Emergence of carp edema virus (CEV) and its significance to European common carp and koi Cyprinus carpio

Potential to grow carp oedema virus (genogroup I) in monolayers of carp-derived primary cells with further implication in cell analysis

Carp oedema virus (CEV) has distinct molecularly identified genogroups of viral mutations, denoted as I, IIa, and IIb. Failure to propagate CEV in vitro limits studies towards understanding its interactions with host cells. Here, virus isolates belonging to genogroup I collected during natural outbreaks in the Czech Republic were employed for routine CEV cultivation in monolayers of carp-derived primary cells, common carp brain (CCB) cells, and epithelioma papulosum cyprinid (EPC) cells. Induction of cytopathic effects (CPEs) was observed and recorded in affected cells. Cell survival rate was evaluated under serial dilutions of the CEV inoculum. Virus cell entry was quantified and visualized by qPCR and transmission electron microscopy, respectively. Study findings indicate primary gills epithelia likely present the most suitable matrix for CEV growth in vitro. Cells of the head kidney and spleen facilitate virus entry with microscopically confirmed CPEs and the presence of cytoplasmic pleomorphic virus particles. Cells of the trunk kidney and gonads are unlikely to permit virus cell entry and CPEs development. Although CEV cultivation in cell lines was inconclusive, EPC cells were CEV permissible. Monolayers of carp-derived primary cells show promise for CEV cultivation that could enable elaborate study of mechanisms underlying cellular binding and responses.

Stressing out-carp edema virus induces stress and modulates immune response in common carp

Introduction

Carp edema virus (CEV) is a fish poxvirus that primarily infects the gills of common carp. CEV causes koi sleepy disease (KSD), which is highly contagious and can result in mortality of up to 100%.

Methods

In the present study, we analyzed the stress and immune responses during KSD in two strains of common carp with different resistance to CEV: susceptible koi and resistant Amur sazan. Experiments were performed at two temperatures: 12°C and 18°C. In the case of koi carp, we also analyzed the effect of supplementation of 0.6% NaCl into tank water, which prevents mortality of the CEV-infected fish (salt rescue model).

Results

We found that CEV-infected koi kept at 18°C had the highest viral load, which correlated with the most severe histopathological changes in the gills. CEV infection resulted in the activation of stress response reflected by the upregulated expression of genes involved in stress response in the stress axis organs and increased levels of cortisol and glucose in the blood plasma. These changes were the most pronounced in CEV-infected koi kept at 18°C. At both temperatures, the activation of antiviral immune response was observed in koi kept under freshwater and NaCl conditions upon CEV infection. Interestingly, a clear downregulation of the expression of adaptive immune genes was observed in CEV-infected koi kept under freshwater at 18°C.

Conclusion

CEV induces a stress response and modulates adaptive immune response in koi, and this is correlated with the level of viral load and disease development.

Assessing tropism and genetic traits of carp oedema virus isolates to enhance detection strategies

Introduction

Carp oedema virus (CEV) is a relatively understudied poxvirus. It exhibits an affinity for gill and skin epithelial cells. Investigations were conducted into selected aspects of CEV biology, with a focus on determining cell and tissue tropism of CEV, acquiring gene sequences and updating CEV tests in fish tissues.

Material and Methods

A total of 238 common carp tissue samples from nine aquaculture farms were analysed. The study evaluated the efficacy of intermediate detection of CEV by real-time PCR and in situ hybridisation. The genes encoding protein P4a were sequenced, analysed and aligned in a phylogenetic tree using a molecular evolution model.

Results

In situ hybridisation revealed the necessity to validate the Centre for Environment, Fisheries and Aquaculture Science protocols for sampling for CEV detection and to use the tissues for which the virus has the highest tropism, namely the skin and kidneys, rather than solely the gills. The level of genetic variability was determined, and it was shown that CEV mutates systematically. The creation of two distinct phylogenetic clades confirms certain strains' description as Polish isolates.

Conclusion

Determining the localisation of CEV genetic material in organs and tissues is pivotal for shaping the World Organisation for Animal Health guidelines. The utility of molecular diagnostics has been demonstrated in the skin and kidney of carp, in addition to the gills, impelling their inclusion in diagnostic protocols. The clusters identified in the phylogenetic tree offer valuable insights for developing the current PCR primers. The prevalence of CEV infection in aquaculture, juxtaposed with its notably lower detection in wild fish, underscores the significance of mandatory molecular diagnostic testing for CEV in carp farming.

Mucosal immune responses of gut IgM in common carp (Cyprinus carpio) following infection with spring viremia of carp virus (SVCV)

Immunoglobulin M (IgM) specifically recognizes various antigens and can activate complement, mediate cytotoxicity, opsonize and agglutinate pathogens to induce phagocytosis, all of which play an important role in immunity. However, the IgM response of common carp (Cyprinus carpio) in the intestinal mucosa after viral infection has not been thoroughly. Therefore, we successfully produced an anti-carp IgM monoclonal antibody and developed a model of viral infection to study the kinetics of immune responses after viral infection. Our results showed that the expression of IL1-β and Igs were dramatically increased, implying that common carp exhibited a significant innate and adaptive immune response to viral infection. Furthermore, we found that the IgM responses varied between the two infection strategies. At 14 days post-infection (DPI), a significant population of IgM+ B cells were observed in the gut, accompanied by a sharp rise in IgM levels. The immune response to secondary infection started at 7 DPI, suggesting that the IgM response is faster in the gut after re-infection. Importantly, we also explored the variability of different gut compartments to viral infection, and result revealed a stronger immune response in the hindgut than in the foregut and midgut. Overall, our findings indicate that IgM plays an important role in the intestinal immune response following primary and secondary viral infection, in which the hindgut plays a major immune function.

