A mortality event in wrasse species (Labridae) associated with the presence of viral haemorrhagic septicaemia virus

Diversity, evolution, and emergence of fish viruses

The production of aquatic animals has more than doubled over the last 50 years and is anticipated to continually increase. While fish are recognized as a valuable and sustainable source of nutrition, particularly in the context of human population growth and climate change, the rapid expansion of aquaculture coincides with the emergence of highly pathogenic viruses that often spread globally through aquacultural practices. Here, we provide an overview of the fish virome and its relevance for disease emergence, with a focus on the insights gained through metagenomic sequencing, noting potential areas for future study. In particular, we describe the diversity and evolution of fish viruses, for which the majority have no known disease associations, and demonstrate how viruses emerge in fish populations, most notably at an expanding domestic-wild interface. We also show how wild fish are a powerful and tractable model system to study virus ecology and evolution more broadly and can be used to identify the major factors that shape vertebrate viromes. Central to this is a process of virus-host co-divergence that proceeds over many millions of years, combined with ongoing cross-species virus transmission.

Epitope mapping of the monoclonal antibody IP5B11 used for detection of viral haemorrhagic septicaemia virus facilitated by genome sequencing of carpione novirhabdovirus

The monoclonal antibody (mAb) IP5B11, which is used worldwide for the diagnosis of viral haemorrhagic septicaemia (VHS) in fish, reacts with all genotypes of VHS virus (VHSV). The mAb exceptionally also reacts with the carpione rhabdovirus (CarRV). Following next generation genome sequencing of CarRV and N protein sequence alignment including five kinds of fish novirhabdoviruses, the epitope recognized by mAb IP5B11 was identified. Dot blot analysis confirmed the epitope of mAb IP5B11 to be associated with the region N219 to N233 of the N protein of VHSV. Phylogenetic analysis identified CarRV as a new member of the fish novirhabdoviruses.

Development of a recombinase polymerase amplification assay for viral haemorrhagic septicemia virus

Viral diseases of fish cause significant economic losses in the aquaculture industry. Viral haemorrhagic septicemia virus (VHSV) is one of the most important viral diseases that affects more than 80 fish species. Detection of the disease, especially in the field, is critical to managing disease prevention and control programmes. Recombinase polymerase amplification (RPA) is an isothermal method with a very short amplification period and a single incubation temperature ranging from 37 to 42°C, which is a good alternative to the polymerase chain reaction (PCR). This study aimed to develop an RPA assay as sensitive as a real-time RT-PCR to detect VHSV. For this purpose, primers and probes are designed for the same targeted region of gG of VHSV. The ssRNA standards were prepared to find the detection limits of the assay. Detection limits were found ten-fold differences between real-time RT-PCR and real-time RT-RPA. While the detection limit of the RT-PCR was found as 95.5 viral RNA molecules/reaction in 95% probit value, the detection limit of RT-RPA was found as 943.75 viral RNA molecules/reaction in 95% probit value using ssRNA standards. These results show that RPA is a suitable test for VHSV Ie detection.

Establishment and Characterization of a Novel Gill Cell Line, LG-1, from Atlantic Lumpfish (Cyclopterus lumpus L.)

The use of lumpfish (Cyclopterus lumpus) as a cleaner fish to fight sea lice infestation in farmed Atlantic salmon has become increasingly common. Still, tools to increase our knowledge about lumpfish biology are lacking. Here, we successfully established and characterized the first Lumpfish Gill cell line (LG-1). LG-1 are adherent, homogenous and have a flat, stretched-out and almost transparent appearance. Transmission electron microscopy revealed cellular protrusions and desmosome-like structures that, together with their ability to generate a transcellular epithelial/endothelial resistance, suggest an epithelial or endothelial cell type. Furthermore, the cells exert Cytochrome P450 1A activity. LG-1 supported the propagation of several viruses that may lead to severe infectious diseases with high mortalities in fish farming, including viral hemorrhagic septicemia virus (VHSV) and infectious hematopoietic necrosis virus (IHNV). Altogether, our data indicate that the LG-1 cell line originates from an epithelial or endothelial cell type and will be a valuable in vitro research tool to study gill cell function as well as host-pathogen interactions in lumpfish.

