Detection of Cyprinid herpesvirus 2 in association with an Aeromonas sobria infection of Carassius carassius (L.), in Italy

Effect of temperature on the protective efficacy of a live attenuated vaccine against herpesviral haematopoietic necrosis in goldfish

The live attenuated vaccine P7-P8 strain against herpesviral haematopoietic necrosis, which is caused by cyprinid herpesvirus 2 (CyHV-2), exhibits high protective efficacy in goldfish at 25°C, the predominant temperature for this disease; however, the effect of water temperature during the vaccination period on efficacy has not been determined. In this study, an in vitro experiment revealed that the vaccine strain grew between 15 and 30°C in the goldfish cell line RyuF-2. Subsequent in vivo efficacy tests were conducted with vaccination temperatures ranging from 15 to 30°C. During the vaccination period, organs were sampled to determine the vaccine growth dynamics. Blood plasma was collected to assess anti-CyHV-2 antibody titres. The protective efficacy of the vaccine at 15, 20, 25, and 30°C after subsequent virulent CyHV-2 challenge resulted in a relative percentage survival of 73.3%, 77.8%, 100%, and 77.8%, respectively, which indicated that the vaccine is effective over this temperature range. The vaccine virus load in the spleen was lowest at 15°C (103.7 DNA copies/mg) and highest at 25°C (106.5 DNA copies/mg). This indicates that the vaccine virus load over 104 DNA copies/mg may elicit sufficient acquired immunity. No significant differences in antibody titre were observed between groups, which suggests that cell-mediated immunity can be fundamentally involved in protection.

In Vivo Imaging Sheds Light on the Susceptibility and Permissivity of Carassius auratus to Cyprinid Herpesvirus 2 According to Developmental Stage

Cyprinid herpesvirus 2 (CyHV-2) is a virus that causes mass mortality in economically important Carassius spp. However, there have been no comprehensive studies into host susceptibility or permissivity with respect to developmental stage, and the major portal of viral entry into the host is still unclear. To help bridge these knowledge gaps, we developed the first ever recombinant strain of CyHV-2 expressing bioluminescent and fluorescent reporter genes. Infection of Carassius auratus hosts with this recombinant by immersion facilitated the exploitation of various in vivo imaging techniques to establish the spatiotemporal aspects of CyHV-2 replication at larval, juvenile, and adult developmental stages. While less susceptible than later developmental stages, larvae were most permissive to CyHV-2 replication, leading to rapid systemic infection and high mortality. Permissivity to CyHV-2 decreased with advancing development, with adults being the least permissive and, thus, also exhibiting the least mortality. Across all developmental stages, the skin was the most susceptible and permissive organ to infection at the earliest sampling points post-infection, indicating that it represents the major portal of entry into these hosts. Collectively these findings provide important fundamental insights into CyHV-2 pathogenesis and epidemiology in Carassius auratus with high relevance to other related economically important virus-host models.

Transcriptomes of Zebrafish in Early Stages of Multiple Viral Invasions Reveal the Role of Sterols in Innate Immune Switch-On

Although it is widely accepted that in the early stages of virus infection, fish pattern recognition receptors are the first to identify viruses and initiate innate immune responses, this process has never been thoroughly investigated. In this study, we infected larval zebrafish with four different viruses and analyzed whole-fish expression profiles from five groups of fish, including controls, at 10 h after infection. At this early stage of virus infection, 60.28% of the differentially expressed genes displayed the same expression pattern across all viruses, with the majority of immune-related genes downregulated and genes associated with protein synthesis and sterol synthesis upregulated. Furthermore, these protein synthesis- and sterol synthesis-related genes were strongly positively correlated in the expression pattern of the rare key upregulated immune genes, IRF3 and IRF7, which were not positively correlated with any known pattern recognition receptor gene. We hypothesize that viral infection triggered a large amount of protein synthesis that stressed the endoplasmic reticulum and the organism responded to this stress by suppressing the body's immune system while also mediating an increase in steroids. The increase in sterols then participates the activation of IRF3 and IRF7 and triggers the fish's innate immunological response to the virus infection.

Evaluation of protective effects of heat-inactivated cyprinid herpesvirus-2 (CyHV-2) vaccine against herpesviral hematopoietic necrosis disease (HVHND) in goldfish (Carassius auratus)

Cyprinid herpesvirus-2 (CyHV-2) is an important virus that causes herpesviral hematopoietic necrosis disease (HVHND) leading to huge economic losses in goldfish (Carassius auratus). However, until now no proper prophylactic measure or treatment is available for CyHV-2 infection in goldfish. Hence, in this experiment, we developed a heat-inactivated CyHV-2 vaccine and evaluated its performance in goldfish. Initially, CyHV-2 was propagated in the fantail goldfish fin (FtGF) cell line and the titer of the viral inoculum was 10^7.8 TCID₅₀/ml. Subsequently, various temperatures (40 °C, 50 °C, 60 °C, 70 °C, and 80 °C) were evaluated to achieve the complete inactivation of CyHV-2. Only the viral inoculum inactivated at 80 °C for 1 h did not show any cytopathic effect in the FtGF cell line after five blind passages. Hence the heat-inactivated CyHV-2 vaccine developed at 80 °C was further used for immunization trials in goldfish. The experimental goldfish were intraperitoneally immunized with 300 μL of the heat-inactivated CyHV-2 vaccine. Subsequently, the kidney and spleen tissues were sampled at various time points post-vaccination (6th hr, 2nd day, 4th day, 6th day, 10th day, 16th day, and 30th day) to evaluate the expression of immune genes (IL-12, IL-10, IFN-γ, CD8, and CD4). A significant upregulation of immune genes was observed at various time points in the kidney and spleen tissue of the vaccinated goldfish. Furthermore, in order to study the efficacy of the vaccine, the experimental fish were challenged with CyHV-2 (10^7.8 TCID₅₀/ml) after the 30th day post-vaccination. The survival of the fish in the vaccine group (86.7%) was significantly higher compared to the non-vaccinated group (20%). Moreover, the relative percentage survival of the vaccinated group was 83.34%. In spite of the single dose, the heat-killed vaccine developed in the present study elicited the immune response and offered better protection in goldfish against CyHV-2. However, further large-scale field performance evaluation studies are necessary to develop this vaccine on a commercial scale.

