Development of real-time recombinase polymerase amplification (RPA) and RPA combined with lateral flow dipstick (LFD) assays for the rapid and sensitive detection of cyprinid herpesvirus 3

In this issue, we established rapid, cost-effective, and simple detection methods including recombines polymerase amplification with lateral flow dipstick (RPA-LFD) and real-time RPA for cyprinid herpesvirus 3(CyHV-3), and evaluated their sensitivity, specificity, and applicability, the real-time RPA method could achieve sensitive diagnosis of CyHV-3 within 1.3 copies per reaction, respectively. The real-time RPA method is 10-fold more sensitive than RPA-LFD method. The exact number of CyHV-3 can be calculated in each sample by real-time RPA. The sera from koi also can be tested in these methods. In addition, no cross-reaction was observed with other related pathogens, including carp oedema virus (CEV), spring viraemia of carp virus (SVCV), cyprinid herpesvirus 1(CyHV-1), cyprinid herpesvirus 2(CyHV-2), type I grass carp reovirus (GCRV-I), type II GCRV (GCRV-II), type III GCRV (GCRV-III), and Aeromonas hydrophila.

Comparative transcriptional analysis between virulent isolate HN1307 and avirulent isolate GD1108 of grass carp reovirus genotype II

As a widespread epidemic virus, genotype II of the grass carp reovirus poses a significant threat to the grass carp farming industry in China. Different genotype II isolates cause different degrees of virulence, although the underlying pathogenic mechanisms remain largely unknown. In this work, infections of grass carp with the virulent isolate grass carp reovirus (GCRV)-HN1307 and the avirulent isolate GCRV-GD1108 were performed to reveal a possible mutual transcriptional discrepancy. More differentially expressed genes (DEGs) were identified in the HN1307-infected group, which defined a grossly similar gene ontology (GO) pattern and different pathway landscape as the GD1108-infected group. Gene set enrichment analysis revealed that pathways related to innate immunity and metabolism were reciprocally activated and suppressed, respectively, following infection withHN1307, compared with GD1108. The trend analysis further indicated that immune-related pathways were involved in one of the four statistically significant profiles. Network analysis of transcription factor-gene interactions and protein-protein interactions on the immune-related profile suggested that among the core transcriptional factors (TFs) (UBTF, HCFC1, MAZ, MAX, and NRF1) and the hub proteins (Tlr3, Tlr7, Tlr9, Irf3, and Irf7), the latter were highly enriched in the toll-like receptor signaling pathway. Real-time quantitative PCR performed on the selected mRNAs validated the relative expression. This work will provide insights into the distinct transcriptional signatures from avirulent and virulent isolates of GCRV, which may contribute to the development of products for prevention.

Purification and concentration of infectious koi herpesvirus using steric exclusion chromatography

Koi herpesvirus (KHV) is the causative agent of a koi herpesvirus disease (KHVD) inducing high mortality rates in common carp and koi (Cyprinus carpio). No widespread effective vaccination strategy has been implemented yet, which is partly due to side effects of the immunized fish. In this study, we present an evaluation of the purification of infectious KHV from host cell protein and DNA, using the steric exclusion chromatography. The method is related to conventional polyethylene glycol (PEG) precipitation implemented in a chromatographic set-up and has been applied for infectious virus particle purification with high recoveries and impurity removal. Here, we achieved a yield of up to 55% of infectious KHV by using 12% PEG (molecular weight of 6 kDa) at pH 7.0. The recoveries were higher when using chromatographic cellulose membranes with 3-5 μm pores in diameter instead of 1 μm. The losses were assumed to originate from dense KHV precipitates retained on the membranes. Additionally, the use of >0.6 M NaCl was shown to inactivate infectious KHV. In summary, we propose a first step towards a purification procedure for infectious KHV with a possible implementation in fish vaccine manufacturing.

Development of a rapid and sensitive reverse transcription real-time quantitative PCR assay for detection and quantification of grass carp reovirus II

Hemorrhagic disease of grass carp, which is induced by grass carp reovirus II (GCRV-II), leads to mass mortality in grass carp culture and causes enormous economic loss. However, there is currently no quantitative analysis method for the detection of GCRV-II, which is greatly restricted the etiological and epidemiological study of the disease. In this study a reverse transcription TaqMan PCR (RT-qPCR) assay was developed for the quantitative detection of GCRV-II. The probe and primers targeted location is the segment 6 (S6) region of the GCRV-II genome which is highly conserved. Standard curves were drawn and criteria were confirmed after the determination of the optimum reaction conditions. The species-specific assay showed that the method is highly specific and has no cross reactions with other pathogens. The assay was sufficiently sensitive to detect as low as 10 copies of virus RNA. Moreover, the method has a very good repeatability for batches and inter-batches sample detection. Then the method was applied to detect the virus in tissue samples from clinically infected grass carp, compared with conventional RT-seminested PCR, the RT-qPCR represents a specific value for detection rate of positive samples. In summary, the RT-qPCR was applied and achieved high sensitivity and specificity for GCRV-II detection.

New Insights into Lymphocystis Disease Virus Genome Diversity

Lymphocystis disease viruses (LCDVs) are viruses that infect bony fish which has been found in different locations across the globe. Four virus species have been classified by the International Committee on Taxonomy of Viruses (ICTV), despite remarkable discrepancies in genome size. Whole genome sequencing and phylogenetic analysis of LCDVs from wild fish from the North Sea and partial sequences from gilthead sea bream of an aquafarm located in the Aegean Sea in Turkey confirm that the LCDV1 genome at 100 kb is approximately half the size of the genomes of LCDV2-4. Since the fish species, of which LCDV1 was isolated, differ taxonomically at the order level, co-speciation can be excluded as the driver of the adaptation of the genome of this nucleocytoplasmic large DNA virus, but may represent an adaptation to the lifestyle of this demersal fish in the northeast Atlantic.

