by sleeping disease virus (SDV). In this study a detailed clinical, histological, virological and serological description of the experimental reproduction of SD in 1-year-old rainbow trout exposed to SDV was carried out. Two hundred disease-free fish were intraperitoneally inoculated with a SDV isolate and 100 fish were inoculated with an uninfected cell culture lysate as a negative control. Infected and control fish were randomly removed at days 4, 7, 14, 21, 42 and 70 post-infection. Blood and tissues were collected for virus isolation, histopathological examination and serum neutralization. SDV was detected in serum, kidney and brain of infected fish from 4 to 21 days post-infection (dpi). Characteristic pathological lesions were observed in infected fish as early as 7 dpi. Lesions were first detected in exocrine pancreas and subsequently observed in heart and skeletal muscle. Neutralizing antibodies to SDV were detected in infected fish from 14 to 70 dpi. Infected fish displayed typical signs of SD 1-month pi and the mortality reached 18.7% within 44 days. This study experimentally reproduced all the pathognomonic features of natural outbreaks of SD in 1-year-old rainbow trout.

Élimination des cations métalliques divalents : complexation par l'alginate de sodium et ultrafiltration

Depuis quelques années la pollution par les métaux lourds et devenue un problème important pour la protection de l'environnement et de nombreuses méthodes ont été développées pour éliminer les métaux toxiques présents dans l'eau.

Parmi les différents procédés utilisés, la complexation-ultrafiltration est bien connue et de nombreuses études sur ce sujet sont décrites dans la littérature. Cependant, le choix de nouveaux macroligands hydrosolubles demeure important pour développer cette technologie.

L'un des objectifs de ce travail était de montrer que dans ce procédé un biopolymère peut remplacer un macroligand de synthèse. Les expériences ont été menées avec de l'alginate de sodium, polysaccharide extrait des algues brunes, et porteur de groupements carboxyliques et hydroxydes capables de complexer les cations.

Notre étude se divise en trois parties. Après avoir décrit, dans la première, le matériau et les méthodes utilisées, nous étudions dans la seconde les conditions de l'ultrafiltration (seuil de coupure, pression appliquée, pH, concentration ), avant de discuter dans la troisième les résultats obtenus dans le traitement de solutions contenant Cd2+, Cu2+, Mn2+ and Pb2+.