Nodavirus provokes subclinical encephalitis and retinochoroiditis in adult farmed Atlantic cod, Gadus morhua L

Betanodavirus meningoencephalitis in an Atlantic blue marlin

Viral nervous necrosis (viral encephalopathy and retinopathy) is caused by piscine nodavirus (Nodaviridae, Betanodavirus). Since 1986, this highly infectious virus has caused mass mortalities of up to 100% in farmed saltwater and freshwater fish around the world (with the exception of South America and Antarctica), affecting >60 species across 10 orders. The Atlantic blue marlin (Makaira nigricans Lacépède, 1802) is a top-level predator found throughout the tropical waters of the Atlantic and Indo-Pacific oceans. Despite their popularity as a sportfish, relatively little is known about the Atlantic blue marlin and other billfish. We describe here chronic betanodavirus infection in a juvenile Atlantic blue marlin, which is, to our knowledge, the first report of disease in M. nigricans.

Nervous Necrosis Virus Modulation of European Sea Bass (Dicentrarchus labrax, L.) Immune Genes and Transcriptome towards Establishment of Virus Carrier State

Viral infections of teleost fish have great environmental and economic implications in aquaculture. Nervous necrosis virus (NNV) is a pathogen affecting more than 120 different species, causing high mortality and morbidity. Herein, we studied the course of NNV experimental infection of D. labrax, focusing on survivors which indicated viral carrier state. To determine the carrier state of D. labrax head kidney, we performed a gene expression analysis of selected immune-related genes and we profiled its transcriptome 14 days post infection (dpi). All tested genes showed clear differentiations in expression levels while most of them were up-regulated 14 dpi suggesting that their role is not limited in early antiviral responses, but they are also implicated in disease persistence. To gain a better understanding of the fish that survived the acute infection but still maintained a high viral load, we studied the differential expression of 124 up-regulated and 48 down-regulated genes in D. labrax head kidney, at 14 dpi. Concluding, the NNV virus persistent profile was assessed in D. labrax, where immune-related gene modification was intense (14 dpi) and the head kidney transcriptome profile at this time point offered a glimpse into host attempts to control the infection in asymptomatic carriers.

Betanodavirus and VER Disease: A 30-year Research Review

The outbreaks of viral encephalopathy and retinopathy (VER), caused by nervous necrosis virus (NNV), represent one of the main infectious threats for marine aquaculture worldwide. Since the first description of the disease at the end of the 1980s, a considerable amount of research has gone into understanding the mechanisms involved in fish infection, developing reliable diagnostic methods, and control measures, and several comprehensive reviews have been published to date. This review focuses on host–virus interaction and epidemiological aspects, comprising viral distribution and transmission as well as the continuously increasing host range (177 susceptible marine species and epizootic outbreaks reported in 62 of them), with special emphasis on genotypes and the effect of global warming on NNV infection, but also including the latest findings in the NNV life cycle and virulence as well as diagnostic methods and VER disease control.

Isolation and initial characterization of new betanodaviruses in shellfish

Betanodaviruses cause the disease viral nervous necrosis (VNN) in finfish. Using a novel approach with two consecutive PCRs, detection semi-nested two-step RT-PCR (DSN-2 RT-PCR) and discriminative multiplex two-step RT-PCR (DMT-2 RT-PCR), we have identified the presence of a new type of betanodavirus in shellfish and called it Korean shellfish nervous necrosis virus (KSNNV). Partial nucleotide sequences of the T4 region in RNA2 fragment of KSNNVs were 73%-75% homologous to those of other reported genotypes and formed a new cluster of betanodavirus in phylogenetic tree analysis. Successful isolation of KSNNV was achieved in two of six shellfish samples containing high concentrations of virus using the blind passage method, and the typical shapes of betanodavirus were confirmed in KSNNV-KOR1 by electron microscopy. In the experimental infection test, seven of 14 fish species showed susceptibility to KSNNV-KOR1 isolate but without clinical signs or death. Although the range of susceptible host species was not significantly different from the RGNNV type, the concentration of KSNNV in the brain of infected fish (10² -10⁵ copies/mg brain) was much lower compared to that found in sevenband grouper (Epinephelus septemfasciatus Thunberg) sampled in the moribund stage with RGNNV infection (10⁶ -10⁷ copies/mg brain). However, histopathological analyses showed the presence of multiple vacuoles in brains of all KSNNV-infected fish at 14 days postinjection. In detection test, as a single or multiple type with the other genotype(s) (RGNNV or BFNNV), the prevalence of KSNNV was 8.4% and 8.7% in domestic (62 of 741 samples) and Chinese samples (12 of 138 samples), respectively, but not in finfish. We propose that KSNNVs obtained from shellfish be classified into a separate and new genotype of betanodavirus.