Hemolysin from Aeromonas hydrophila enhances the host's serum enzyme activity and regulates transcriptional responses in the spleen of Cyprinus rubrofuscus

Aeromonas hydrophila is a conditional pathogen impacting public hygiene and safety. Hemolysin is a virulence factor of Aeromonas hydrophila that causes erythrocyte hemolysis, yet its transcriptional response to Cyprinus rubrofuscus remains unknown. Our investigation confirmed the hemolysis of hemolysin from A. hydrophila. Serum enzyme activity was evaluated weekly after C. rubrofuscus were immunized with hemolysin Ahh1. The results showed that the hemolysin enhances the serum superoxide dismutase (SOD), lysozyme (LZM), and catalase (CAT) activity, which reached a maximum on day 14. To elucidate the molecular interaction between hemolysin from A. hydrophila and the host, we performed transcriptome sequencing on the spleen of C. rubrofuscus 14 days post hemolysin infection. The total number of clean reads was 41.37 Gb, resulting in 79,832 unigenes with an N50 length of 1863 bp. There were 1982 significantly differentially expressed genes (DEGs), including 1083 upregulated genes and 899 downregulated genes. Transcript levels of the genes, such as LA6BL, CD2, and NLRC5, were significantly downregulated, while those of IL11, IL1R2, and IL8 were dramatically upregulated. The DEGs were mainly enriched in the immune disease, viral protein interaction with cytokine and cytokine receptor, and toll-like receptor pathways, suggesting that hemolysin stimulation can activate the transcriptional responses. RT-qPCR experiments results of seven genes, IL-8, STAT2, CTSK, PRF1, CXCL9, TLR5, and SACS, showed that their expression was highly concordant with RNA-seq data. We clarified for the first time the key genes and signaling pathways response to hemolysin from A. hydrophila, which offers strategies for treating and preventing diseases.

Genotyping tool for salmonid gill pox virus (SGPV) obtained from farmed and wild Atlantic salmon (Salmo salar)

Poxviruses are common viruses found in vertebrate species. In 2006, the first poxvirus associated with salmon, salmonid gill poxvirus (SGPV), was identified during an outbreak of gill disease at a smolt production site in northern Norway and at two marine farms in western Norway. Poxviruses had previously been detected in ayu (Plecoglossus altivelis) and koi carp (Cyprinus carpio). In all three fish species, poxviruses are associated with gill disease. It has not been possible to culture SGPV from Norway, and little is known about its virulence. However, the association between SGPV and gill disease in salmon has shown the need for molecular tools to identify reservoirs and transmission routes. Sequencing the genome of a second isolate of SGPV has made it possible to compare variable regions between two strains of the virus, showing the presence of a large number of variable regions that exhibit both variable numbers of tandem repeats and intra-ORF variation. We present eight regions that are suitable for distinguishing strains of SGPV and determining their phylogenetic relationship, and these were used to compare SGPV isolates obtained from both farmed and wild salmon in fresh and sea water. The prevalence of the virus was found to be higher in wild salmon in rivers than in returning wild salmon collected from traps in Norwegian fjords. Genotyping based on the eight selected variable regions, suggests the presence of geographically distinct isolates in freshwater among both farmed and wild salmon, while SGPV from marine farms shows high local diversity and a wide geographical distribution of similar strains of the virus.

Distribution of carp edema virus in organs of infected juvenile common carp

Introduction

The disease caused by carp edema virus (CEV) manifests with lethargy as a primary sign; this observation in koi in Japan gained the disease the name koi sleepy disease (KSD). In the years following the discovery of the virus in Japan, KSD cases have been noted in the UK in koi and common carp. Conducting research in order to expand knowledge of the processes of distribution of CEV in infected fish organs will be helpful for eradication and diagnostic purposes.

Material and Methods

Carp edema virus-affected fish with clinical signs of KSD were experimentally cohabited with common carp fry (30 fish). Three fish were euthanised by bath in a 0.5 g L-1 tricaine solution at one week intervals (7, 14, 21 and 28 days post cohabitation). Tissue samples from the brain, gills, spleen, kidney, intestines and skin were collected, and the total DNA was extracted and tested by real-time PCR.

Results

By the seventh day post infection, CEV DNA was most often found in the skin, gills and brain and less frequently in the kidney and intestines. In many of the common carp fry, CEV DNA could typically be found in several organs of each individual fish, although it was only found in one sample of spleen tissue.

Conclusion

In this experimental study the pathogenesis of the CEV infection process was shown, the high infectivity of CEV was confirmed and the best organs were determined for sampling in CEV-infection experimentation. The real-time PCR method used in our cohabitation experiments was shown to be useful at the clinical and asymptomatic stage of virus infection.

Dietary Olive Leaf Extract Differentially Modulates Antioxidant Defense of Normal and Aeromonas hydrophila-Infected Common Carp (Cyprinus carpio) via Keap1/Nrf2 Pathway Signaling: A Phytochemical and Biological Link