Development and characterization of monoclonal antibodies specific for cyprinid herpesvirus 2

Infections of Cyprinid herpesvirus 2 in goldfish and farmed crucian carp (Carassius auratus gibelio) are still an urgent problem worldwide. Detection and prevention are necessary for the control of haematopoietic necrosis disease caused by CyHV-2. Although many sensitive molecular diagnostic methods have been developed, effective immunodiagnosis and neutralization approaches based on monoclonal antibodies (MAbs) against CyHV-2 are still important to CyHV-2 study. In this experiment, purified CyHV-2 was used as antigens to produce monoclonal antibodies (Mabs). Six Mabs bound to different proteins were selected by Dot-blot screening and Western-blot analysis, and no one had cross-reactivity with closely related koi herpesvirus. Among them, Mabs 2E1-B10, 1F5-A3 and 4C4-A7 belonged to IgG₁ isotype, while other three Mabs 3G9-B11, 3B4-G5 and 4F4-B7 belonged to IgM isotype. These six Mabs all could specifically detect CyHV-2 in CyHV-2 infected caudal fin of Carassius auratus gibelio (GiCF) cells by immunofluorescence assays. Then, the neutralization ability was tested in vitro, and the result showed that all six Mabs can attenuate CPE by CyHV-2 in vitro among which 2E1-B10 had the best neutralization ability. The virus proteins recognized by these six Mabs were identified by mass spectrometry identification, and the result showed they probably were ORF88, ORF55R, ORF115 and ORF151R. This study is the first to prepare Mabs by purifying CyHV-2, which will provide a practical basis for the in-depth study of CyHV-2 virus.

Complete Genome and Molecular Characterization of a New Cyprinid Herpesvirus 2 (CyHV-2) SH-01 Strain Isolated from Cultured Crucian Carp

Cyprinid herpesvirus 2 (CyHV-2) is a causative factor of herpesviral hematopoietic necrosis (HVHN) in farmed crucian carp (Carassius carassius) and goldfish (Carassius auratus). In this study, we analyzed the genomic characteristics of a new strain, CyHV-2 SH-01, isolated during outbreaks in crucian carp at a local fish farm near Shanghai, China. CyHV-2 SH-01 exhibited a high sensitivity to goldfish and crucian carp in our previous research. The complete genome of SH-01 is 290,428 bp with 154 potential open reading frames (ORFs) and terminal repeat (TR) regions at both ends. Compared to the sequenced genomes of other CyHVs, Carassius auratus herpesvirus (CaHV) and Anguillid herpesvirus 1 (AngHV-1), several variations were found in SH-01, including nucleotide mutations, deletions, and insertions, as well as gene duplications, rearrangements, and horizontal transfers. Overall, the genome of SH-01 shares 99.60% of its identity with that of ST-J1. Genomic collinearity analysis showed that SH-01 has a high degree of collinearity with another three CyHV-2 isolates, and it is generally closely related to CaHV, CyHV-1, and CyHV-3, although it contains many differences in locally collinear blocks (LCBs). The lowest degree of collinearity was found with AngHV-1, despite some homologous LCBs, indicating that they are evolutionarily the most distantly related. The results provide new clues to better understand the CyHV-2 genome through sequencing and sequence mining.

Generation and application of a monoclonal antibody specific for the ORF121 of cyprinid herpesvirus 2

Cyprinid herpesvirus 2 (CyHV-2) is a viral pathogen worldwide and causing high mortality on goldfish and silver crucian carp (Carassius auratus gibelio). In order to establish a stable and sensitive immunological diagnostic approach, the recombinant ORF121 protein encoded by the CyHV-2 ORF121 gene, was selected as a capture antigen to identify cells and tissues infected with CyHV-2 by immunological methods in this study. Firstly, the open reading frame of CyHV-2 ORF121 was cloned into the PGEX-4T-3 vector and expressed in Escherichia coli. Purified recombinant ORF121 protein was then used as an antigen to prepare monoclonal antibodies, and an efficient hybridoma cell line was selected by dot-blot assay. The resulting mAb-3D9 was applied to detect CyHV-2 in infected caudal fin of Carassius auratus gibelio (GiCF) cells and fish tissues by western blotting, immunofluorescence assays and immunohistological asays. The monoclonal antibody could specifically identify CyHV-2 in infected GiCF cells and the gills, the kidney and the spleen tissues, and it could attenuate CPE by CyHV-2 in vitro, suggesting it can be applied for CyHV-2 detection in the crucian carp and ORF121 may be a candidate vaccine against CyHV-2.

Immune gene expression and protective effects in goldfish (Carassius auratus L.) immunized with formalin-inactivated cyprinid herpesvirus-2 (CyHV-2) vaccine

The goldfish hematopoietic necrosis viral disease (GHNVD) has led to worldwide economic losses in goldfish aquaculture. The present study has focused on the development of an inactivated vaccine for the cyprinid herpesvirus (CyHV-2) and to check the immunogenicity of the vaccine in the host. The fantail goldfish fin (FtGF) cell line was used in the propagation of the CyHV-2 and the viral titer obtained were of 10^7.8 TCID₅₀/ml. Followed by the virus was inactivated using 0.1% formalin for 2 days. Various concentrations of formalin-inactivated CyHV-2 (1%, 0.7%, 0.5%, 0.3% and 0.1%) were studied in the FtGF cell line. Morphological changes were observed in the FtGF cell line in all other concentrations of formalin except 0.1% formalin-inactivated CyHV-2 vaccine. The goldfishes were intraperitoneally injected with 300 μl of vaccine and various immune gene responses were studied for a period of 30 days. The gene expression of the adaptive markers CD8, CD4, IFN-ϒ, the cytokines (IL-10, IL-12) was studied in kidney and spleen tissues. Formalin-inactivated CyHV-2 vaccine showed a significant up-regulation of the genes CD8 and IFN-ϒ by the 6th hr post-vaccination onwards. The experimental fish were challenged intraperitoneally with CyHV-2 virus of concentration 10^7.8 TCID₅₀/ml after 30 days of post-vaccination. A significant difference in cumulative mortality rate was observed for the vaccinated fishes from the unvaccinated fishes. The relative percent survival for formalin immunized fish was 74.03%. Our results have proven that the formalin-inactivated vaccines were efficient and it resulted in triggering the immune gene expression in goldfish. The development and further enhanced studies for this vaccine will lead to a promising low-cost commercial vaccine for CyHV-2 viral infection.