Betanodavirus: Dissection of the viral life cycle

Progressive research has been recently made in dissecting the molecular biology of Betanodavirus life cycle, the causative pathogen of viral encephalopathy and retinopathy in economic important marine fish species. Establishment of betanodavirus infectious clone allows the manipulation of virus genome for functional genomic study, which elucidates the biological event of the viral life cycle at molecular level. The betanodavirus strategizes its replication by expressing anti-apoptosis/antinecrotic proteins to maintain the cell viability during early infection. Subsequently utilizes and controls the biological machinery of the infected cells for viral genome replication. Towards the late phase of infection, mass production of capsid protein for virion assembly induces the activation of host apoptosis pathway. It eventually leads to the cell lysis and death, which the lysis of cell contributes to the accomplishment of viral shedding that completes a viral life cycle. The recent efforts to dissect the entire betanodavirus life cycle are currently reviewed.

Viral Protein Kinetics of Piscine Orthoreovirus Infection in Atlantic Salmon Blood Cells

Piscine orthoreovirus (PRV) is ubiquitous in farmed Atlantic salmon (Salmo salar) and the cause of heart and skeletal muscle inflammation. Erythrocytes are important target cells for PRV. We have investigated the kinetics of PRV infection in salmon blood cells. The findings indicate that PRV causes an acute infection of blood cells lasting 1-2 weeks, before it subsides into persistence. A high production of viral proteins occurred initially in the acute phase which significantly correlated with antiviral gene transcription. Globular viral factories organized by the non-structural protein µNS were also observed initially, but were not evident at later stages. Interactions between µNS and the PRV structural proteins λ1, µ1, σ1 and σ3 were demonstrated. Different size variants of µNS and the outer capsid protein µ1 appeared at specific time points during infection. Maximal viral protein load was observed five weeks post cohabitant challenge and was undetectable from seven weeks post challenge. In contrast, viral RNA at a high level could be detected throughout the eight-week trial. A proteolytic cleavage fragment of the µ1 protein was the only viral protein detectable after seven weeks post challenge, indicating that this µ1 fragment may be involved in the mechanisms of persistent infection.

Epidemiological characterization of VNNV in hatchery-reared and wild marine fish on Hainan Island, China, and experimental infection of golden pompano (Trachinotus ovatus) juveniles

The current epidemiological situation of viral nervous necrosis virus (VNNV) on Hainan Island was investigated. A total of 490 hatchery-reared fish and 652 wild fish were sampled for VNNV detection from March 2013 to May 2014. Positive detection rates of 84.53% (153/181) and 0.97 % (3/309) were obtained in diseased and healthy hatchery-reared samples, respectively, by conventional RT-PCR. However, using more-sensitive nested RT-PCR, the positive detection rates in healthy hatchery-reared fish reached up to 64.08% (198/309), suggesting that asymptomatic VNNV carriers commonly exist among larvae and juveniles breeding on Hainan Island. In wild-fish samples, 2.6% (17/652) and 34.2% (223/652) positive detection rates were observed using RT-PCR and nested RT-PCR, respectively, indicating that wild fish may be a potential reservoir for VNNV. Phylogenetic analysis showed that all 52 VNNV isolates from cultured fish belong to the RGNNV genotype, but 2 out of 48 VNNV isolates from wild fish samples were found to be of the SJNNV genotype. This study is the first to confirm the existence of SJNNV-genotype VNNV in China. Golden pompano, an important fish species for culture, was selected as a fish model to investigate the optimal conditions for RGNNV disease progression in artificial infection experiments. The effects of temperature, salinity, and fish size were evaluated. Results showed that 28 °C and 20 ‰ are the optimal infection temperature and salinity, respectively, and golden pompano juveniles with small body sizes are more susceptible to RGNNV. These findings are highly consistent with those conditions involved in the natural outbreak of RGNNV.