Olive leaves are an immense source of antioxidant and antimicrobial bioactive constituents. This study investigated the effects of dietary incorporation of olive leaf extract (OLE) on the growth performance, hematobiochemical parameters, immune response, antioxidant defense, histopathological changes, and some growth- and immune-related genes in the common carp (Cyprinus carpio). A total of 180 fish were allocated into four groups with triplicate each. The control group received the basal diet without OLE, while the other three groups were fed a basal diet with the OLE at 0.1, 0.2, and 0.3%, respectively. The feeding study lasted for 8 weeks, then fish were challenged with Aeromonas hydrophila. The results revealed that the group supplied with the 0.1% OLE significantly exhibited a higher final body weight (FBW), weight gain (WG%), and specific growth rate (SGR) with a decreased feed conversion ratio (FCR) compared to the other groups (p < 0.05). An increase in immune response was also observed in the fish from this group, with higher lysosome activity, immunoglobulin (IgM), and respiratory burst than nonsupplemented fish, both before and after the A. hydrophila challenge (p < 0.05). Similarly, the supplementation of the 0.1% OLE also promoted the C. carpio's digestive capacity pre- and post-challenge, presenting the highest activity of protease and alkaline phosphatase (p < 0.05). In addition, this dose of the OLE enhanced fish antioxidant capacity through an increase in the activity of superoxide dismutase (SOD) and glutathione peroxidase (GPx) and decreased hepatic lipid peroxidation end products (malondialdehyde-MDA), when compared to the control group, both pre- and post-infection (p < 0.05). Concomitantly with the superior immune response and antioxidant capacity, the fish fed the 0.1% OLE revealed the highest survival rate after the challenge with A. hydrophila (p < 0.05). A significant remarkable upregulation of the hepatic sod, nrf2, and protein kinase C transcription levels was detected as a vital approach for the prevention of both oxidative stress and inflammation compared to the infected unsupplied control group (p < 0.05). Interestingly, HPLC and UPLC-ESI-MS/MS analyses recognized that oleuropein is the main constituent (20.4%) with other 45 compounds in addition to tentative identification of two new compounds, namely oleuroside-10-carboxylic acid (I) and demethyl oleuroside-10-carboxylic acid (II). These constituents may be responsible for the OLE exerted potential effects. To conclude, the OLE at a dose range of 0.66-0.83 g/kg w/w can be included in the C. carpio diet to improve the growth, antioxidant capacity, and immune response under normal health conditions along with regulating the infection-associated pro-inflammatory gene expressions, thus enhancing resistance against A. hydrophila.

Clinical and Laboratory Parameters of Carp Edema Virus Disease: A Case Report

In the present study, we describe a natural outbreak of carp edema virus disease (CEVD) in koi carp, concentrating on clinical manifestation, gross and microscopic pathology, immunological parameters, viral diagnostics, and phylogenetic analysis. Examination of white blood cell parameters showed increased monocyte and decreased lymphocyte counts in CEV-affected fish compared to healthy control fish. Regarding immune system functioning, the present work shows, for the first time, enhanced phagocytic activity in CEV-affected fish. Respiratory burst of phagocytes was strongly increased in diseased fish, the increase being attributed to an increased phagocyte count rather than enhancement of their metabolic activity. The present work also newly shows histopathological changes in the pancreatic tissue of diseased koi.

Characteristics of pathology and transcriptome profiling reveal features of immune response of acutely infected and asymptomatic infected of carp edema virus in Koi

Koi sleepy disease (KSD) is a high mortality and infection viral disease caused by carp edema virus (CEV), which was a serious threat to aquaculture of common carp and export trade of Koi worldwide. Asymptomatic infection is an important cause of the difficulty in preventing KSD and its worldwide spread, because asymptomatic infection can be activated under appropriate condition. However, the understanding of the molecular correlates of these infections is still unknown. The purpose of this study was to compare the pathology change, enzyme activity, immunoglobulin activity, host and viral gene expression differences in acutely infected and cohabiting asymptomatic Koi infected with CEV. Healthy Koi were used as a control. The gross pathology, histopathology and ultrastructural pathology showed the difference and characteristics damage to the tissues of Koi under different infection conditions. Periodic Acid-Schiff stain (PAS), enzyme activity and immunoglobulin activity revealed changes in the immune response of gill tissue between acutely infected, asymptomatic infected and healthy Koi. A total of 111 and 2484 upregulated genes and 257 and 4940 downregulated genes were founded in healthy Koi vs asymptomatic infected Koi and healthy Koi vs acutely infected Koi, respectively. Additionally, 878 upregulated genes and 1089 downregulated genes were identified in asymptomatic vs. acutely infected Koi. Immune gene categories and their corresponding genes in different comparison groups were revealed. A total of 3, 59 and 28 immune-related genes were identified in the group of healthy Koi vs asymptomatic infected Koi, healthy Koi vs acutely infected Koi and asymptomatic infected Koi vs acutely infected Koi, respectively. Nineteen immune-related genes have the same expression manner both in healthy Koi vs acutely infected Koi and asymptomatic Koi vs acutely infected Koi, while 9 immune-related genes were differentially expressed only in asymptomatic Koi vs acutely infected Koi, which may play a role in viral reactivation. In addition, 8 differentially expressed genes (DEGs) were validated by quantitative reverse transcription PCR (RT-qPCR), and the results were consistent with the RNA-Seq results. In conclusion, the data obtained in this study provide new evidence for further elucidating CEV-host interactions and the CEV infection mechanism and will facilitate the implementation of integrated strategies for controlling CEV infection and spread.

Fish immune responses to natural infection with carp edema virus (Koi sleepy disease): An emerging fish disease in India

Emerging pathogen, carp edema virus (CEV) causes koi sleepy disease (KSD) in Koi and common carp causing severe mortalities worldwide. In the present study, a total of 150 fish species belonging to eight different families were sampled from the ornamental fish retailers and farms, located in Karnataka, India. The OIE protocol viz., level-I, II and III diagnoses confirmed the infection of CEV in 10 koi fish. Interestingly, other fish species belonging to different fish family including cyprinidae family were negative to CEV. Further, CEV infection was confirmed by sequencing (partial 4a gene); it showed the similarity with that of CEV reported from India and Germany strains with similarity of 97.4-99.94% and belonged to genogroup IIa. TEM analysis of purified CEV, in vivo cohabitation and tissue infection experiments confirmed the CEV infection. In addition, viral load was significantly higher (10^6-7 copies) in koi collected from Dakshina Kannada than of Bengaluru (10^3-4 copies). To understand the host-pathogen interaction, different organs such as gill, kidney, liver and spleen from naturally (CEV) infected koi were used to study the immune gene responses by using eight innate and one adaptive immune response. Results indicated that TNF-α, RohTNF-α, iNOS, IFN-γ and IL-10, and catalyze β-2M of MHC class I pathway genes were upregulated in koi. Higher expression of immune genes during the CEV infection may have inhibited viral replication and mount an antigenic adaptive response. Similar to other viral infections, interferon-γ play an important role during poxvirus infections. Quantification of immune genes in infected fish will provide insights into the host responses and provide valuable information to devise intervention strategies to prevent and control disease due to CEV.