Production and application of monoclonal antibodies against ORF66 of cyprinid herpesvirus 2

Cyprinid herpesvirus 2(CyHV-2)is the main pathogen causing haematopoietic necrosis disease of goldfish (Carassius auratus auratus) and gibel carp (Carassius auratus gibelio), which has caused huge economic losses to aquaculture industry of goldfish and gibel carp around the world. Currently, various detection methods based on nucleic acids have been established for the detection of CyHV-2. However, there is still a lack of rapid and effective immunological detection technology. In this study, anti-CyHV-2 ORF66 monoclonal antibodies (MAbs) were prepared to use the recombinant ORF66 protein as the antigen. Firstly, the open reading frame of CyHV-2 ORF66 was cloned into the pET-28a vector and expressed in Escherichia coli. Three MAbs (2F11, 2G8, and 3D6) against recombinant ORF66 protein were developed by immunization of Balb/C mice. Among them, MAb-2F11 belonged to the IgG2b isotype, 2G8 and 3D6 belonged to the IgG1 isotype. Western blotting analysis was performed to assess the ability of the MAbs to bind to the ORF66 recombinant protein and CyHV-2 nucleocapsid protein ORF66. In addition, the MAb-2F11 was used to detect the virus particles that infected in cell line and tissues of gibel carp virus infection by immunological methods. These results indicated that the anti-CyHV-2 ORF66 MAb-2F11 prepared in this study could not only detect the presence of the virus but also provide a research tool for further studying the role of ORF66 in the process of CyHV-2 infection.

Development and evaluation of a convenient immunochromatographic strip test for rapid detection of cyprinid herpesvirus 2 (CyHV-2)

Cyprinid herpesvirus 2 (CyHV-2) has become a serious threat to the gibel carp Carassius auratus gibelio industry and has led to enormous losses worldwide. We have therefore developed an immunochromatographic strip (ICS) to enable rapid on-site detection of CyHV-2 by aquaculture facility staff. The ICS employs 2 monoclonal antibodies (MAbs 2C3-1E6 and 3H2-1G5) against the ORF25 protein, a CyHV-2 membrane protein, as the capture and detection antibodies, respectively. Indirect immunofluorescence assay (IIFA) and Western blotting of CyHV-2-infected fathead minnow cells indicated that the 2 MAbs could specifically bind CyHV-2 by recognizing ORF25 antigen. Sandwich ELISA showed that the detection limit of ORF25 protein halved when MAb 2C3-1E6 served as the capture antibody compared to MAb 3H2-1G5. The test for detecting purified CyHV-2 using the ICS could be completed in 10 min and the sensitivity was 1 µg ml-1. Sensitivity of the ICS remained stable following storage at 4, 25 and 37°C for 6 mo. Tissue homogenate from gibel carp with and without obvious gill hemorrhages was subjected to CyHV-2 detection using the ICS: the results were in good accordance with conventional PCR. Our ICS does not require highly trained technicians or specialized equipment, making it suitable for rapid diagnosis of CyHV-2 infection both in the laboratory and in the field.

Comparative sensitivity of three new cell lines developed from gill, liver and brain tissues of goldfish, Carassius auratus (L.) to cyprinid herpesvirus-2 (CyHV-2)

Cyprinid herpesvirus 2 (CyHV-2) is the etiological agent of Goldfish herpesviral haematopoietic necrosis (GHVHN) in goldfish. In this study, three new cell lines including Fantail goldfish gill (FtGG), Fantail goldfish liver (FtGL) and Fantail goldfish brain (FtGB) had been established and characterized from the gill, liver and brain tissue of C. auratus respectively. Cell lines were optimally grown at 28 °C in Leibovitz-15 (L-15) medium supplemented with 10 % fetal bovine serum (FBS). The PDT during exponential growth of FtGG, FtGL and FtGB cells were determined to be 41.47 h, 63.43 h and 79.79 h respectively. Karyotyping analysis of cell lines remained diploid (2n = 100). The revival rate was 82 %, 72 % and 70 % in FtGG, FtGL and FtGB cells respectively after 6 months of cryopreservation. All the three cells showed similar cytopathic effect (CPE) between 3-5 days post-infection (dpi) with CyHV-2 and complete destruction of the monolayer was observed at 8-10 dpi. The viral titers of CyHV-2 in FtGG, FtGL and FtGB reached 10^7.375±0.35 TCID₅₀ ml^-1, 10⁴·^55±0.070 TCID₅₀ ml^-1 and 10^6.45±0.070 TCID₅₀ ml^-1 respectively. These newly established cell lines will be a useful diagnostic tool for viral diseases in this fish species and also for the isolation and study of goldfish viruses in future.

Transcriptomic and proteomic analyses reveal new insights into the regulation of immune pathways during cyprinid herpesvirus 2 infection in vitro

Carassius auratus gibelio is susceptible to the herpesviral hematopoietic necrosis (HVHN) disease caused by cyprinid herpesvirus 2 (CyHV-2) infection during the breeding process. Nevertheless, the report on biological response of CyHV-2 with C. auratus gibelio was limited, especially in vitro. In this study, host gene expression profiling was mostly analyzed in caudal fin cells of Carassius auratus gibelio (GiCF) underlying CyHV-2 infection. Transcriptomics and proteomics were employed to study the differential expression gene and revealed the host genes involved in pathway during the CyHV-2 infection. Transcriptome analysis revealed that compared with the control group, there were 11 335 and 19 421 differentially expressed unigenes at 48 h and at 96 h, respectively. Furthermore, proteome analysis showed that there were a total of 9008 proteins, among which 169 proteins were differential expression in the 48 h group and 502 proteins in the 96 h group. Notably, 10 and 158 differentially co-expressed genes at mRNA and protein levels (cDEGs) were reliably quantified at 48 h and 96 h, respectively. Interestingly, significantly different expressed genes both in the transcriptome and the proteome were identified, including GNG7, Hsp90a, THBS1 and RRM2. The result suggested that PI3k-AKT pathway was activated, but the p53 signaling pathway was suppressed. The above result will lay the foundation for understanding the mechanisms of host defense virus invasion during CyHV-2 infection.