Simultaneous Isolation and Identification of Largemouth Bass Virus and Rhabdovirus from Moribund Largemouth Bass (Micropterus salmoides)

Largemouth bass is an important commercially farmed fish in China, but the rapid expansion of its breeding has resulted in increased incidence of diseases caused by bacteria, viruses and parasites. In this study, moribund largemouth bass containing ulcer foci on body surfaces indicated the most likely pathogens were iridovirus and rhabdovirus members and this was confirmed using a combination of immunohistochemistry, cell culture, electron microscopy and conserved gene sequence analysis. We identified that these fish had been co-infected with these viruses. We observed bullet-shaped virions (100−140 nm long and 50−100 nm in diameter) along with hexagonal virions with 140 nm diameters in cell culture inoculated with tissue homogenates. The viruses were plaque purified and a comparison of the highly conserved regions of the genome of these viruses indicated that they are most similar to largemouth bass virus (LMBV) and hybrid snakehead rhabdovirus (HSHRV), respectively. Regression infection experiments indicated fish mortalities for LMBV-FS2021 and HSHRV-MS2021 were 86.7 and 11.1%, respectively. While co-infection resulted in 93.3% mortality that was significantly (p < 0.05) higher than the single infections even though the viral loads differed by >100-fold. Overall, we simultaneously isolated and identified LMBV and a HSHRV-like virus from diseased largemouth bass, and our results can provide novel ideas for the prevention and treatment of combined virus infection especially in largemouth bass.

Establishment and evaluation of qPCR and real-time recombinase-aided amplification assays for detection of largemouth bass ranavirus

Largemouth bass ranavirus disease (LMBVD) caused by largemouth bass ranavirus (LMBV) has resulted in severe economic losses in the largemouth bass (Micropterus salmoides) farming industry in China. Early and accurate diagnosis is the key measure for the prevention and control of LMBVD. In this study, a quantitative polymerase chain reaction (qPCR) and a real-time recombinase-aided amplification (real-time RAA) assay were established for the detection of LMBV. The sensitivity and specificity of these two methods, and the efficacy for detection of LMBV from clinical samples were also evaluated. Results showed that the real-time RAA reaction was completed in <30 min at 39℃ with a detection limit of 58.3 copies, while qPCR reaction required 60 min with a detection limit of 5.8 copies. Both methods were specific for LMBV, where no cross-reactions observed with the other tested fish pathogens. Comparing the amplification results of both assays to the results obtained by virus isolation using 53 clinical tissue samples, results showed that the clinical sensitivity of real-time RAA and qPCR were 93.75% and 100% respectively, and the clinical specificity of both were 100%. Our results showed that qPCR is more suitable for quantitative analysis and accurate detection of LMBV in the laboratory, while real-time RAA is more suitable as a point-of-care diagnostic tool for on-site detection and screening of LMBV under farm conditions and in poorly equipped laboratories.

Recombinant Lactococcus lactis Expressing Grass Carp Reovirus VP6 Induces Mucosal Immunity Against Grass Carp Reovirus Infection

Grass carp haemorrhagic disease caused by grass carp reovirus II is a serious disease of the aquaculture industry and vaccination is the only effective method of GCRV protection. In this study, Lactococcus lactis was used as oral vaccine delivery to express the GCRV II VP6 protein. We evaluated the protective efficacy of the live vaccine strain to induce mucosal immune protection. After oral administration, the recombinant strains remained in the hindgut for antigen presentation and increased the survival rate 46.7% and the relative percent survival 42.9%, respectively versus control vaccination. Though L. lactis alone can induce the inflammatory response by stimulating the mucosal immune system, the recombinant L. lactis expressing VP6 greatly enhanced nonspecific immune responses via expression of immune related genes of the fish. Furthermore, both systemic and mucosal immunity was elicited following oral immunization with the recombinant strain and this strain also elicited an inflammatory response and cellular immunity to enhance the protective effect. L. lactis can therefore be utilized as a mucosal immune vector to trigger high levels of immune protection in fish at both the systemic and mucosal levels. L. lactis is a promising candidate for oral vaccine delivery.

Development and comparative evaluation of real-time PCR and real-time RPA assays for detection of tilapia lake virus

Tilapia lake virus (TiLV) is a newly emerged pathogen responsible for high mortality and economic losses in the global tilapia industry. Early and accurate diagnosis is an important priority for TiLV disease control. In order to evaluate the methodology in the molecular diagnosis of TiLV, we compared newly developed quantitative real-time PCR (qPCR) and real-time recombinase polymerase amplification (real-time RPA) assays regarding their sensitivities, specificities and detection effect on clinical samples. Real-time RPA amplified the target pathogen in less than 30 min at 39 °C with a detection limit of 620 copies, while qPCR required about 60 min with a detection limit of 62 copies. Both assays were specific for TiLV and there were no cross-reactions observed with other common fish pathogens. The assays were validated using 35 tissue samples from clinically infected and 60 from artificially infected animals. The sensitivities for the real-time RPA and qPCR assays were 93.33 and 100%, respectively, and the specificity was 100% for both. Both methods have their advantages and can play their roles in different situations. The qPCR is more suitable for quantitative analysis and accurate detection of TiLV in a diagnostic laboratory, whereas real-time RPA is more suitable for the diagnosis of clinical diseases and preliminary screening for TiLV infection in poorly equipped laboratories as well as in fish farms.