Carp edema virus infection associated gill pathobiome: A case report

Understanding disease aetiology and pathologic mechanisms is essential for fish health evaluation. Carp edema virus (CEV) is the causative agent of a disease (CEVD) responsible for high mortality rates in both wild and cultured common carp Cyprinus carpio. Inspection of two carp specimens from a pond with high fish mortality revealed CEV infection in both the host and its ectoparasite (Argulus foliaceus). In addition to flavobacteria, well known to be associated with gill lesions, we found that free-living eukaryotes (amoebae and ciliates) and a temporary parasite (Ichthyobodo spp.) colonizing the gills may also contribute to alterations in gill structure and/or function, either directly, through firm (Ichthyobodo) or weak (amoebae) attachment of trophozoites to the gill epithelium, or indirectly, through carriage of pathogenic bacteria. Bacterial assemblages rich in families and genera, with predominance of Cetobacterium spp. in low-intensity alteration of the gill tissue and of Flavobacterium spp. in gills with extensive necrotic lesions, were detected in gills and within the cytoplasm of associated amoebae using high-throughput sequencing. Quantitative PCR indicated F. swingsii as the prevailing flavobacterial species within amoebae from less affected gills and F. psychrophilum within amoebae from extensively affected gills. This case study suggests that eukaryotic organisms as part of the gill pathobiome may also contribute to irreversible gill lesions seen in CEVD. Emphasizing the complexity of mutual relationships between bacterial assemblages and eukaryotic co-pathogens, further studies regarding factors that trigger pathology and influence severity in the CEV-positive carp are needed.

Impact of climate driven temperature increase on inland aquaculture: application to land-based production of common carp (Cyprinus carpio L.)

Climate change will expose the food producing sector to a range of challenges. Inland aquaculture farms are particularly vulnerable, due to the difficulty in changing their location, and therefore require specific tools to predict the influence of direct and indirect effects on production, environment, and economic feasibility. The objective of our study was to apply a simple set of models to produce a set of growth, risk and suitability maps for stakeholders within the common carp sector in Poland, to assist decision making under two different scenarios of climate change: a moderate situation (RCP 4.5) and an extreme situation (RCP 8.5). We used present (2000-2019) and future projections (2080-2099) for water surface temperature based on land surface temperature data from regionally downscaled climate models to draw maps to: i) show optimal temperature conditions for carp growth, ii) assess risk of disease outbreak caused by three important common carp pathogens: Cyprinid herpesvirus 3 (CyHV-3), carp edema virus (CEV) and spring viremia of carp (SVCV), and iii) predict potential suitability changes of carp farming in Poland. The study identified areas with the most and least favourable temperature conditions for carp growth, as well as those areas with the highest/lowest number of days with suitable temperatures for virus infection. These suitability maps showed the combined effect of direct and indirect effects of climate change projections under RCP 8.5 and RCP 4.5 scenarios. The approach applied herein will be of use worldwide for analysing the risks of temperature increase to land-based aquaculture, and the results presented are important for carp farmers in Poland and elsewhere, industry in general, and government stakeholders, to understand the direct and indirect effects of climate change on the triple bottom line of people, planet, and profit. This article is protected by copyright. All rights reserved.

Cytokines Studied in Carp (Cyprinus carpio L.) in Response to Important Diseases

Cytokines belong to the most widely studied group of intracellular molecules involved in the function of the immune system. Their secretion is induced by various infectious stimuli. Cytokine release by host cells has been extensively used as a powerful tool for studying immune reactions in the early stages of viral and bacterial infections. Recently, research attention has shifted to the investigation of cytokine responses using mRNA expression, an essential mechanism related to pathogenic and nonpathogenic-immune stimulants in fish. This review represents the current knowledge of cytokine responses to infectious diseases in the common carp (Cyprinus carpio L.). Given the paucity of literature on cytokine responses to many infections in carp, only select viral diseases, such as koi herpesvirus disease (KHVD), spring viremia of carp (SVC), and carp edema virus disease (CEVD), are discussed. Aeromonas hydrophila is one of the most studied bacterial pathogens associated with cytokine responses in common carp. Therefore, the cytokine-based immunoreactivity raised by this specific bacterial pathogen is also highlighted in this review.

Genetic diversity of the carp oedema virus in France

The koi sleepy disease of carp caused by the carp oedema virus (CEV) was observed on farms and in ponds in France since the 2010s. Samples of CEV collected in France over a period of eight years were characterized at the molecular level by sequencing the partial p4a gene. All the sequences, except one, fell into two well-defined genogroups. Sequences obtained from CEV detected in common carp generally clustered in genogroup I and sequences from CEV detected in the koi were assigned to genogroup II. A particular sample was different to the others and represented a putative new genogroup possibly arose from a recombination event between a genogroup II sequence and one from an unknown genogroup. Compared with sequences from CEV of other countries, most of the French sequences exhibited high degree of DNA identities with those published previously, indicating identical sources of viruses. The sequence diversity suggests multiple introductions of the viruses in France. Among the French sequences, two genogroup-specific molecular markers were identified. One was an insertion/deletion identified within a microsatellite and other was a group of single nucleotide polymorphisms. CEV seems to generate genetic diversity via diverse mechanisms: substitutions, indels and recombination events.