Reactivation of cyprinid herpesvirus 2 (CyHV-2) in asymptomatic surviving goldfish Carassius auratus (L.) under immunosuppression

Cyprinid herpesvirus 2 (CyHV-2) is a highly contagious pathogen of goldfish (Carassius auratus) and Prussian carp (Carassius auratus gibelio) causing herpesviral hematopoietic necrosis. Our previous study revealed that CyHV-2 can persistently infect the kidney and spleen of goldfish that recovered from a primary infection. In this study, we tried to identify the cells persistently infected with the virus in surviving fish and investigated virus reactivation in the survivors injected with immunosuppressants, namely dexamethasone (Dex) and cyclosporine A (CsA). Virus DNA was detected from the monocytes that were isolated from the trunk kidney of the asymptomatic survivors, suggesting that monocytes/macrophages are major cells that may be persistently infected with CyHV-2. A significant increase of virus DNA levels was detected in the group injected with Dex at 10 and 21 days post-injection (dpi). In the fish group injected with CsA, the virus DNA level was the same as that in the control group at 10 dpi but increased in some organs at 21 dpi. Compared with Dex-injected fish at 10 dpi, the group injected with both Dex and CsA showed a greater increase in virus DNA levels. The gene expression of phagocytosis-associated genes, major histocompatibility complex (MHC) class II and p47^phox, and anti-virus antibody levels increased in the CsA group due to virus reactivation in the infected cells but not in the Dex and Dex & CsA groups, indicating that Dex effectively suppressed monocyte/macrophage function and antibody production. In addition, recombinant interferon γ (IFNγ) supplementation in the kidney leukocyte culture that was isolated from survivors showed a reduction of virus DNA. CsA may inhibit T-helper 1 (Th1) cells and consequently IFNγ production, causing a synergetic effect with Dex on virus reactivation. The results suggest that the activity of monocytes/macrophages stimulated by IFNγ can relate to virus latency and reactivation in asymptomatic virus carriers.

Screening for protective antigens of Cyprinid herpesvirus 2 and construction of DNA vaccines

BACKGROUND

In recent years, crucian carp hematopoietic necrosis caused by Cyprinid herpesvirus 2 (CyHV-2) infection has caused an enormous economic loss to the aquaculture industry.

METHODS

In this study antigenic epitope analysis was performed on the membrane proteins of CyHV-2, and 8 antigen-rich peptide fragments were selected for prokaryotic expression. Then, the immunogenicity of the recombinant proteins was analyzed. On this basis, DNA vaccines were constructed for immunization of hybridized Prussian carps. The protective effect of DNA vaccines against challenge in hybridized Prussian carps was evaluated.

RESULTS

The results showed that all 8 recombinant proteins were successfully expressed. Among the recombinant proteins, ORF16, tORF25, tORF64, and ORF146, gave a positive serum reaction with CyHV-2. Of the four proteins used for the immunization of silver crucian carps, the antibody titer induced by tORF25 was the highest. The DNA vaccine, pEGFP-N1-ORF25, was constructed based on ORF25 and able to induce production of specific antibodies in carps, while up-regulating the expression of MHCⅠ, IL-1β, C3, and TF-A in the kidneys of carps. Moreover, the immunoprotective rate was increased to 70% in hybridized Prussian carps.

CONCLUSION

The results showed that the DNA vaccine constructed based on the ORF25 gene had a greater immune protective effect and can be used as a candidate vaccine for immunoprotection against CyHV-2.

Evaluation of Cyprinid Herpesvirus 2 Latency and Reactivation in Carassius gibel

Cyprinid herpesvirus 2 (CyHV-2, species Cyprinid herpesvirus 2) causes severe mortality in ornamental goldfish, crucian carp (Carassius auratus), and gibel carp (Carassius gibelio). It has been shown that the genomic DNA of CyHV-2 could be detected in subclinical fish, which implied that CyHV-2 could establish persistent infection. In this study, the latency of CyHV-2 was investigated in the survival fish after primary infection. CyHV-2 genomic DNA was detected in multiple tissues of acute infection samples; however, detection of CyHV-2 DNA was significantly reduced in fish recovered from the primary infection on day 300 postinfection. No active viral gene transcription, such as DNA polymerase and ORF99, was detected in recovered fish. Following temperature stress, an increase of CyHV-2 DNA copy numbers and gene transcription were observed in tissues examined, which suggests that CyHV-2 was reactivated under stress. In addition, a cell line (GCBLat1) derived from the brain tissue from CyHV-2-exposed fish harbored CyHV-2 genome but did not produce infectious virions under normal culture conditions. However, CyHV-2 replication and viral gene transcription occurred when GCBLat1 cells were treated with trichostatin A (TSA) or phorbol 12-myristate 13-acetate (TPA). It suggests CyHV-2 can remain latent in vitro and can reactivate under stress condition.

Detection of cyprinid herpesvirus 2 by loop-mediated isothermal amplification in combination with a lateral flow dipstick

We developed a convenient technique to detect Herpesviral haematopoietic necrosis attributed to cyprinid herpes virus 2 (CyHV-2), a serious disease of Crucian carp and goldfish related to high mortality. In the present study, we employed a lateral flow dipstick (LAMP-LFD) to present a loop-mediated isothermal amplification assay. The specificity was ascertained via other six viruses, and the sensitivity was compared using PCR method, which are the reaction conditions changes for the method improved. The results revealed that CyHV-2 performance was observable at 64 °C in a separated tube within 60 min, when the samples hybridized using an FITC-labeled probe. As the LAMP-LFD method's specificity was high, with its sensitivity identical to that of traditional PCR, the overall DNA collected revealed the lowest detection limit of 0.18 pg/μl from goldfish diseased by CyHV-2. In summary, the development of LAMP-LFD's method does not require expensive instruments, and it can be regarded as a fast, simple, and reliable method for CyHV-2 detection.