Novel atypical Aeromonas salmonicida bath challenge model for juvenile ballan wrasse (Labrus bergylta, Ascanius)

Atypical Aeromonas salmonicida (aAs) is currently one of the most routinely recovered bacterial pathogens isolated during disease outbreaks in farmed cleaner fish, ballan wrasse (Labrus bergylta, Ascanius). Vibrionaceae family bacteria have also been isolated from ballan wrasse in Scotland. This study determined the infectivity, pathogenicity and virulence of aAs and Vibrionaceae isolates in juvenile farmed ballan wrasse (n = 50; approx. 2 g) using a bath challenge, and fish were monitored for a period of 16 days. Atypical As caused significant mortalities in contrast to Vibrionaceae isolates. Notably, differential virulence was observed between two aAs vapA type V strains at similar challenge doses. Diseased fish exhibited a systemic infection where aAs was detected in all analysed tissues (liver, spleen and kidney) by PCR and qPCR. Macroscopically, moribund and survivor fish exhibited hepatomegaly and splenomegaly. In moribund and surviving fish, histopathology showed granulomatous hepatitis with eosinophilic granular cells surrounding bacterial colonies and endocarditis along with splenic histiocytosis. This is the first report of a successful aAs bath challenge model for juvenile ballan wrasse which provides an important tool for future studies on vaccine efficacy and immunocompetence.

Health assessment of the cleaner fish ballan wrasse Labrus bergylta from the British south-west coast

Wild-caught ballan wrasse Labrus bergylta are translocated en masse from the British south-west coast to Scotland for use as cleaner fish to tackle Atlantic salmon Salmo salar sea lice infestations; however, very little is known about the background health status of this species. This is the first health assessment of wild ballan wrasse from the British south-west. Wild-caught ballan wrasse (n = 75) from coastal populations off Dorset and Cornwall were subjected to a full health screen for viral, bacterial and parasitic infections and associated pathology. A range of metazoan and protozoan parasites were observed in histological sections, including copepods (sea lice Caligus centrodonti), nematodes, cestodes, digenean metacercariae, Cryptocaryon-like ciliates and an intestinal coccidian (Eimeria sp.) observed in 26.6% of the samples. The mycoplasma Acholeplasma laidlawii was associated with cytopathic effect in cell culture inoculated with tissue homogenates. The opportunistic pathogen Photobacterium damselae damselae was isolated from a single fish with a systemic infection. The isolate was confirmed to possess the virulence factors hlyAch and plpV, previously associated with cell toxicity and pathogenicity to fish. There are no immediate concerns for the continued mass translation of ballan wrasse, however careful monitoring of the population is recommended.

Outbreak of viral haemorrhagic septicaemia (VHS) in lumpfish (Cyclopterus lumpus) in Iceland caused by VHS virus genotype IV

A novel viral haemorrhagic septicaemia virus (VHSV) of genotype IV was isolated from wild lumpfish (Cyclopterus lumpus), brought to a land-based farm in Iceland, to serve as broodfish. Two groups of lumpfish juveniles, kept in tanks in the same facility, got infected. The virus isolated was identified as VHSV by ELISA and real-time RT-PCR. Phylogenetic analysis, based on the glycoprotein (G) gene sequences, may indicate a novel subgroup of VHSV genotype IV. In controlled laboratory exposure studies with this new isolate, there was 3% survival in the I.P. injection challenged group while there was 90% survival in the immersion group. VHSV was not re-isolated from fish challenged by immersion. In a cohabitation trial, lumpfish infected I.P. (shedders) were placed in tanks with naïve lumpfish as well as naïve Atlantic salmon (Salmo salar L.). 10% of the lumpfish shedders and 43%-50% of the cohabiting lumpfish survived after 4 weeks. 80%-92% of the Atlantic salmon survived, but no viral RNA was detected by real-time RT-PCR nor VHSV was isolated from Atlantic salmon. This is the first isolation of a notifiable virus in Iceland and the first report of VHSV of genotype IV in European waters.