Infectious hematopoietic necrosis virus (IHNV) persistence in Sockeye Salmon: influence on brain transcriptome and subsequent response to the viral mimic poly(I:C)

Background

Sockeye Salmon are an iconic species widely distributed throughout the North Pacific. A devastating pathogen of Sockeye Salmon is infectious hematopoietic necrosis virus (IHNV, genus Novirhabdovirus, family Rhabdoviridae). It has been postulated that IHNV is maintained in salmon populations by persisting over the life of its host and/or by residing in natural reservoirs other than its susceptible hosts. Herein we demonstrate the presence of IHNV in the brain of Sockeye Salmon that survived an experimentally-induced outbreak, suggesting the presence of viral persistence in this susceptible species. To understand the viral persistent state in Sockeye Salmon we profiled the transcriptome to evaluate the host response in asymptomatic carriers and to determine what effects (if any) IHNV exposure may have on subsequent virus challenges.

Results

A laboratory disease model to simulate a natural IHNV outbreak in Sockeye Salmon resulted in over a third of the population incurring acute IHN disease and mortality during the first four months after initial exposure. Nine months post IHNV exposure, despite the absence of disease and mortality, a small percentage (<4 %) of the surviving population contained IHNV in brain. Transcriptome analysis in brain of asymptomatic virus carriers and survivors without virus exhibited distinct transcriptional profiles in comparison to naïve fish. Characteristic for carriers was the up-regulation of genes involved in antibody production and antigen presentation. In both carriers and survivors a down-regulation of genes related to cholesterol biosynthesis, resembling an antiviral mechanism observed in higher vertebrates was revealed along with differences in nervous system development. Moreover, following challenge with poly(I:C), survivors and carriers displayed an elevated antiviral immune response in comparison to naïve fish.

Conclusions

IHN virus persistence was identified in Sockeye Salmon where it elicited a unique brain transcriptome profile suggesting an ongoing adaptive immune response. IHNV carriers remained uncompromised in mounting efficient innate antiviral responses when exposed to a viral mimic. The capacity of IHNV to reside in asymptomatic hosts supports a virus carrier hypothesis and if proven infectious, could have significant epidemiological consequences towards maintaining and spreading IHNV among susceptible host populations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1759-y) contains supplementary material, which is available to authorized users.

Influence of temperature on Betanodavirus infection in Senegalese sole (Solea senegalensis)

In this study Senegalese sole juveniles were experimentally infected with a reassortant Betanodavirus strain at three different temperatures: 22 °C, 18 °C and 16 °C by bath challenge and cohabitation. The results obtained showed that virus virulence decreased by reducing the water temperature. At 22 °C mortalities reached 100%, at 18 °C they ranged from 75 to 80% and at 16 °C only 8% of the fish died. In addition, horizontal transmission was demonstrated regardless of the rearing temperature. At 16 °C active viral replication was detected up to 66 days post-infection, but no signs of the disease were observed and only a very low level of mortality was recorded. The increase in water temperature from 16 to 22 °C caused a quick rise in the viral load and a subsequent outbreak of mortalities. These findings demonstrate that this reassortant Betanodavirus strain can cause a persistent infection in Senegalese sole at low temperatures (16 °C) for long periods of time, and when temperature increases the virus is able to trigger an acute infection and provoke high mortalities.

First report of viral nervous necrosis-induced mass mortality in hatchery-reared larvae of clownfish, Amphiprion sebae Bleeker

Frequent mortality was observed in the larval rearing facility of marine clownfish, Amphiprion sebae Bleeker. Mortality resulted in 80% loss of larval stock. Moribund larvae showed clinical signs typical of viral nervous necrosis, such as uncoordinated, corkscrew-like swimming behaviour, hypersensitivity to stimuli, darkening of body and assembly into large groups, similar to bunches of grapes. The aetiology of the disease was confirmed by gross observation of clinical signs, histopathology and molecular diagnosis. Histological studies revealed severe vacuolation in the brain and in the bipolar and ganglion layers of the eye. Molecular diagnosis by reverse transcription-polymerase chain reaction (RT-PCR) specific to piscine nodavirus yielded a positive result. The partial nucleotide sequences of the PCR-amplified fragment were 97-98% similar to other betanodavirus isolates reported globally and more closely aligned with red-spotted grouper nervous necrosis virus (RGNNV). This is the first report of susceptibility of clownfish, A. sebae, to betanodavirus and the presence of the RGNNV in India.