Investigation of bacterial diversity and pathogen abundances in gibel carp (Carassius auratus gibelio) ponds during a cyprinid herpesvirus 2 outbreak

Cyprinid herpesvirus 2 (CyHV-2) infection is detrimental to gibel carp health and may result in severe economic loss in freshwater aquaculture. However, information regarding the interaction of this pathogen with the aquatic environment is scarce. In this study, quantitative polymerase chain reaction (qPCR) and high-throughput sequencing were used to determine the abundances of pathogens and bacterial community compositions in two aquaculture ponds in Jiangsu Province, China. The results indicate that the concentrations of six selected pathogens were higher in the water than in the sediment and that these concentrations peaked during disease outbreak. In total, 8,326 and 18,244 operational taxonomic units were identified from water and sediment samples, respectively. The dominant phyla were Proteobacteria, Actinobacteria, Cyanobacteria, Bacteroidetes, and Chlorobi in water samples and Proteobacteria, Firmicutes, Actinobacteria, Chloroflexi, and Bacteroidetes in sediment samples. Bacterial communities were similar at the phylum level in different ponds, although significant differences were observed at the genus level. In addition, bacterial diversity was associated with environmental factors (temperature, chemical oxygen demand, NO2- -N, NO3- -N, and NH4+ -N) in the pond where the outbreak occurred. Additionally, CyHV-2 abundance was positively correlated with dissolved oxygen levels and Aeromonas spp. abundance in pond water (p < .01). This study provides comprehensive insight into the mechanisms of interaction between potential pathogens and the freshwater environment of aquaculture ponds during CyHV-2 disease outbreaks. Furthermore, the results from this study can contribute to improvement of the aquatic environment and establishment of disease prevention and control measures.

Morphological, microbiological and ultrastructural aspects of sepsis by Aeromonas hydrophila in Piaractus mesopotamicus

Aeromonas bacteria can cause an infection characterized by septicemia and is one of the most common pathogens in tropical fish. This disease is responsible for high morbidity and mortality rates, causing considerable losses in aquaculture. Thus, the understanding of its pathophysiology is crucial to develop control strategies of this bacterial infection in farmed fish. This study aimed to characterize early pathological aspects of acute sepsis in pacu (Piaractus mesopotamicus) experimentally infected with Aeromonas hydrophila. A total of 160 juvenile pacus were inoculated intraperitoneally with A. hydrophila (1.78 x 10⁹ CFU/mL) and at 0 (control), 1, 3, 6, and 9 hours post-inoculation (hpi), animals were anesthetized and samples were collected for microbiological, light microscopy and transmission electron microscopy (TEM) analyzes. The results showed the occurrence of hemodynamic alterations, such as hemorrhage and congestion, which were observed mainly after 6 and 9 hpi. It was possible to re-isolate Aeromonas at all sampling times except in control group. However, just after 9 hpi it was possible to find the bacteria in all fish and tissues. Light microscopy analyses revealed a degenerative process, necrosis and vascular damage mainly at 6 and 9 hpi. According to the ultrastructural examination, areas of cellular death were identified in all examined tissues, especially at 6 and 9 hpi. However, the most severe, related to necrosis, were observed after 6 and 9 hpi. The findings suggested that this bacterium spreads in the first hpi through the fish organs, mainly affecting spleen, liver and kidney, causing irreversible lesions at the molecular level.

Development of two brain cell lines from goldfish and silver crucian carp and viral susceptibility to Cyprinid herpesivirus-2

Two cell lines were established from silver crucian carp and goldfish brain tissue and used as the biological tool for monitoring viral diseases. Characterization including optimal growth kinetics study, karyotyping, and mitochondrial ribosomal RNA (rRNA) genotyping were performed. The primary cultures of these cells were generated by the explant technique using the medium 199 supplemented with 20 % fetal bovine serum and epidermal/fibroblast growth factors. The cells grew over the range of 15 to 30°C, while the optimal temperature for culture was 30°C. The cell lines were maintained in vitro and could be subcultured over 40 times. Following cryopreservation in liquid nitrogen, thawed cells exhibit viability of > 90 % after a 13-mo period of storage. The chromosome count of two cell lines were determined to be 154 and 110, respectively, which agreed well with triploid crucian carp brain cells and diploid goldfish brain cells. Polymerase chain reaction amplification and sequence analysis indicated 100 % and 94% match with known crucian carp mitochondrial DNA sequences. Cytopathic effect was continuously observed in both cell lines over 10 passages after inoculation with tissue homogenates of sick or died goldfish from cyprinid herpesvirus 2 (CyHV-2) outbreaks. These newly established cell lines could be a diagnostic tool for viral diseases in fish species.

Persistence of cyprinid herpesvirus 2 in asymptomatic goldfish Carassius auratus (L.) that survived an experimental infection

Cyprinid herpesvirus 2 (CyHV-2) is the causative agent of herpesviral haematopoietic necrosis (HVHN) in goldfish, Carassius auratus, and Prussian carp, C. auratus gibelio. In this study, we investigated virus persistence in goldfish experimentally infected with CyHV-2. Virus DNA presence in organs was monitored in survivors reared at a virus permissive temperature and also in survivors treated with a non-permissive temperature for 4 days, initiated at three different time points post-infection in order to obtain fish with different virus loads. We detected virus DNA in all organs tested at 51 days post-infection (dpi) and in the spleen, trunk kidney and gills of survivors at 81 dpi, although the virus load in fish influenced the subsequent number of organs that tested positive for virus DNA. In addition, some organs dissected from four out of five asymptomatic survivors tested positive by PCR following incubation in vitro in a medium for 5 days. Following inoculation with the homogenate of PCR-positive kidney incubated in vitro, one of the three inoculated fish died, showing that the detected virus by PCR produced infectious particles. This study suggests that CyHV-2 can establish a persistent infection in some organs, especially the spleen and trunk kidney, and that asymptomatic surviving fish can be a source of infection.

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.