Establishment of a rare minnow (Gobiocypris rarus) model for evaluation of experimental vaccines against a disease induced by grass carp reovirus genotype II

Vaccination is the most effective way to control the grass carp haemorrhagic disease (GCHD) with the primary pathogen grass carp reovirus genotype II (GCRV-II). However, due to the large difference in breeding conditions and unclear genetic background of grass carp, the results of the experiment were not reliable, which further hinders the effective prevention and control of GCHD. The rare minnow (Gobiocypris rarus) is highly sensitive to GCRV. Its small size, easy feeding, transparent egg membrane, and annual spawning are in line with the necessary conditions for an experimental aquatic animals culture object. In this study, immunogenicity and protective effects of attenuated and inactivated viruses for grass carp and rare minnow were evaluated in parallel. The expression of immune-related genes increased statistically significant after immunization. With the rise of specific serum antibody titers, the results of rare minnow and grass carp were consistent. In addition, there was no significant residue of adjuvant observed in both fish species injected with an adjuvanted and inactivated virus. Challenge of immunized grass carp and rare minnow with the isolate HuNan1307 resulted in protection rates of 95.8% and 92.6% for attenuated virus, 81.4% and 77.7% for inactivated virus, respectively, as well as the viral load changed consistently. The results indicated that rare minnow can be used as a model for evaluation of experimental vaccines against GCHD.

Special Issue "Emerging Viruses in Aquaculture"

According to the 2018 FAO report on aquaculture, there are 598 species of finfish, molluscs, crustaceans, and other organisms used in aquafarming around the world [...].

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.

Development of a double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) for the detection of KHV

Koi herpesvirus disease (KHVD) caused by the koi herpesvirus (KHV) is difficult to diagnose in live fish, presenting a challenge to the koi industry. The enzyme-linked immunosorbent assay (ELISA) method cannot be widely used to detect KHV because few commercial anti-KHV antibody exists. Here, we developed an anti-ORF132 polyclonal antibody and confirmed its reactivity via indirect immunofluorescence assay and Western blotting. A double-antibody sandwich ELISA (DAS-ELISA) was established to detect KHV, monoclonal antibody 1B71B4 against ORF92 was used as the capture antibody, and the detection antibody was the polyclonal antibody against the truncated ORF132. The lowest limit was 1.56 ng/ml KHV. Furthermore, the DAS-ELISA reacted with KHV isolates, while no cross-reactions occurred with carp oedema virus, spring viraemia of carp virus, frog virus 3 and grass carp reovirus. Two hundred koi serum samples from Guangdong, China, were used in the DAS-ELISA test, and the positive rate of the koi sera was 13%. The clinical sensitivity and specificity of the DAS-ELISA relative to the traditional PCR method were 66.7% and 97.6%, respectively. Our findings may be useful for diagnosing and preventing KHVD in koi and common carp.

Antiviral Actions of 25-Hydroxycholesterol in Fish Vary With the Virus-Host Combination

Cholesterol is essential for building and maintaining cell membranes and is critical for several steps in the replication cycle of viruses, especially for enveloped viruses. In mammalian cells virus infections lead to the accumulation of the oxysterol 25-hydroxycholesterol (25HC), an antiviral factor, which is produced from cholesterol by the cholesterol 25 hydroxylase (CH25H). Antiviral responses based on CH25H are not well studied in fish. Therefore, in the present study putative genes encoding for CH25H were identified and amplified in common carp and rainbow trout cells and an HPLC-MS method was applied for determination of oxysterol concentrations in these cells under virus infection. Our results give some evidence that the activation of CH25H could be a part of the antiviral response against a broad spectrum of viruses infecting fish, in both common carp and rainbow trout cells in vitro. Quantification of oxysterols showed that fibroblastic cells are capable of producing 25HC and its metabolite 7α,25diHC. The oxysterol 25HC showed an antiviral activity by blocking the entry of cyprinid herpesvirus 3 (CyHV-3) into KFC cells, but not spring viremia of carp virus (SVCV) or common carp paramyxovirus (Para) in the same cells, or viral haemorrhagic septicaemia virus (VHSV) and infectious pancreatic necrosis virus (IPNV) into RTG-2 cells. Despite the fact that the CH25H based antiviral response coincides with type I IFN responses, the stimulation of salmonid cells with recombinant type I IFN proteins from rainbow trout could not induce ch25h_b gene expression. This provided further evidence, that the CH25H-response is not type I IFN dependent. Interestingly, the susceptibility of CyHV-3 to 25HC is counteracted by a downregulation of the expression of the ch25h_b gene in carp fibroblasts during CyHV-3 infection. This shows a unique interplay between oxysterol based immune responses and immunomodulatory abilities of certain viruses.

Cell Culture-Derived Tilapia Lake Virus-Inactivated Vaccine Containing Montanide Adjuvant Provides High Protection against Viral Challenge for Tilapia

Tilapia lake virus (TiLV) is a newly emerging pathogen responsible for high mortality and economic losses in the global tilapia industry. Currently, no antiviral therapy or vaccines are available for the control of this disease. The goal of the present study was to evaluate the immunological effects and protective efficacy of formaldehyde- and β-propiolactone-inactivated vaccines against TiLV in the presence and absence of the Montanide IMS 1312 VG adjuvant in tilapia. We found that β-propiolactone inactivation of viral particles generated a vaccine with a higher protection efficacy against virus challenge than did formaldehyde. The relative percent survivals of vaccinated fish at doses of 10⁸, 10⁷, and 10⁶ 50% tissue culture infectious dose (TCID₅₀)/mL were 42.9%, 28.5%, and 14.3% in the absence of the adjuvant and 85.7%, 64.3%, and 32.1% in its presence, respectively. The vaccine generated specific IgM and neutralizing antibodies against TiLV at 3 weeks following immunization that were significantly increased after a second booster immunization. The steady state mRNA levels of the genes tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β), interferon γ (IFN-γ), cluster of differentiation 4 (CD4), major histocompatibility complex (MHC)-Ia, and MHC-II were all increased and indicated successful immune stimulation against TiLV. The vaccine also significantly lowered the viral loads and resulted in significant increases in survival, indicating that the vaccine may also inhibit viral proliferation as well as stimulate a protective antibody response. The β-propiolactone-inactivated TiLV vaccine coupled with the adjuvant Montanide IMS 1312 VG and booster immunizations can provide a high level of protection from virus challenge in tilapia.