Isolation of salmonid alphavirus subtype 6 from wild-caught ballan wrasse, Labrus bergylta (Ascanius)

The use of cleaner fish as a biological control for sea lice in Atlantic salmon aquaculture has increased in recent years. Wild-caught wrasse are commonly used as cleaner fish in Europe. In Ireland, samples of wrasse from each fishing area are screened for potential pathogens prior to their deployment into sea cages. Salmonid alphavirus was isolated from a pooled sample of ballan wrasse, showing no signs of disease, caught from the NW of Ireland. Partial sequencing of the E2 and nsP3 genes showed that it was closely related to the previously reported SAV subtype 6. This represents only the second isolation of this subtype and the first from a wild fish species, namely ballan wrasse.

Presence of selected pathogens on the gills of five wrasse species in western Norway

The objective of this study was to identify gill pathogens in Labridae (wrasse) species used as cleaner fish to control salmon louse in western Norwegian aquaculture. Wrasse are often moved over long distances, raising issues of fish health, welfare and pathogen transmission. Histological examination and real-time RT-PCR analysis of the gills from Centrolabrus exoletus, Ctenolabrus rupestris, Labrus bergylta, L. mixtus and Symphodus melops revealed several pathogens: a new species of Ichthyobodo, Paramoeba perurans, microsporidia, trichodinids, Hatschekia spp., Candidatus Similichlamydia labri and 2 putative new species of Chlamydiae. Cand. S. labri or closely related bacteria were present on most wrasse specimens. Epitheliocysts on the gills of L. mixtus contained large inclusions (120 µm) with actiniae radiating from the inclusion membrane. A possible member of the Candidatus family Parilichlamydiaceae was present at a high prevalence on the gills of L. mixtus, L. bergylta and C. rupestris. Sequencing the 16S rRNA gene showed 93.9% similarity to Cand. S. labri and 96.8% similarity to Cand. Parilichlamydia carangidicola from the gills of Seriola lalandi. This bacterium probably represents a new species within the order Chlamydiales, family Cand. Parilichlamydiaceae. The other Chlamydiae detected on gills of S. melops could represent a new species in Cand. genus Syngnamydia. Ichthyobodo sp. and Paranucleospora theridion were detected on the gills of nearly all individuals, while Paramoeba spp. were detected on the gills of L. bergylta and L. mixtus. Trichodinids, microsporidia and parasitic copepods had low prevalence. Viral haemorrhagic septicaemia virus was not detected.

Viral haemorrhagic septicaemia virus (VHSV) remains viable for several days but at low levels in the water flea Moina macrocopa

Viral haemorrhagic septicaemia virus (VHSV) Genotype IVb has been isolated from amphipods belonging to the genus Diporeia, but it has yet to be established whether crustacean zooplankton act as vectors of this virus for fish species. Therefore, we evaluated the viability of infectious VHSV in the water flea Moina macrocopa. VHSV was re-isolated from replicate groups of M. macrocopa that had been immersed with 108.0, 107.0, and 105.0 TCID50 ml-1 of VHSV (DK-3592B, Genotype Ia). Furthermore, 40 M. macrocopa that had been immersed with 108.0 TCID50 ml-1 of VHSV for 72 h had VHSV titers of 102.7-104.3 TCID50. Thus, VHSV was clearly taken up by M. macrocopa and remained viable in this crustacean for several days. However, no mortality was observed over a 28 d period in rainbow trout Oncorhynchus mykiss that were fed VHSV-contaminated M. macrocopa for 14 d, and we found that the virus titer significantly decreased after a 4 h incubation with pyloric caecal extracts from rainbow trout, indicating that passage through the gut is likely to result in a significant decrease in viral titer. This may explain why consumption of prey containing low levels of VHSV did not result in clinical VHS.

A historical review of the key bacterial and viral pathogens of Scottish wild fish

Thousands of Scottish wild fish were screened for pathogens by Marine Scotland Science. A systematic review of published and unpublished data on six key pathogens (Renibacterium salmoninarum, Aeromonas salmonicida, IPNV, ISAV, SAV and VHSV) found in Scottish wild and farmed fish was undertaken. Despite many reported cases in farmed fish, there was a limited number of positive samples from Scottish wild fish, however, there was evidence for interactions between wild and farmed fish. A slightly elevated IPNV prevalence was reported in wild marine fish caught close to Atlantic salmon, Salmo salar L., farms that had undergone clinical IPN. Salmonid alphavirus was isolated from wild marine fish caught near Atlantic salmon farms with a SAV infection history. Isolations of VHSV were made from cleaner wrasse (Labridae) used on Scottish Atlantic salmon farms and VHSV was detected in local wild marine fish. However, these pathogens have been detected in wild marine fish caught remotely from aquaculture sites. These data suggest that despite the large number of samples taken, there is limited evidence for clinical disease in wild fish due to these pathogens (although BKD and furunculosis historically occurred) and they are likely to have had a minimal impact on Scottish wild fish.