Detection methods of Cyprinid herpesvirus 2 infection in silver crucian carp (Carassius auratus gibelio) via a pORF72 monoclonal antibody

Cyprinid herpesvirus 2 (CyHV-2) is the main pathogen responsible for causing haematopoietic necrosis disease in Carassius auratus gibelio. Although many nucleic acid-based diagnostic methods have been applied, no stable and sensitive immunological diagnostic approaches have been reported. In this study, to detect CyHV-2 in clinical samples using immunological methods, recombinant ORF72 protein (pORF72), encoded by the CyHV-2 ORF72 gene, was used as a capture antigen to identify blood and tissues infected with CyHV-2. First, ORF72 gene was amplified from the CyHV-2 genome and cloned into a PGEX-4t-3 expression vector to produce pORF72 in Escherichia coli. The purified pORF72 was used as an immunogen to prepare monoclonal antibodies. The Western blotting assays revealed that the monoclonal antibody could specifically identify the pORF72. Furthermore, an immunohistochemical protocol and a blood smear method were established to detect CyHV-2 in carps. The results indicate that the monoclonal antibody against pORF72 could be utilized as an effective detection tool for haematopoietic necrosis disease in Carassius auratus gibelio.

Importation of CyHV-2-infected goldfish into the Netherlands

Cyprinid herpesvirus 2 (CyHV-2) is known as the causative agent of herpesviral haematopoietic necrosis in goldfish Carassius auratus auratus. However, the virus has also been detected in Prussian carp C. gibelio and crucian carp C. carassius from European and Asian countries. To prevent spread of the causative virus to other areas, investigation of the risk factors of spread of this virus is important. In this study, 8 batches of goldfish imported into the Netherlands by airfreight from Asia and the Middle East were investigated for the presence of the virus. CyHV-2 DNA was detected by PCR in the pooled kidneys of 4 of the 8 imported goldfish batches, of which 1 was from a CyHV-2 disease case at a Dutch importer's quarantine facility. Sequence analysis of the CyHV-2 strains from this study and from previous reports showed that there were at least 6 different lengths in the mA region, resulting in tentatively at least 4 genotypes. Virus isolation was positive for only 1 (Amsterdam Schiphol-1 [AMS-1]) of the 8 samples. It was shown that the AMS-1 isolate was highly virulent to Ryukin goldfish after 100.3 TCID50 fish-1 intraperitoneal injection. The viral titre of the AMS-1 isolate for goldfish fin cells at several temperatures was similar to that of a Japanese CyHV-2 isolate. Our results prove that one of the routes of spread of various CyHV-2 strains is through the global trade of apparently healthy infected goldfish.

Susceptibility of isogeneic ginbuna Carassius auratus langsdorfii Temminck et Schlegel to cyprinid herpesvirus-2 (CyHV-2) as a model species

Herpesviral haematopoietic necrosis (HVHN), caused by cyprinid herpesvirus-2 (CyHV-2), has affected the commercial production of the goldfish Carassius auratus and gibelio carp Carassius auratus gibelio. High water temperature treatments are reported to reduce the mortality rate of infected goldfish and elicit immunity in the survivors. To define the mechanism by which this intervention induces resistance, clonal ginbuna Carassius auratus langsdorfii, which is closely related to both species and has been used in fish immunology, may represent a promising model species. In this study, we investigated the susceptibility of clonal ginbuna strains to CyHV-2 and the effect of high water temperature treatment on infected ginbuna and goldfish. Experimental intraperitoneal infection with CyHV-2 at 25 °C caused 100% mortality in ginbuna strains, which was accompanied by histopathological changes typical of HVHN. Both infected ginbuna S3n strain and goldfish, exposed to high temperature for 6 days [shifting from 25 °C (permissive) to 34 °C (non-permissive)], showed reduced mortalities after the 1st inoculation, and subsequent 2nd virus challenge to 0%, indicating induction of immunity. It was concluded that ginbuna showed a similar susceptibility and disease development in CyHV-2 infection compared to goldfish, suggesting that ginbuna can be a useful fish model for the study of CyHV-2 infection and immunity.

Hematological and Histological Changes in Prussian Carp Carassius gibelio Infected with Cyprinid Herpesvirus 2

Outbreaks of cyprinid herpesvirus 2 (CyHV-2) disease, also known as herpesviral hematopoietic necrosis, among cultured Prussian Carp Carassius gibelio has occurred each year in Jiangsu province, China, since 2009. In autumn 2014, hematological, blood biochemical, and histological changes in naturally infected moribund Prussian Carp were investigated after CyHV-2 was confirmed as the sole etiologic agent by etiological analyses. Total erythrocyte count, total leukocyte count, hemoglobin concentration, and thrombocyte count were significantly reduced (P < 0.01), whereas erythrocyte osmotic brittleness was significantly increased (P < 0.01) in infected fish compared with control fish. In addition, monocyte count was higher (P < 0.01) and lymphocyte count was lower (P < 0.01) in diseased fish than in control fish. The blood biochemical analyses indicated significant increases (P < 0.01) in the activities of alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, and lactate dehydrogenase as well as in the levels of total protein, globulin, total bilirubin, creatinine, and urea along with significant decreases (P < 0.01) in glucose and albumin in the diseased group. Histopathological examinations indicated that the kidneys and spleens of moribund Prussian Carp were the most severely lesioned organs, followed by the gills and hearts. Hypertrophied nuclei with marginated chromatin also appeared in the hearts and renal tubular epithelia from diseased fish. Pathological analysis of blood cells showed that approximately 78% of erythrocytes and 94% of leukocytes were lesioned with different levels of degeneration and necrosis in the diseased group. These previously unreported observations may be useful in the diagnosis of CyHV-2 disease. Received May 11, 2015; accepted March 24, 2016.