Koi sleepy disease as a pathophysiological and immunological consequence of a branchial infection of common carp with carp edema virus

Gills of fish are involved in respiration, excretion and osmoregulation. Due to numerous interactions between these processes, branchial diseases have serious implications on fish health. Here, "koi sleepy disease" (KSD), caused by carp edema virus (CEV) infection was used to study physiological, immunological and metabolic consequences of a gill disease in fish. A metabolome analysis shows that the moderately hypoxic-tolerant carp can compensate the respiratory compromise related to this infection by various adaptations in their metabolism. Instead, the disease is accompanied by a massive disturbance of the osmotic balance with hyponatremia as low as 71.65 mmol L^-1, and an accumulation of ammonia in circulatory blood causing a hyperammonemia as high as 1123.24 µmol L^-1. At water conditions with increased ambient salt, the hydro-mineral balance and the ammonia excretion were restored. Importantly, both hyponatremia and hyperammonemia in KSD-affected carp can be linked to an immunosuppression leading to a four-fold drop in the number of white blood cells, and significant downregulation of cd4, tcr a2 and igm expression in gills, which can be evaded by increasing the ion concentration in water. This shows that the complex host-pathogen interactions within the gills can have immunosuppressive consequences, which have not previously been addressed in fish. Furthermore, it makes the CEV infection of carp a powerful model for studying interdependent pathological and immunological effects of a branchial disease in fish.

It is everywhere-A survey on the presence of carp edema virus in carp populations in Germany

Carp edema virus (CEV) is the causative agent of koi sleepy disease (KSD), a serious gill disease affecting common carp, Cyprinus carpio, and its ornamental variety, koi. After recent detections of the virus in various countries around the world, KSD has emerged as a new global disease in carp. However, the prevalence of the infection in carp populations in a given geographical region has not been studied thoroughly. The present communication reports an investigation into the presence of CEV in carp and koi populations in Germany. For this purpose, gill samples collected from carp and koi populations suffering from gill diseases or collected for a routine examination of their health status were tested for the presence of CEV by PCR. In total, 651 fish samples from 401 carp or koi cases were examined in 2015 and 2016, additional 118 samples from previous studies were included in the examination. CEV was detected in archive samples from carp dating back to 2007, and in koi samples dating back to 2009. From 2015 to 2016, CEV was detected in 69% of cases from carp populations examined from the main carp-producing areas in Germany, and in 41% of the examined cases from koi populations from all over Germany. Clinical KSD occurred mainly from April to June in carp populations at water temperatures ranging from 8 to 12°C and in koi populations at water temperatures ranging from 18 to 22°C. Most fish from clinically affected carp or koi populations harboured high virus loads of above 10,000 copies of CEV-specific DNA per 250 ng DNA, while gills from fish of other fish species from the ponds, including goldfish, grass carp and European perch were found CEV negative or harboured a low virus load. A phylogenetic analysis revealed the presence of multiple CEV variants from genogroup I in carp and genogroup II in koi populations in Germany. Genetically identical genogroup I isolates were detected in carp from different geographical locations in Germany and in other European carp populations. Some German genogroup II variants were identical to variants previously recorded from koi in Asian and other European countries. The data presented here show that CEV is highly prevalent in German common carp and koi populations and implies the spreading of this virus by intense trading of common carp and koi without necessary risk mitigating measures. As infections with this virus may induce serious disease, CEV diagnostic should be included in health surveillance and disease monitoring programmes.

Carp edema virus-related mortality in wild adult common carp (cyprinus carpio) in Italy

Mortality in wild fish populations represents a challenging issue for public fish health inspectors. When a single fish species is involved, an infective aetiology is frequently suspected, with focus on viral notifiable diseases. However, other viral agents not subjected to regulation and causing mortality in common carp have been reported such as carp edema virus (CEV). In mid-June 2020, a severe common carp mortality was observed in an artificial lake in north-east of Italy. Sleepy fish were noted some days before the beginning of the mortality itself, which lasted several days and involved over 340 adult specimens. During the outbreak, water temperature was around 15°C, water quality was normal, and no adverse meteorological events were reported in the area. Four specimens, which showed severe cutaneous hyperaemia and increased mucus production on skin and gills, were tested by bacteriological methods and virological analysis targeting the main carp pathogens. Molecular analysis performed on gills, kidney and brains from all the fish analysed resulted positive for CEV, which, based on anamnestic information and laboratory findings, was considered the responsible for the mortality event herein described.

Carp Edema Virus Infection Is Associated With Severe Metabolic Disturbance in Fish

Significant mortalities associated with emerging viral diseases are challenging the economy of common carp aquaculture. As such, there is an increased need to disentangle how infected fish cope with progressive disease pathology and lose the ability for homeostatic maintenance of key physiological parameters. A natural carp edema virus (CEV) infection outbreak at a carp fish farm provided an opportunity to examine diseased and healthy carp in the same storage pond, thereby contributing to our better understanding of CEV disease pathophysiology. The disease status of fish was determined using PCR-based virus identification combined with analysis of gill pathology. Compared with healthy control carp, the blood chemistry profile of CEV-infected fish revealed major disruptions in electrolyte and acid-base balance (i.e., hyponatraemia, hypochloraemia, hyperphosphatemia, elevated pH, base excess, and anion gap and decreased partial dissolved carbon dioxide). In addition, we recorded hyperproteinaemia, hyperalbuminaemia, hypotonic dehydration, endogenous hyperammonaemia, and decreased lactate along with increased creatinine, alkaline phosphatase, alanine aminotransferase, and aspartate aminotransferase. Red blood cell associated hematology variables were also elevated. The multivariate pattern of responses for blood chemistry variables (driven by sodium, pH, partial dissolved carbon dioxide, ammonia, and albumin in the principal component analysis) clearly discriminated between CEV-infected and control carp. To conclude, we show that CEV infection in carp exerts complex adverse effects and results in severe metabolic disturbance due to the impaired gill respiratory and excretory functioning.