High nervous necrosis virus (NNV) diversity in wild wrasse (Labridae) in Norway and Sweden

Wild goldsinny wrasse Ctenolabrus rupestris, corkwing wrasse Symphodus melops and ballan wrasse Labrus bergylta were collected at 8 sampling sites in Sweden and Norway during summer 2014. Brain tissue from 466 wrasses were analyzed for nervous necrosis virus (NNV) infections by real-time RT-PCR, and positive samples were subjected to sequencing and phylogenetic analysis of partial segments of the RNA2 and RNA1 genes. This study shows that NNV is present in wild ballan, corkwing and goldsinny wrasse along the coastline of Sweden and Norway. The overall prevalence in the sampled labrids was 6.7%. Prevalence was 6.4% in goldsinny, 6.3% in corkwing and 18% in ballan wrasse. The wrasse RNA2 NNV sequences revealed high genetic variability and were divided into 3 clusters within the cold water barfin flounder NNV (BFNNV) and warm water cluster red-spotted grouper NNV (RGNNV) genogroups. Within the BFNNV genogroup, wrasse NNVs clustered in 2 sub-genogroups, with grey mullet NNV (GMNNV) and with Atlantic halibut NNV (AHNNV). These groups were previously dominated by virus originating from Atlantic cod Gadus morhua and Atlantic halibut Hippoglossus hippoglossus from the northeast Atlantic. The presence of NNV in wild wrasse and the surprising high genetic variability observed in this study should be considered before moving wild-caught wrasse between geographically distant sites. The results show that use of wild-caught wrasse as brood fish in wrasse farming represents a risk of introducing NNV into aquaculture.

The Promise of Whole Genome Pathogen Sequencing for the Molecular Epidemiology of Emerging Aquaculture Pathogens

Aquaculture is the fastest growing food-producing sector, and the sustainability of this industry is critical both for global food security and economic welfare. The management of infectious disease represents a key challenge. Here, we discuss the opportunities afforded by whole genome sequencing of bacterial and viral pathogens of aquaculture to mitigate disease emergence and spread. We outline, by way of comparison, how sequencing technology is transforming the molecular epidemiology of pathogens of public health importance, emphasizing the importance of community-oriented databases and analysis tools.

Potential distribution of the viral haemorrhagic septicaemia virus in the Great Lakes region

Viral haemorrhagic septicaemia virus (VHSV) genotype IVb has been responsible for large-scale fish mortality events in the Great Lakes of North America. Anticipating the areas of potential VHSV occurrence is key to designing epidemiological surveillance and disease prevention strategies in the Great Lakes basin. We explored the environmental features that could shape the distribution of VHSV, based on remote sensing and climate data via ecological niche modelling. Variables included temperature measured during the day and night, precipitation, vegetation, bathymetry, solar radiation and topographic wetness. VHSV occurrences were obtained from available reports of virus confirmation in laboratory facilities. We fit a Maxent model using VHSV-IVb reports and environmental variables under different parameterizations to identify the best model to determine potential VHSV occurrence based on environmental suitability. VHSV reports were generated from both passive and active surveillance. VHSV occurrences were most abundant near shore sites. We were, however, able to capture the environmental signature of VHSV based on the environmental variables employed in our model, allowing us to identify patterns of VHSV potential occurrence. Our findings suggest that VHSV is not at an ecological equilibrium and more areas could be affected, including areas not in close geographic proximity to past VHSV reports.