Integrated analysis of mRNA-miRNA expression in Tilapia infected with Tilapia lake virus (TiLV) and identifies primarily immuneresponse genes

We investigated differential gene expression in Tilapia infected with the Tilapia Lake virus (TiLV).We used high-throughput sequencing to identify mRNAs and miRNAs involved in TiLV infection progression We identified 25,359 differentially expressed genes that included 863 new genes. We identified 1770, 4142 and 4947 differently expressed genes comparing non-infected controls with 24 and 120 h infections and between the infected groups, respectively. These genes were enriched to 291 GO terms and 62 KEGG pathways and included immune system progress and virion genes. High-throughput miRNA sequencing identified 316 conserved miRNAs, 525 known miRNAs and 592 novel miRNAs. Furthermore, 138, 198 and 153 differently expressed miRNAs were found between the 3 groups listed above, respectively. Target prediction revealed numerous genes including erythropoietin isoform X2, double-stranded RNA-specific adenosine deaminase isoform X1, bone morphogenetic protein 4 and tapasin-related protein that are involved in immune responsiveness. Moreover, these target genes overlapped with differentially expressed mRNAs obtained from RNA-seq. These target genes were significantly enriched to GO terms and KEGG pathways including immune system progress, virion and Wnt signaling pathways. Expression patterns of differentially expressed mRNA and miRNAs were validated in 20 mRNA and 19 miRNAs by qRT-PCR. We also were able to construct a miRNA-mRNA target network that can further understand the molecular mechanisms on the pathogenesis of TiLV and guide future research in developing effective agents and strategies to combat TiLV infections in Tilapia.

Koi herpesvirus (KHV) and KHV disease (KHVD) - a recently updated overview

Over the last years, there has been an enormous increase in the knowledge on koi herpesvirus (KHV), koi herpesvirus disease (KHVD), pathogenesis and virus variants. Different KHV lineages have clearly been identified, possible genomic changes during replication in different cell cultures at different temperatures but also in several hosts have been identified, a persistent stage of infection has been specified and it has been shown that infection with KHV is not host specific at all, but KHVD is. Additionally, it has been shown that it is possible to combat KHVD by immunization with inactivated and attenuated live vaccines using different delivery systems but also to benefit from alternative treatments with e.g. exopolysaccharids obtained from Arthrospira platensis.

Carbon nanotube-based DNA vaccine against koi herpesvirus given by intramuscular injection

Koi herpesvirus (KHV) also named Cyprinid Herpesvirus 3 (CyHV-3) is one of the most threatening pathogens affecting common carp production as well as the valued ornamental koi carp. The current commercial vaccines available are costly and potentially cause severe stress caused by live virus. KHV ORF149 gene has been proved encoding one of the main immunogenic proteins for KHV. In this study, we coupled a plasmid expression vector for ORF149 to single walled carbon nanotubes (SWCNTs) for an anti-KHV vaccine. The vaccine conferred an 81.9% protection against intraperitoneal challenge with KHV. Importantly, SWCNTs as a promising vehicle can enhanced the protective effects 33.9% over that of the naked DNA vaccine at the same dose. The protection was longer and serum antibody production, enzyme activities and immune-related gene expression were all induced in fish vaccinated with the nanotube-DNA vaccine compared with the DNA alone. Thereby, this study demonstrates that the ORF149 DNA vaccine loaded onto SWCNTs as a novel vaccine might provide an effective method of coping with KHV disease using intra-muscular vaccination.

Comparison of the blood parameters and histopathology between grass carp infected with a virulent and avirulent isolates of genotype II grass carp reovirus

Grass carp hemorrhagic disease caused by grass carp reovirus (GCRV) is the most important disease for grass carp aquaculture. Its typical clinical symptom is haemorrhaging, although the mechanism was remained unclear. In this study, we investigated the differences in blood parameters and histopathological features between grass carp infected with a virulent and avirulent isolates of genotype II GCRV. Infection with the virulent isolate resulted in increases in 8 routine blood and 2 serum biochemical parameters (P < 0.05); while 9 routine blood and 5 biochemical parameters were significantly decreased (P < 0.05) compared with fish infected with the avirulent isolate. The majority of these alterations were related to hemorrhage, inflammatory reactions and organic damage. The histopathologic changes were primarily vasodilation and hyperaemia in multiple organs, lymphocyte and macrophage infiltration as well as severe vacuolar degeneration in spleen, kidney and liver. The histopathology changes in fish infected with the avirulent isolate were minimal. These results indicated that the pathogenicity of GCRV was primarily reflected in destruction of the blood circulatory system and parenchymatous organs. This study lays the foundation for further research on the pathogenesis of bleeding caused by GCRV infection and the use of blood parameters and histopathology as tools for disease diagnosis.