Uncovering the first occurrence of Tilapia parvovirus in Thailand in tilapia during co-infection with Tilapia tilapinevirus

The recently discovered Tilapia parvovirus (TiPV) was the first Parvovirus confirmed to infect fish, causing mortality outbreaks in farmed adult Nile tilapia in China. Severe mortality outbreaks caused by Tilapia tilapinevirus (TiLV) to farmed tilapia in Thailand revealed the concomitant occurrence of TiPV. Out of ten fish farms screened, TiPV was detected in one site rearing juvenile red hybrid tilapia. Clinical signs included abnormal swimming, scale protrusion, skin and muscle haemorrhaging, exophthalmia and generalized anaemia. Histological findings showed extensive infiltration of lymphocytes, with increased melanomacrophage centres in the anterior kidney and spleen, erythrocyte depletion in the spleen and hepatic syncytial cells. Both TiLV and TiPV were systemically distributed in the body of moribund fish. The analysis of the near-complete TiPV genome isolated from Thailand revealed 98.74% sequence identity to the formerly isolated from China, together with a highly conserved and comparable genomic organization and with a 3 nucleotides deletion in the 5-UTR. The viral genome structure was highly conserved for each of its components, with nucleotide and amino acid identity ranging from 100% for ORF1 to 97% for ORF2, and with conserved HuH and Walker loop motifs within NS1. Taken together, our results document the first detection of TiPV outside China, thus for the first time in Thailand. Moreover, TiPV was detected for the first time during a natural occurrence in farmed red hybrid tilapia and involved in co-infection pattern with TiLV. Diagnostic investigations during tilapia disease outbreaks should include the screening for TiPV. Further studies are needed to elucidate TiPV genomic variance, pathobiology, including focussing on the outcomes of TiLV-TiPV co-infection patterns, necessary to enable risk assessment for the worldwide spreading of TiPV and to design adequate control measures against these emerging viruses in tilapia.

Carp edema virus and immune response in carp (Cyprinus carpio): Current knowledge

The importance of world aquaculture production grows annually together with the increasing need to feed the global human population. Common carp (Cyprinus carpio) is one of the most important freshwater fish in global aquaculture. Unfortunately, carp production is affected by numerous diseases of which viral diseases are the most serious. Koi herpesvirus disease (KHVD), spring viraemia of carp (SVC), and during the last decades also koi sleepy disease (KSD) are currently the most harmful viral diseases of common carp. This review summarizes current knowledge about carp edema virus (CEV), aetiological agent causing KSD, and about the disease itself. Furthermore, the article is focused on summarizing the available information about the antiviral immune response of common carp, like production of class I interferons (IFNs), activation of cytotoxic cells, and production of antibodies by B cells focusing on anti-CEV immunity.

A Seasonal Study of Koi Herpesvirus and Koi Sleepy Disease Outbreaks in the United Kingdom in 2018 Using a Pond-Side Test

Simple Summary

Cyprinid herpesvirus (CyHV)-3 and carp edema virus (CEV), the causative agents of koi herpesvirus disease and koi sleepy disease, respectively, are emerging DNA viruses infecting koi and common carp. Similarities in their clinical presentation present difficulties for its on-site identification based on gross pathology. Fluorescence real-time loop-mediated isothermal amplification (LAMP) assays for detecting CyHV-3 and CEV DNA were designed to use border inspection posts and local testing by national authorities for outbreak control. The limit of these tests’ detection (10² and 10³ viral copies for CyHV-3 and CEV, respectively) allows for the amplification of viral DNA in clinical samples in less than 20 min. The assays’ field performance was tested with 63 common carp mucus swabs taken during disease investigations in 2018, and the results validated with the reference laboratory analysis. Overall, the good performance, ease of use, and cost-effectiveness of these tests make them good candidates for a point of care test. However, further work is required to incorporate reliable internal controls and improve the sensitivity of these tests’ asymptomatic testing.

Abstract

Fluorescence real-time LAMP assays were designed for the orf43 gene of CyHV-3 European genotype and the p4a gene of the CEV genogroup I. A third LAMP assay to detect the ef1a gene of the host common carp was designed as an internal control. The limit of detection was 10² and 10³ viral copies under 25 min for CyHV-3 and CEV, respectively. The specificity of the CyHV-3 LAMP assay was 95.6% of 72 fish herpesviruses tested. Sixty-three non-lethal common carp mucus swabs were collected across 16 sites during disease investigations. DNA extractions were performed in under 10 min using the QuickExtract™ digestion buffer. The LAMP amplification of CyHV-3 DNA in mucus swabs from clinical cases was detected from 4 to 13 min in 13 sites, while a co-infection of CyHV-3 and CEV was confirmed by LAMP in a single site. The LAMP results agreed with the results of the reference laboratory. The common carp ef1a was amplified only in 61% of the mucus swabs collected, preventing its use as a robust internal control to distinguish false negatives from invalid tests. After further optimization, these tests could be implemented for border inspection posts surveillance and decentralizing testing during disease outbreaks.

First Evidence of Carp Edema Virus Infection of Koi Cyprinus carpio in Chiang Mai Province, Thailand

The presence of carp edema virus (CEV) was confirmed in imported ornamental koi in Chiang Mai province, Thailand. The koi showed lethargy, loss of swimming activity, were lying at the bottom of the pond, and gasping at the water's surface. Some clinical signs such as skin hemorrhages and ulcers, swelling of the primary gill lamella, and necrosis of gill tissue, presented. Clinical examination showed co-infection by opportunistic pathogens including Dactylogyrus sp., Gyrodactylus sp. and Saprolegnia sp. on the skin and gills. Histopathologically, the gill of infected fish showed severe necrosis of epithelial cells and infiltrating of eosinophilic granular cells. Electron microscope examination detected few numbers of virions were present in the cytoplasm of gill tissue which showed an electron dense core with surface membranes worn by surface globular units. Molecular detection of CEV DNA from gill samples of fish was performed by polymerase chain reaction (PCR) and confirmed by nested-PCR. Phylogenetic analyses revealed that CEV isolate had 99.8% homology with the CEV isolated from South Korea (KY946715) and Germany (KY550420), and was assigned to genogroup IIa. In conclusion, this report confirmed the presence of CEV infection of koi Cyprinus carpio in Chiang Mai province, Thailand using pathological and molecular approaches.