Detection of goldfish haematopoietic necrosis herpes virus (Cyprinid herpesvirus-2) with multi-drug resistant Aeromonas hydrophila infection in goldfish: First evidence of any viral disease outbreak in ornamental freshwater aquaculture farms in India

This outbreak report details of a mortality event where Cyprinid herpes virus-2 (CyHV-2) was detected in association with multidrug-resistant Aeromonas hydrophila infection in goldfish, Carassius auratus, from commercial farms. The goldfish exhibited large scale haemorrhages on the body, fins and gills, lepidorthosis, necrosed gills, protruded anus and shrunken eyes. White nodular necrotic foci in spleen and kidneys were noticed, along with necrosis and fusion of gill lamellae. Transmission electron microscopy of affected tissues revealed the presence of mature virus particles. Involvement of CyHV-2 was confirmed by PCR, sequencing and observed cytopathic effect in koi carp fin cell line along with experimental infection study. A bacterium isolated from the internal organs of affected fish was found to be pathogenic Aeromonas hydrophila having resistance to more than 10 classes of antibiotics. We postulate that CyHV-2 was the primary etiological agent responsible for this outbreak with secondary infection by A. hydrophila. The experimental infection trials in Labeo rohita and koi carp by intraperitoneal challenge with CyHV-2 tissue homogenates failed to reproduce the disease in those co-cultured fish species. This is the first report of a viral disease outbreak in organised earthen ornamental fish farms in India and bears further investigation.

First detection of Cyprinid Herpesvirus 2 (CyHV-2) in goldfish (Carassius auratus) in France

Massive mortalities of Carassius auratus (L.) occurred in a farm in France during summer 2014. Fish presented anorexia, loss of scales and large amounts of mucus on the gills. Necrosis of the distal tip of the filament and the lamellae, combined with fusion of the lamellae, was observed, as well as necrosis in the hematopoietic organs and in the digestive tract. The histological examination led to hypothesize the implication of a virus in the mortality. The presence of cyprinid herpesvirus 2 (CyHV-2) in dead fish was demonstrated by amplification and sequencing of portions of the DNA polymerase and helicase genes, both sequences exhibiting 100% identity with CyHV-2 from Japan. In an attempt to find genetic markers of variation, two regions containing tandem repeats in the Japanese genome were amplified from a virus-positive sample from the present outbreak. A first region (mB) was fully identical to the Japanese isolate. However, the second region (mA) exhibited a range of deletions and substitutions compared to CyHV-2 from Japan. This is the first report of CyHV-2 in France in association with mortality of goldfish and the first identification of a molecular marker for its tracing.

Detection of cyprinid herpesvirus 2 in peripheral blood cells of silver crucian carp, Carassius auratus gibelio (Bloch), suggests its potential in viral diagnosis

Epidemics caused by cyprinid herpesvirus 2 (CyHV-2) in domestic cyprinid species have been reported in both European and Asian countries. Although the mechanisms remain unknown, acute CyHV-2 infections generally result in high mortality, and the surviving carps become chronic carriers displaying no external clinical signs. In this study, in situ hybridization analysis showed that CyHV-2 tended to infect peripheral blood cells during either acute or chronic infections in silver crucian carp, Carassius auratus gibelio (Bloch). Laboratory challenge experiments coupled with real-time PCR quantification assays further indicated that steady-state levels of the viral genomic copy number in fish serum exhibited a typical 'one-step' growth curve post-viral challenge. Transcriptional expression of open reading frames (ORF) 121, which was selected due to its highest transcriptional levels in almost all tested tissues, was monitored to represent the replication kinetics of CyHV-2 in peripheral blood cells. Similar kinetic curve of active viral gene transcription in blood cells was obtained as that of serum viral load, indicating that CyHV-2 replicated in peripheral blood cells as well as in other well-characterized tissues. This study should pave the way for designing non-invasive and cost-effective serum diagnostic methods for quick detection of CyHV-2 infection.

Herpesviral Hematopoietic Necrosis in Goldfish in Switzerland: Early Lesions in Clinically Normal Goldfish (Carassius auratus)

Cyprinid herpesvirus 2 is a pathogen of goldfish, inducing a disease referred to as herpesviral hematopoietic necrosis. The disease is described so far in Japan, North America, Taiwan, Australia, the United Kingdom, and recently also Italy. Here the authors describe histologic lesions in clinically affected fish in comparison with clinically normal but virus DNA-positive goldfish in Switzerland. While necrosis or enhanced single-cell necrosis in the hematopoietic tissue in the pronephros or mesonephros was evident in dead and sick animals, in clinically normal goldfish, only single-cell necrosis was observed. Virus DNA was demonstrated in dead as well as clinically affected and subclinically infected goldfish by polymerase chain reaction and in situ hybridization. This study identifies the presence of goldfish herpesvirus in Switzerland and highlights the fact that the virus might be more widespread than assumed, as clinically normal goldfish can also carry cyprinid herpesvirus 2, showing histologically similar lesions but of lesser extent and severity.

CyHV-2 ORF104 activates the p38 MAPK pathway

Cyprinid herpesvirus 2 (CyHV-2) is the pathogen responsible for herpesviral hematopoietic necrosis disease, which causes huge losses on aquaculture. So far the studies of CyHV-2 mainly focus on the identification and detection of this virus, but little is known about the role of specific CyHV-2 genes in the infection process. Based on the genomic information, CyHV-2 ORF104 encodes a kinase-like protein, which is highly conserved among the three CyHVs. Our study was initiated to investigate the role of kinase-like protein ORF104 during virus infection. Subcellular localization study showed that ORF104 was mainly expressed in the nucleus in both human HEK293T and fish EPC cells. However, deletion of the putative nuclear localization signal of ORF104 (ORF104M) resulted in the cytoplasmic distribution in HEK293T. We then examined whether MAPKs were involved in the ORF104-mediated signaling pathway by overexpressing ORF104 and ORF104M in HEK293T. Overexpression of ORF104 and ORF104M resulted in the up-regulation of p38 phosphorylation, but not JNK or ERK, indicating that ORF104 specifically activates p38 signaling pathway. In vivo study showed that CyHV-2 infection enhanced p38 phosphorylation in gibel carp (Carassius auratus gibelio). Interestingly, p38 inhibitor SB203580 strongly reduced fish death caused by CyHV-2 infection. Therefore, our study for the first time reveals the function of ORF104 during CyHV-2 infection, indicating that ORF104 is a potential vaccine candidate for CyHV-2.