Establishment of a brain cell line obtained from hybrids of Channa argus ×Channa maculata for the detection of tilapia lake virus

A brain cell line (CAMB) derived from hybrid snakehead (Channa argus (♂) × Channa maculata (♀)) was established by trypsin and collagenase combined digestion. The culturing conditions and cell biological characteristics were systematically studied. For growth of the cells, M199 medium containing 10% fetal bovine serum was used and at 27 °C incubated. Based on morphological analysis, CAMB cells were confirmed to be epithelial. The cell line has been subcultured more than 80 times since its initial primary culture. Chromosome analysis revealed that CAMB cells had an abnormal chromosome number 2n = 64, whereas the chromosome number in the hybrid snakehead was 45. The suitability of CAMB for tilapia lake virus (TiLV) was demonstrated. A CPE was observed after infection with TiLV-2017A. The highest TiLV titer was observed after 12 days post infection (dpi) and reached 107.2 TCID50/mL. The virus replication was confirmed by electron microscopic observations. Additionally, immunofluorescence assay confirmed the presence of TiLV-2017A after infection of CAMB. Therefore, CAMB cells can be a useful tool for the investigation of the pathogenesis of the TiLV induced disease in tilapia.

Establishment of a cell line from egg of rare minnow Gobiocypris rarus for propagation of grass carp reovirus genotype II

The rare minnow, Gobiocypris rarus, is small experimental fish proven to be sensitive to Grass Carp Reovirus (GCRV) infection. In present study we established a new cell (GrE) from eggs of G. rarus. GrE cells grew well at 28 °C in M199 medium containing 10% fetal bovine serum, and has been subcultured for over 70 passages. Chromosome analysis indicated that 40% of the cells were diploid 2n = 66 while the chromosome number of the fish is 2n = 50. Viral replication in GrE cells was confirmed by transmission electron microscopy, immunofluorescence assays and virus titration experiments. GrE cells and Cyenopharyngodon idellus kidney cells were infected with two GCRV genotypes while the virus copies of GCRV II in GrE peaked at 2.25 × 10⁵ on 12th dpi. In vivo challenge experiments using GCRV I and II isolates at generations 1 and 20 indicated that GCRV II reproduce similar symptoms and histopathological changes of the disease in the rare minnow. These results indicated that GrE is permissive for GCRV genotype II propagation and can be used for pathogenesis studies and vaccine development of the predominant genotype of GCRV.

Cyprinid herpesvirus 3 Evolves In Vitro through an Assemblage of Haplotypes that Alternatively Become Dominant or Under-Represented

Viruses are able to evolve in vitro by mutations after serial passages in cell cultures, which can lead to either a loss, or an increase, of virulence. Cyprinid herpesvirus 3 (CyHV-3), a 295-kb double-stranded DNA virus, is the etiological agent of the koi herpesvirus disease (KHVD). To assess the influence of serial passages, an isolate of CyHV-3 (KHV-T) was passaged 99 times onto common carp brain (CCB) cells, and virus virulence was evaluated during passages through the experimental infections of common carp. After 78 CCB passages, the isolate was much less virulent than the original form. A comparative genomic analysis of these three forms of KHV-T (P0, P78 and P99) revealed a limited number of variations. The largest one was a deletion of 1363 bp in the predicted ORF150, which was detected in P78, but not in P99. This unexpected finding was confirmed by conventional PCR and digital PCR. The results presented here primarily suggest that, CyHV-3 evolves, at least in vitro, through an assemblage of haplotypes that alternatively become dominant or under-represented.

Development of a simple and rapid reverse transcription-loopmediated isothermal amplification (RT-LAMP) assay for sensitive detection of tilapia lake virus

Recently, substantial mortality of farmed and wild tilapia caused by tilapia lake virus (TiLV) infection has been observed worldwide. However, sensitive and reliable diagnostic method is limited. A reverse transcription-loopmediated isothermal amplification (RT-LAMP) assay has been applied for the detection of TiLV nucleotide sequence. Six primers targeting two locations on the target gene based on a highly conserved sequence in the segment 1 (S1) region of the TiLV genome have been designed. The optimized RT-LAMP reaction was maintained at the isothermal condition of 63°C for 45 min. And the amplifications could be verified by turbidity or a colour change with the addition of SYBR Green I. Subsequently, RT-LAMP products could be observed by a ladder pattern following gel electrophoresis. The species-specific assay showed that the method was sensitive enough to detect as low as 1.6 copies of viral particle, and the assay was highly specific because no cross-reactivity was observed with other pathogens, and had a diagnostic sensitivity and specificity of 100% when TiLV-positive samples and non-target virus were tested. In summary, all the results demonstrate that this RT-LAMP is a rapid, effective and sensitive method for TiLV detection in tilapia aquaculture.

Type I interferon responses of common carp strains with different levels of resistance to koi herpesvirus disease during infection with CyHV-3 or SVCV

Carp from breeding strains with different genetic background present diverse levels of resistance to viral pathogens. Carp strains of Asian origin, currently being treated as Cyprinus rubrofuscus L., especially Amur wild carp (AS), were proven to be more resistant to koi herpesvirus disease (KHVD; caused by cyprinid herpesvirus 3, CyHV-3) than strains originating from Europe and belonging to Cyprinus carpio L., like the Prerov scale carp (PS) or koi carp from a breed in the Czech Republic. We hypothesised that it can be associated with a higher magnitude of type I interferon (IFN) response as a first line of innate defence mechanisms against viral infections. To evaluate this hypothesis, four strains of common carp (AS, Rop, PS and koi) were challenged using two viral infection models: Rhabdovirus SVCV (spring viremia of carp virus) and alloherpesvirus CyHV-3. The infection with SVCV induced a low mortality rates and the most resistant was the Rop strain (no mortalities), whereas the PS strain was the most susceptible (survival rate of 78%). During CyHV-3 infection, Rop and AS strains performed better (survival rates of 78% and 53%, respectively) than PS and koi strains (survival rates of 35% and 10%, respectively). The evaluation of virus loads and virus replication showed significant differences between the carp strains, which correlated with the mortality rate. The evaluation of type I IFN responses showed that there were fundamental differences between the virus infection models. While responses to the SVCV were high, the CyHV-3 generally induced low responses. Furthermore, the results demonstrated that the magnitude of type I IFN responses did not correlate with a higher resistance in infected carp. In the case of a CyHV-3 infection, reduced type I IFN responses could be related to the potential ability of the virus to interfere with cellular sensing of foreign nucleic acids. Taken together, the results broaden our understanding of how common carp from different genetic strains interact with various viral pathogens.