Characterisation of a monoclonal antibody detecting Atlantic salmon endothelial and red blood cells, and its association with the infectious salmon anaemia virus cell receptor

Endothelial cells (ECs) line the luminal surfaces of the cardiovascular system and play an important role in cardiovascular functions such as regulation of haemostasis and vasomotor tone. A number of fish and mammalian viruses target these cells in the course of their infection. Infectious salmon anaemia virus (ISAV) attacks ECs and red blood cells (RBCs) of farmed Atlantic salmon (Salmo salar L.), producing the severe disease of infectious salmon anaemia (ISA). The investigation of ISA has up to now been hampered by the lack of a functional marker for ECs in Atlantic salmon in situ. In this study, we report the characterisation and use of a novel monoclonal antibody (MAb) detecting Atlantic salmon ECs (e.g. vessel endothelium, endocardial cells and scavenger ECs) and RBCs. The antibody can be used with immunohistochemistry, IFAT and on Western blots. It appears that the epitope recognised by the antibody is associated with the ISAV cellular receptor. Besides being a tool to identify ECs in situ, it could be useful in further studies of the pathogenicity of ISA. Finally, the detection of an epitope shared by ECs and RBCs agrees with recent findings that these cells share a common origin, thus the MAb can potentially be used to study the ontogeny of these cells in Atlantic salmon.

Presence and interaction of inflammatory cells in the spleen of Atlantic cod, Gadus morhua L., infected with Francisella noatunensis

Serious infectious diseases, accompanied by macrophage-dominated chronic inflammation, are common in farmed Atlantic cod. To increase knowledge relating to morphological aspects of such inflammatory responses, cod were challenged with Francisella noatunensis, an important bacterial pathogen of this fish species. Tissue and cell dynamics in the spleen were examined sequentially over 60 days. Small clusters of mainly macrophage-like cells (MLCs) staining for non-specific esterase and acid phosphatase developed with time. These foci were transiently infiltrated by pleomorphic proliferating cells of unknown nature and by granulocyte-like cells (GCLCs) staining for peroxidase and lysozyme. The latter cell type, which appeared to be resident in the red pulp of control fish, migrated into the inflammatory foci of infected fish. Cells expressing genes encoding IFN-γ and IL-8 increased in number during the study period. Bacteria were detected only in the MLCs and their number increased despite the extensive inflammation. Our results demonstrate an intimate spatial relationship in inflammatory foci between at least three cell types. The presence of GCLCs, together with MLCs, suggests pyogranulomatous inflammation as a more appropriate descriptive term than granulomatous inflammation.

Impact of asymptomatic nodavirus carrier state and intraperitoneal viral mimic injection on brain transcript expression in Atlantic cod (Gadus morhua)

Nodaviruses and other RNA viruses have a profoundly negative impact on the global aquaculture industry. Nodaviruses target nervous tissue causing viral nervous necrosis, a disease characterized by neurological damage, swimming abnormalities, and morbidity. This study used functional genomic techniques to study the Atlantic cod (Gadus morhua) brain transcript expression responses to asymptomatic high nodavirus carrier state and intraperitoneal injection of polyriboinosinic polyribocytidylic acid (pIC). Reciprocal suppression subtractive hybridization (SSH) cDNA libraries enriched for virus-responsive brain transcripts were constructed and characterized. We generated 1,938 expressed sequence tags (ESTs) from a forward brain SSH library (enriched for transcripts upregulated by nodavirus and/or pIC) and 1,980 ESTs from a reverse brain SSH library (enriched for transcripts downregulated by nodavirus and/or pIC). To examine the effect of nodavirus carrier state on individual brain gene expression in asymptomatic cod, 27 transcripts of interest were selected for quantitative reverse transcription-polymerase chain reaction (QPCR) studies. Transcripts found to be >10-fold upregulated in individuals with a high nodavirus carrier state relative to those in a no/low nodavirus carrier state were identified as ISG15, IL8, DHX58 (alias LGP2), ZNFX1, RSAD2 (alias viperin), and SACS (sacsin, alias spastic ataxia of Charlevoix-Saguenay). These and other SSH-identified transcripts were also found by QPCR to be significantly (P < 0.05) upregulated by pIC compared with saline-injected controls within 72 h of injection. Several transcripts identified in the reverse SSH library, including two putative ubiquitination pathway members (HERC4 and SUMO2), were found to be significantly (P < 0.05) downregulated in individuals with a high nodavirus carrier state. Our data shows that Atlantic cod brains have a strong interferon pathway response to asymptomatic high nodavirus carrier state and that many interferon pathway and other immune relevant transcripts are significantly induced in brain by both nodavirus and pIC.