In vitro characteristics of cyprinid herpesvirus 2: effect of kidney extract supplementation on growth

Herpesviral hematopoietic necrosis caused by goldfish hematopoietic necrosis virus (now identified as cyprinid herpesvirus 2, CyHV-2) has contributed to economic losses in goldfish Carassius auratus culture and is becoming a major obstacle in Prussian carp C. gibelio aquaculture in China. Several reports have described difficulties in culturing the virus, with the total loss of infectivity within several passages in cell culture. We succeeded in propagating CyHV-2 with a high infectious titer in a RyuF-2 cell line newly derived from the fin of the Ryukin goldfish variety using culture medium supplemented with 0.2% healthy goldfish kidney extract. The addition of kidney extract to the medium enabled rapid virus growth, resulting in the completion of cytopathic effect (CPE) within 4 to 6 d at 25°C. The extract also enabled reproducible virus culture with a titer of 105-6 TCID50 ml-1. The virus cultured using this protocol showed pathogenicity in goldfish after intraperitoneal injection. The virus grew in RyuF-2 cells at 15, 20, 25, 30, and 32°C but not at 34°C or higher. Higher incubation temperatures allowed earlier development of CPE, but culture at 30 and 32°C yielded a lower virus titer than that obtained at other temperatures because of heat inactivation of the propagated virus during cultivation. Cell lines derived from goldfish and ginbuna C. langsdorfii showed high susceptibility to the virus; cell lines from carp were susceptible to the virus using a medium containing goldfish kidney extract, but EPC, FHM, and BF-2 cell lines did not produce any CPE, even in the presence of the extract.

Molecular characterisation and prevalence of a new genotype of Cyprinid herpesvirus 2 in mainland China

Cyprinid herpesvirus 2 (CyHV-2, species Cyprinid herpesvirus 2) has been confirmed as a causative agent of the acute haematopoietic necrosis disease outbreak in farmed goldfish (Carassius auratus L.) and gibel carp (Carassius auratus gibelio Bloch). In this study, we present the genomic characteristics of a variant CyHV-2 strain (SY-C1) isolated from farmed gibel carp in mainland China and its comparative genomics analysis with the CyHV-2 reference strain ST-J1. Overall, the full-length genome of SY-C1 shares 98.8% homology with that of ST-J1. Sequence comparisons between SY-C1 and ST-J1 indicate that the variations include single-nucleotide mutations, insertions, deletions, and rearrangements, which suggested that SY-C1 is different from ST-J1 and represents a new genotype. Therefore, we propose that the identified CyHV-2 can be divided into 2 different genotypes and be named China genotype (C genotype) and Japan genotype (J genotype) according to their isolation loci. Furthermore, epidemiological surveys indicate that the dominant genotype of CyHV-2 circulating in mainland China is closer to the China genotype than the Japan genotype.

Effects of experimentally induced infections of goldfish Carassius auratus with cyprinid herpesvirus 2 (CyHV-2) at various water temperatures

In this study, we examined the influence of water temperature on the development of herpesviral haematopoietic necrosis (HVHN) in goldfish Carassius auratus after experimentally induced infection with cyprinid herpesvirus 2 (CyHV-2). In Expt 1, Ryukin goldfish were infected with CyHV-2 by intraperitoneal injection and maintained at 4 different water temperatures. Cumulative mortalities of the 15, 20, 25 and 30°C groups were 10, 90, 90 and 60%, respectively. Therefore, the temperature range of 20-25°C is considered highly permissive for HVHN. One of 6 surviving fish of the 15°C group died after a rapid temperature increase to 25°C at 30 d post infection. All 3 Edonishiki goldfish, co-reared with the surviving Ryukin in tanks where the water temperature was increased from 15 to 25°C, died. In Expt 2, Edonishiki goldfish were exposed to CyHV-2 by bath immersion at 13 or 24°C, resulting in cumulative mortalities of 0 and 87%, respectively, at 28 d post-exposure. No mortality of the surviving Edonishiki in the 13°C treatment was observed when the water temperature was increased to 24°C. In addition, in Expt 2, no mortality was observed in any Ranchu co-reared with CyHV-2-immersed Edonishiki in the group where water temperature was increased from 13 to 24°C, even after re-immersion challenge with CyHV-2. It is interesting to note that CyHV-2 DNA was detected in the kidneys of 4 of the 5 surviving Ranchu co-reared with the CyHV-2-immersed Edonishiki group where the water temperature was increased from 13 to 24°C. Therefore, it is likely that the surviving Edonishiki of the 13°C group were virus carriers. This study indicates that most fish infected with CyHV-2 at 13-15°C acquire resistance to HVHN, but as carriers they are able to infect naïve fish.

Cyprinid herpesvirus-2 causing mass mortality in goldfish: applying electron microscopy to histological samples for diagnostic virology

In June 2013, a major fish kill of adult goldfish Carassius auratus occurred in Runnemede Lake, New Jersey, USA: an estimated 3000 to 5000 fish died within ~5 d. Necropsy of 4 moribund fish revealed severely pale gills, and histopathology showed type I and II fusion of the gills, diffuse necrosis of hematopoietic tissue in anterior and posterior kidney, and multifocal necrosis of the spleen. Within necrotic areas, pyknosis and enlarged nuclei with marginalized chromatin were observed. Cyprinid herpesvirus-2, the etiological agent for herpesviral hematopoietic necrosis disease, was confirmed in all 4 fish using PCR. We assessed the efficacy of identifying herpesviral infections (viral morphogenesis and cellular ultrastructure) using transmission electron microscopy (TEM) when applied to tissues fixed in 10% neutral buffered formalin (NBF) and tissue that was removed from paraffin blocks. Both sample types could be used to detect the virus within cells at similar concentrations. Tissues reprocessed from 10% NBF contained all the known stages of viral morphogenesis including empty capsids, capsids with an inner linear concentric density, capsids with an electron-dense core, and in the cytoplasm, mature capsids containing an envelope. Paraffin-embedded tissues showed similar stages, but viral capsids with an inner linear concentric density were rare and mature enveloped virions were not observed. In previously paraffin-embedded tissues, cellular membranes were not preserved, making identification of cell types and organelles difficult, whereas membrane preservation was good in tissues processed from 10% NBF. The results demonstrated that routinely fixed and paraffin-embedded samples can be successfully utilized to diagnose herpesviruses, and formalin-fixed tissue could be used to describe viral morphogenesis by TEM, making this a useful and reliable method for diagnostic virology when other samples are not available.