New cell lines for efficient propagation of koi herpesvirus and infectious salmon anaemia virus

The production of piscine viruses, in particular of koi herpesvirus (KHV, CyHV-3) and infectious salmon anaemia virus (ISAV), is still challenging due to the limited susceptibility of available cell lines to these viruses. A number of cell lines from different fish species were compared to standard diagnostic cell lines for KHV and ISAV regarding their capability to exhibit a cytopathic effect (CPE) and to accumulate virus. Two cell lines, so far undescribed, appeared to be useful for diagnostic purposes. Fr994, a cell line derived from ovaries of rainbow trout (Oncorhynchus mykiss), produced constantly high ISA virus (ISAV) titres and developed a pronounced CPE even at high cell passage numbers, while standard cell lines are reported to gradually loose these properties upon propagation. Another cell line isolated from the head kidney of common carp (Cyprinus carpio), KoK, showed a KHV induced CPE earlier than the standard cell line used for diagnostics. A third cell line, named Fin-4, established from the fin epithelium of rainbow trout did not promote efficient replication of tested viruses, but showed antigen sampling properties and might be useful as an in vitro model for virus uptake or phagocytosis.

Characterization of gene deletion mutants of Cyprinid herpesvirus 3 (koi herpesvirus) lacking the immunogenic envelope glycoproteins pORF25, pORF65, pORF148 and pORF149

Cyprinid herpesvirus 3 (CyHV-3) or koi herpesvirus is a global pathogen causing mass mortality in koi and common carp, against which improved vaccines are urgently needed. In this study we investigated the role of four nonessential, but immunogenic envelope glycoproteins encoded by members of the ORF25 gene family (ORF25, ORF65, ORF148 and ORF149) during CyHV-3 replication. Single deletion of ORF65 did not affect in vitro replication, and deletion of ORF148 even slightly enhanced virus growth on common carp brain (CCB) cells. Deletions of ORF25 or ORF149 led to reduced plaque sizes and virus titers, which was due to delayed entry into host cells. An ORF148/ORF149 double deletion mutant exhibited wild-type like growth indicating opposing functions of the two proteins. Electron microscopy of CCB cells infected with either mutant did not indicate any effects on virion formation and maturation in nucleus or cytoplasm, nor on release of enveloped particles. The ORF148, ORF149 and double deletion mutants were also tested in animal experiments using juvenile carp, and proved to be insufficiently attenuated for use as live virus vaccines. However, surviving fish were protected against challenge with wild-type CyHV-3, demonstrating that these antibody inducing proteins are dispensable for an efficient immune response in vivo.

Establishment and characterization of a cell line from tilapia brain for detection of tilapia lake virus

Tilapia lake virus (TiLV) is an emerging disease threatening tilapia culture in many parts of the world. A cell line from the brain of tilapia, which was named TiB, was established, characterized and subcultured with more than 100 passages. The TiB cell line was optimally maintained at 27°C using medium 199 (M199) supplemented with 10% foetal bovine serum (FBS). Chromosome analysis revealed that 60% of TiB cells at passage 5 maintained the modal chromosome number 2n = 44, while at passage 60, there were 43% of TiB cells with the diploid chromosome number 2n = 50. A significant cytopathic effect was observed in TiB cells after infection with tilapia lake virus (TiLV-2017A), and the viral replication in the cells was confirmed by transmission electron microscopy, immunofluorescence assays and viral titres, indicating the susceptibility of TiB cells to TiLV-2017A. The viral titres of TiLV-2017A in TiB cells reached 10^7.43 TCID₅₀ /ml within 10 days. The stable growth and susceptibility to fish viruses make TiB cells a useful tool for fish virus-host cell interaction and for immune response of fish.

Development of a VP38 recombinant protein-based indirect ELISA for detection of antibodies against grass carp reovirus genotype II (iELISA for detection of antibodies against GCRV II)

Currently, serological assays for grass carp reovirus genotype II (GCRV-II) diagnosis are not available. In this study, an indirect enzyme-linked immunosorbent assay (ELISA) for the detection of antibodies against GCRV-II was developed. The structural protein VP38 of GCRV-II was used as the coating antigen. Monoclonal antibodies (mAb) against IgM of grass carp labelled with HRP were used as a secondary antibody. The antigen concentration and serum dilution were optimized using chess board titration. Furthermore, the specificity of indirect ELISA assay was confirmed by cross check with sera positive for other grass carp pathogens. In comparison with results obtained from indirect immunofluorescence assay (IFA) and Western blot by testing of 60 serum samples to evaluate the sensitivity and specificity of the ELISA, agreement between 90% and 96.7% was reached, respectively. A serological survey was performed using the assay with grass carp field serum samples. The seropositive rate of the 242 serum samples was 69.8%. In conclusion, the developed indirect ELISA is a very specific and sensitive test that will be useful for large-scale serological surveys to detect indirectly GCRV II infections as well as to monitor the changes of antibody level after immunization.