Molecular characterization of carp edema virus disease: An emerging threat to koi Cyprinus carpio in China

Carp edema virus disease (CEVD) has resulted in great economic losses in koi (Cyprinus carpio koi) and common carp (Cyprinus carpio carpio) populations in the world. In this study, the diseased koi were diagnosed as CEV infection based on 5' untranslated region (5'UTR) and 4a protein genes by the conventional PCR, nested PCR and quantitative PCR (qPCR) analyses. Phylogenetic tree analysis showed that the TJ201708 strain was classified into the genogroup IIa. Furthermore, qPCR of 5'UTR gene revealed that the lowest detection limit was 4.0 fg/μL. The pathogenicity of CEV for koi was demonstrated in the infection experiments. Histopathological examination revealed the petechial hemorrhages of liver and spleen, vacuolization of lamina propria of intestine and swelling and necrosis of respiratory epithelial cells of gills. To our knowledge, this is the first report the qPCR of 5'UTR gene in the detection of carp edema virus.

Carp oedema virus disease outbreaks in Czech and Slovak aquaculture

This work describes the first confirmed cases of carp oedema virus disease (CEVD) in Slovakia and the Czech Republic and the phylogenetic analysis of Czech and Slovak carp oedema virus (CEV) isolates. Four cases of disease outbreak in the Czech Republic are described, the oldest dating from mid-May 2013 and one case from Slovakia dating from May 2019. In all cases, virus presence was confirmed using nested PCR. PCR products were sequenced and compared with 357-bp nucleotide sequences encoding the CEV P4a protein in GenBank. In four cases of disease outbreak (three common carp breeding facilities and one koi garden pond), CEV detected belonged to genogroup I. In one case (koi garden pond), fish were confirmed as infected with CEV from genogroup II. This work complements data on CEV occurrence in European countries and contributes to a better understanding of the pathways leading to transmission of the virus throughout Europe.

Koi herpesvirus disease outbreak: Input for the implementation of a surveillance program in South Tyrol - Italy

A severe episode of high and abnormal mortality was observed in the population of Cyprinus carpio of Lake Caldaro (South Tyrol, Italy) in summer 2016. The diagnostic investigation carried out led to the identification of Koi Herpesvirus (KHV) as the etiologic agent. Following this disease outbreak and its socio-economic consequences, the local authorities, in agreement with the local Fishing Association, decided to implement a surveillance program for the achievement of KHV-free health status (Category I) in the Province, in accordance to the Implementing Decision (EU) 2015/1554. The selected area was a defined geographical compartment (the Monticolo lakes compartment, South Tyrol, Italy), which is located near Lake Caldaro, where the Koi Herpesvirus disease (KHVD) outbreak had occurred. This area is of particular interest because it supplies other water bodies with juvenile C. carpio individuals; with the achievement of a KHV-free health status, South Tyrol could possibly become independent in the breeding of this fish species. Suitable samples were collected and processed during a two-year period in order to detect the presence/absence of KHV. The same samples were tested for other viruses that can affect carp, namely spring viraemia of carp (SVCV) and carp edema virus (CEV). According to the results, the authors conclude that the Monticolo lakes area should be classified as KHV-free, as no sample has tested positively for the presence of this specific virus (KHV).

Koi herpesvirus and carp edema virus threaten common carp aquaculture in Croatia

Common carp (Cyprinus carpio) is a very important fish species for warm-water aquaculture in Croatia. All Croatian carp farms are subjected to a surveillance programme for the presence of koi herpesvirus (KHV), causing a deadly disease called koi herpesvirus disease (KHVD). However, there is no surveillance for other viral pathogens of importance like carp edema virus (CEV), a causative agent of koi sleepy disease (KSD). During regular testing within the KHVD surveillance programme, we tested samples for CEV simultaneously. The screening indicated possible outbreaks of KHVD and KSD. During 2016, KHVD broke out in an isolated area and soon thereafter a KHV eradication programme was successfully performed. However, during 2018 and 2019, two additional mortality events occurred in lakes in the southern part of Croatia during the spring. Samples from both events tested positive for CEV. An epidemiological investigation confirmed the introduction of infected carps from an infected farm to one of the lakes. To prevent the spreading of CEV into open waters, it is of utmost importance to introduce CEV testing before fish movement or to perform regular testing of all carp farms in the country to determine CEV prevalence for the purpose of implementation of control measures.

First detection of koi herpesvirus and carp oedema virus in Iraq associated with a mass mortality in common carp (Cyprinus carpio)

At the end of October 2018, a mass fish mortality occurred in Iraq, involving thousands of tons of cultured and wild common carp (Cyprinus carpio) along Euphrates and Tigris rivers. Fish were found dead or moribund along rivers coasts, showing lethargy, dyspnoea and flared gills. At necropsy, discoloration patches were noticed on the gills. Wet preparations showed rare metacercariae and Dactylogyrus spp. Samples were subjected to bacteriological tests and virological investigation through real-time PCR and nested PCR. Both were positive for koi herpesvirus (KHV) and carp oedema virus. Results obtained were confirmed by the OIE reference laboratory of KHV disease (KHVD) at Cefas (UK) and by sequence analysis. This is the first report on the detection of both viruses in Iraq.

Koi herpesvirus and carp oedema virus: Infections and coinfections during mortality events of wild common carp in the United States

Koi herpesvirus (KHV; cyprinid herpesvirus-3) and carp oedema virus (CEV) are important viruses of common and koi carp (Cyprinus carpio); however, the distribution of these viruses in wild common carp in North America is largely unknown. During the summers of 2017 and 2018, 27 mass mortalities of common carp were reported from four states in the USA (Minnesota, Iowa, Pennsylvania and Wisconsin), the majority of which were distributed across eight major watersheds in southern Minnesota. Samples from 22 of these mortality events and from five clinically healthy nearby carp populations were screened for KHV, CEV and SVCV using real-time polymerase chain reaction (qPCR). KHV was confirmed in 13 mortality events, CEV in two mortality events and coinfections of KHV/CEV in four mortality events. Nucleotide sequence analysis revealed that the KHV and CEV detected here are closely related to European lineages of these viruses. While molecular detection alone cannot conclusively link either virus with disease, the cases described here expand the known range of two important viruses. This is also the first reported detection of KHV and CEV coinfections in wild carp populations.