Susceptibility of goldsinny wrasse, Ctenolabrus rupestris L. (Labridae), to viral haemorrhagic septicaemia virus (VHSV) genotype III: Experimental challenge and pathology

Cleaner fish, such as wrasse, are being increasingly used to combat the sea lice infestation of Atlantic salmon (Salmo salar L.) in many European countries. To determine susceptibility of the goldsinny wrasse (Ctenolabrus rupestris L.) and pathogenesis of the viral haemorrhagic septicaemia virus (VHSV) genotype III isolate 12-654, previously associated with VHSV infection in the Shetland Islands in 2012, fish were experimentally challenged by intraperitoneal injection (IP), bath immersion and cohabitation routes. Cumulative proportion of moribund wrasse reached 17% following the virus immersion challenge while by the IP-route moribunds exceeded 50% within 14days post-challenge. Typical signs of VHS as reported in rainbow trout (Oncorhynchus mykiss), were not observed in moribund goldsinny wrasse. The most pronounced histopathological changes, consistent regardless of the route of infection, were observed within the heart and included atrium myofibril degeneration, focal infiltration and multifocal necrosis, with prominent swelling of the endocardium and occasional detachment. Pathological changes in the atrium were associated with presence of the viral antigen as confirmed by a positive immunohistochemical staining. Virus clearance and heart tissue recovery were noted although further experiments are required to confirm these observations. The results of a cohabitation experiment confirmed that goldsinny wrasse shed viable virus and therefore represent a risk of virus transmission to other VHSV susceptible species. Similarities between the pathology in goldsinny wrasse induced through the controlled experimental challenges and that of wrasse spp. from an infection occurrence in Shetland are discussed.

A survey of wild marine fish identifies a potential origin of an outbreak of viral haemorrhagic septicaemia in wrasse, Labridae, used as cleaner fish on marine Atlantic salmon, Salmo salar L., farms

Viral haemorrhagic septicaemia virus (VHSV) was isolated from five species of wrasse (Labridae) used as biological controls for parasitic sea lice predominantly, Lepeophtheirus salmonis (Krøyer, 1837), on marine Atlantic salmon, Salmo salar L., farms in Shetland. As part of the epidemiological investigation, 1400 wild marine fish were caught and screened in pools of 10 for VHSV using virus isolation. Eleven pools (8%) were confirmed VHSV positive from: grey gurnard, Eutrigla gurnardus L.; Atlantic herring, Clupea harengus L.; Norway pout, Trisopterus esmarkii (Nilsson); plaice, Pleuronectes platessa L.; sprat, Sprattus sprattus L. and whiting, Merlangius merlangus L. The isolation of VHSV from grey gurnard is the first documented report in this species. Nucleic acid sequencing of the partial nucleocapsid (N) and glycoprotein (G) genes was carried out for viral characterization. Sequence analysis confirmed that all wild isolates were genotype III the same as the wrasse and there was a close genetic similarity between the isolates from wild fish and wrasse on the farms. Infection from these local wild marine fish is the most likely source of VHSV isolated from wrasse on the fish farms.

Epidemiology and Economics Support Decisions about Freedom from Aquatic Animal Disease

In this study, we review the application of epidemiology and economics to decision-making about freedom from aquatic animal disease, at national and regional level, and recent examples from Europe. Epidemiological data (e.g. pathogen prevalence and distribution) determine the technical feasibility and cost of eradication. The eradication of pathogens which exist in wild populations, or in a latent state, is technically difficult, uncertain and expensive. Notably, the eradication of diseases of molluscs is rarely attempted because host populations (farmed and wild) cannot be completely removed from open water systems. Doubt about the success of eradication translates into uncertain ex-ante cost estimates. Additionally, the benefits of an official disease-free status cannot be estimated with any accuracy. For example, in Europe, official freedom from epizootic ulcerative syndrome and white spot syndrome virus has not been pursued, arguably because the evidence does not exist for the benefits (reduced risk of disease in wild populations) to be estimated and thus weighed against the costs of maintaining disease freedom (e.g. restriction on imports). Economic analysis must assess not only whether the benefits of disease freedom outweigh costs, but whether it is the economically optimal disease control option. Government may also want to compare investment in aquatic animal health with other opportunities. As resources become scarce, governments have sought to share costs of disease control with industry, and thus to ensure equity, the distribution benefits must be known so costs can be borne by those who benefit. The economic principles to support decisions about disease freedom are well established, but their application is constrained by lack of epidemiological data, which may explain the lack of economic analysis in support of aquatic animal management in Europe. The integration of epidemiology and economics in disease control planning will identify research aimed at improving the underpinning evidence base.