Aeromonas shuberti as a cause of multi-organ necrosis in internal organs of Nile tilapia, Oreochromis niloticus

A disease with white spots in internal organs of Nile tilapia occurred in Zhanjiang, southern China. Multiple, white nodules, 0.8-2.2 mm in diameter, were scattered throughout the liver, spleen and kidney of diseased fish. Signs of nodules reproduced after artificial infection with the isolated strain. Isolated bacteria were Gram-negative, facultative anaerobic, motile, short rod-shaped, with a length of 1.2-2.2 μm. Morphological and biochemical tests, as well as phylogenetic analysis, all strongly indicated that the isolate from tilapia is identical to Aeromonas schubertii (A. schubertii) which temporary named LF1708 strain. Antibiotic sensitivity assays showed the LF1708 is sensitive to 24 of 27 tested antibiotics. Pathogenicity test revealed that the isolate at the dose of 3.75 × 10⁶ CFU/g killed 100% of experimental tilapia within 2 days and the dose of 1 × 10⁷ CFU/g killed 100% of experimental zebrafish within 1 day. Histopathology of diseased tilapia infected with A. schubertii showed numerous necrotic lesions widely distributed in spleen, liver and kidney, and infiltration with a large number of bacteria. To our knowledge, this was the first report that associated A. schubertii with mortality in tilapia.

Generation of a potential koi herpesvirus live vaccine by simultaneous deletion of the viral thymidine kinase and dUTPase genes

Koi herpesvirus (KHV, Cyprinidherpesvirus 3) causes a fatal disease of koi and common carp. To obtain safe and efficacious live vaccines, we generated deletion mutants of KHV lacking the nonessential genes encoding two enzymes of nucleotide metabolism, thymidine kinase (TK, ORF55) and deoxyuridine-triphosphatase (DUT, ORF123). Since single-deletion mutants based on a KHV isolate from Israel (KHV-I) only exhibited partial attenuation (Fuchs W, Fichtner D, Bergmann SM, Mettenleiter TC. Arch Virol 2011;156 : 1059-1063), a corresponding double mutant was generated and tested in vivo, and shown to be almost avirulent but still protective. To overcome the low in vitro virus titres of KHV-I (≤10⁵ p.f.u. ml^-1), single and double TK and DUT deletions were also introduced into a cell culture-adapted KHV strain from Taiwan (KHV-T). The deletions did not affect in vitro virus replication, and all KHV-T mutants exhibited wild-type-like plaque sizes and titres exceeding 10⁷ p.f.u. ml^-1, as a prerequisite for economic vaccine production. Compared to wild-type and revertant viruses, the single-deletion mutants of KHV-T were significantly attenuated in vivo, and immersion of juvenile carp in water containing high doses of the double mutant caused almost no fatalities. Nevertheless, the deletion mutants induced similar levels of KHV-specific serum antibodies to the parental wild-type virus, and conferred solid protection against disease after challenge with wild-type KHV. For the convenient differentiation of DNA samples prepared from gill swabs of carp infected with wild-type and TK-deleted KHV we developed a triplex real-time PCR. Thus, KHV-TΔDUT/TK might be suitable as a genetic DIVA vaccine in the field.

A new age in AquaMedicine: unconventional approach in studying aquatic diseases

BACKGROUND

Marine and aquaculture industries are important sectors of the food production and global trade. Unfortunately, the fish food industry is challenged with a plethora of infectious pathogens. The freshwater and marine fish communities are rapidly incorporating novel and most up to date techniques for detection, characterization and treatment strategies. Rapid detection of infectious diseases is important in preventing large disease outbreaks.

MAIN TEXT

One hundred forty-six articles including reviews papers were analyzed and their conclusions evaluated in the present paper. This allowed us to describe the most recent development research regarding the control of diseases in the aquatic environment as well as promising avenues that may result in beneficial developments. For the characterization of diseases, traditional sequencing and histological based methods have been augmented with transcriptional and proteomic studies. Recent studies have demonstrated that transcriptional based approaches using qPCR are often synergistic to expression based studies that rely on proteomic-based techniques to better understand pathogen-host interactions. Preventative therapies that rely on prophylactics such as vaccination with protein antigens or attenuated viruses are not always feasible and therefore, the development of therapies based on small nucleotide based medicine is on the horizon. Of those, RNAi or CRISPR/Cas- based therapies show great promise in combating various types of diseases caused by viral and parasitic agents that effect aquatic and fish medicine.

CONCLUSIONS

In our modern times, when the marine industry has become so vital for feed and economic stability, even the most extreme alternative treatment strategies such as the use of small molecules or even the use of disease to control invasive species populations should be considered.

Establishment and characterization of a new cell line from koi brain (Cyprinus carpio L.)

A novel permanently growing brain cell line from koi (Cyprinus carpio L.) (KB cell line) was established, and its suitability for detection of koi herpesvirus (KHV) was demonstrated in this study. The KB cell line was optimally maintained at 27°C in Leibovitz's L-15 medium supplemented with 10% foetal bovine serum (FBS). It was subcultured more than 100 times, and chromosome analysis revealed that 51.54% of KB cells at passage 80 maintained the abnormal diploid chromosome number 2n = 96 while the modal chromosome number was 2n = 100. The cell line was cryopreserved in liquid nitrogen at -196°C and was recovered from storage after 1 year with good cell viability and vitality. The results of virus isolation demonstrated that KB cells were susceptible to KHV, which was shown by the presence of an obvious cytopathic effect and abundant virus particles. The viral titres of KHV in KB reached 10^5.73 TCID₅₀ /0.1 ml within 7 days. Immunofluorescence and Western blot assays confirmed that KB replicated KHV. The newly established KB cell line will serve as a useful tool to elucidate KHV disease (KHVD) pathogenesis.