Identification of new genogroups in Austrian carp edema virus isolates

Carp edema virus disease (CEVD), also known as koi sleepy disease (KSD), is an emerging viral disease caused by the carp edema virus (CEV). It is characterized by lethargic behavior, gill necrosis, and generalized edema, leading to significant morbidity and mortality in common carp and koi Cyprinus carpio. Accurate diagnosis of CEVD relies on amplification of a P4a protein-encoding DNA segment from the CEV genome. A phylogenetic analysis of amplified fragments revealed 3 distinct CEV genogroups: I, IIa, and IIb. We explored the phylogenetic relationship between Austrian CEV isolates with existing CEV genogroups. The phylogenetic analysis (n = 18) established the presence of the 3 extant CEV genogroups as well as 2 new CEV genogroups (IIIa and IIIb) classified to identify the Austrian isolates that were distinct from the existing CEV genogroups. It is evident that CEV infection cases are growing in number each year, which may be due to development of sensitive diagnostic assays, while information regarding the virus is scarce. National and international efforts are required to study the epidemiology of the CEV in major carp-producing countries.

Potential vector species of carp edema virus (CEV)

During a PCR-based CEV survey in Poland in 2015-2017, the virus was detected in many farms both in clinical and asymptomatic cases and in common as well as in koi carp (Cyprinus carpio). In order to evaluate the potential carrier role of fish species that share the same habitats with carp, an experimental trial was performed. Investigations carried out on specimens of bleak (Alburnus alburnus), crucian carp (Carassius carassius), European perch (Perca fluviatilis), Prussian carp (Carassius gibelio), roach (Rutilus rutilus) and tench (Tinca tinca) cohabited with CEV-infected carp yielded positive results. These species of fish were experimentally cohabited with CEV-infected common carp at a temperature of 16°C ± 1. Material from the brain, gills, spleen, kidneys, intestine and skin was investigated for the presence of CEV DNA. Similar investigations were performed with uninfected fish designated controls. Samples were tested for CEV by qPCR.

Quantitative diagnostics of gill diseases in common carp: not as simple as it seems

During a disease outbreak, affected fish exhibit particular clinical signs, and the task in veterinary diagnostics is to identify the causative agent(s) as a prerequisite for appropriate treatment measures. In this study, we present an outbreak of a multifactorial gill disease in a cohort of ornamental koi Cyprinus carpio with gill necrosis as the main exterior clinical sign. By means of pathogen identification and determining pathogen abundance in various tissues, mortality of individual fish was found to be caused by different agents. Three out of 5 diseased individuals suffered from koi herpesvirus disease (KHVD) associated with a systemic infection with cyprinid herpesvirus 3 (CyHV-3), 1 fish succumbed to koi sleepy disease (KSD) caused by a high carp edema virus (CEV) load in the gills co-infected with CyHV-3 and flavobacteria, and the last fish had low loads of both viruses but high flavobacteria and Ichthyobodo burdens and most likely died from an interaction of these bacterial and parasitic agents. The results indicated that correct identification of the agent responsible for the observed clinical signs or mortality during co-infection might require quantitative determination of the abundance of the pathogens as well as detailed knowledge of the infection biology of these pathogens.

First report of carp edema virus in the mortality of wild common carp Cyprinus carpio in North America

Carp edema virus (CEV) is an unclassified poxvirus that infects skin and gill tissue to cause koi sleepy disease. In the USA, CEV was first detected in 1996 in a California koi wholesaler, and has since been reported sporadically only within imported and domestic koi. Common carp Cyprinus carpio are a non-native species now present in most waterways in the USA. In May 2017, >526 large adult common carp in spawning condition died in Mill Pond, Park Ridge, NJ, USA. The water temperature during the kill was 15°C and the affected fish displayed marked lethargy prior to death. The presence of CEV was confirmed by endpoint PCR, real-time quantitative PCR (qPCR), and transmission electron microscopy (TEM), making this the first report of CEV associated with a wild carp kill in North America. Phylogenetic analysis of a region of the 4a gene encoding the major core protein clustered the CEV strain among others in genogroup I, which includes CEV strains previously detected in common carp cultured in Europe. Gill histopathology included severe lamellar fusion and apoptosis in the interlamellar region and TEM identified cytoplasmic virions consistent in morphology with CEV in the branchial epithelial cells. Five months following the mortality, surviving fish were collected and screened for CEV by purifying and concentrating virus from the gills and testing with qPCR. No evidence of CEV was found, supporting previous studies showing CEV is not detectable in gills after abatement of clinical signs.

Flavobacteria as secondary pathogens in carp suffering from koi sleepy disease

Koi sleepy disease (KSD) is a disease with increasing importance in global common carp aquaculture. Despite the fact that carp edema virus (CEV) is most likely the causative agent of KSD, the disease often presents itself as multifactorial with several parasites and bacteria species present on gills, skin or in internal organs. Therefore, in this study, we analysed and presented initial results on an interaction of flavobacteria and CEV in the development of clinical KSD in carp suffering from proliferative gill disease. We examined selected field samples from Germany and Hungary and confirmed the presence of CEV and flavobacteria co-infections in subset of the samples. In several infection experiments, we studied the transfer and dynamics of both infections. Furthermore, we analysed which Flavobacterium species could be isolated from KSD-affected fish and concluded that Flavobacterium branchiophilum is a possible copathogen. Antibiotic treatment experiments showed that CEV seems to be the primary pathogen causing an insult to the gills of carp and by these enabling other pathogens, including F. branchiophilum, to establish co-infections. Despite the fact that F. branchiophilum co-infection is not required for the development of clinical KSD, it could contribute to the pathological changes recorded during the outbreaks.