Development of indirect immunofluorescence assay for TCID50 measurement of grass carp reovirus genotype II without cytopathic effect onto cells

Grass carp reovirus (GCRV) caused severe hemorrhagic disease with significant losses of fingerling and yearling grass carp, Cyenopharyngodon idellus, in southeast Asian. It was first isolated in 1983 in China, and clade analysis of the different GCRV isolates indicates there are at least three different genotypes I, II, and III. In recent years, GCRV genotype II has been determined as a dominant virus type which cause severe obvious clinical signs in fish but no cytopathic effect onto presently available cell culture. TCID₅₀ is one of standard method to quantity infectious virus particles. In the present study, an indirect immunofluorescence assay (IFA) was developed using antibody against a protein encoded by segment 10 of GCRV genotype II. Moreover, the specific assay to differentitate GCRV of different genotypes and a sensitive assay for determination of GCRV genotype II were developed respectively. The results showed the IFA only can recognize genotype II virus at the lowest initial concentration of 550 genomic copies/ml. Furthermore, comparison of results obtained from qPCR and the TCID₅₀ assay combined IFA was conducted. The results indicated that TCID₅₀ of GCRV isolates JX0901 and HZ08 differs with 2 log steps reduction in the numbers of viruses compared with the number of genome copies detected by qPCR. The immunofluorescence assay developed is sensitive, specific, and the TCID₅₀ combined with IFA will be a standardizable technique for the quantitation and detection of infectious GCRV in cell culture without cytolysis.

Validation of a KHV antibody enzyme-linked immunosorbent assay (ELISA)

Koi herpesvirus (KHV) causes KHV disease (KHVD). The virus is highly contagious in carp or koi and can induce a high mortality. Latency and, in some cases, a lack of signs presents a challenge for virus detection. Appropriate immunological detection methods for anti-KHV antibodies have not yet been fully validated for KHV. Therefore, it was developed and validated an enzyme-linked immunosorbent assay (ELISA) to detect KHV antibodies. The assay was optimized with respect to plates, buffers, antigens and assay conditions. It demonstrated high diagnostic and analytical sensitivity and specificity and was particularly useful at the pond or farm levels. Considering the scale of the carp and koi industry worldwide, this assay represents an important practical tool for the indirect detection of KHV, also in the absence of clinical signs.

Detection of Herpesvirus anguillae (AngHV-1) in European eel Anguilla anguilla (L.) originating from northern Poland-assessment of suitability of selected diagnostic methods

The Community Action Plan requests EU member states to implement measures that ensure the recovery of the severely depleted European eel stocks. One of the main threats is posed by Anguillid herpesvirus 1 (AngHV-1) leading to increased mortality in both wild and farmed eels. Following recommendations of the OIE to minimize the risk of obtaining false-negative results, the main aim of the study was to optimize diagnostic methods for AngHV-1 detection using conventional PCR, nested PCR and in situ hybridization assay. While 53.3% of the individual organ samples were tested positive for AngHV-1 by PCR, the additional virus analysis via nested PCR revealed that the actual prevalence was 93.3%. In the cell cultivation passages, a cytopathic effect was hardly found in the first two rounds. In the third passage onto cell cultures, a lytic CPE was detected. The identification and confirmation of the viruses obtained from cell cultures as well as directly from the organ tissues were proceeded by PCR, nested PCR and sequencing of the PCR products. While no positive signal was detectable in the first round by PCR using samples from the third cell culture passages, the nested PCR provided weak but visible positive signals.

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

Carp edema virus disease (CEVD), also known as koi sleepy disease, is caused by a poxvirus associated with outbreaks of clinical disease in koi and common carp Cyprinus carpio. Originally characterised in Japan in the 1970s, international trade in koi has led to the spread of CEV, although the first recognised outbreak of the disease outside of Japan was not reported until 1996 in the USA. In Europe, the disease was first recognised in 2009 and, as detection and diagnosis have improved, more EU member states have reported CEV associated with disease outbreaks. Although the structure of the CEV genome is not yet elucidated, molecular epidemiology studies have suggested distinct geographical populations of CEV infecting both koi and common carp. Detection and identification of cases of CEVD in common carp were unreliable using the original PCR primers. New primers for conventional and quantitative PCR (qPCR) have been designed that improve detection, and their sequences are provided in this paper. The qPCR primers have successfully detected CEV DNA in archive material from investigations of unexplained carp mortalities conducted >15 yr ago. Improvement in disease management and control is possible, and the principles of biosecurity, good health management and disease surveillance, applied to koi herpesvirus disease, can be equally applied to CEVD. However, further research studies are needed to fill the knowledge gaps in the disease pathogenesis and epidemiology that, currently, prevent an accurate assessment of the likely impact of CEVD on European koi and common carp aquaculture and on wild carp stocks.

Antiviral activity of exopolysaccharides from Arthrospira platensis against koi herpesvirus

Although koi herpesvirus (KHV) has a history of causing severe economic losses in common carp and koi farms, there are still no treatments available on the market. Thus, the aim of this study was to test exopolysaccharides (EPS) for its antiviral activity against KHV, by monitoring inhibition and cytotoxic effects in common carp brain cells. These substances can be easily extracted from extracellular algae supernatant and were identified as groups of sulphated polysaccharides. In order to reach this aim, Arthrospira platensis, which is well known for its antiviral activity of intra- and extracellular compounds towards mammalian herpesviruses, was investigated as standard organism and compared to commercial antiviral drug, ganciclovir, which inhibits the viral DNA polymerization. The antiviral activity of polysaccharides of A. platensis against KHV was confirmed in vitro using qualitative assessment of KHV life cycle genes, and it was found by RT-PCR that EPS, applied at a concentration of >18 μg mL^-1 and a multiplicity of infection (MOI) of 0.45 of KHV, suppressed the viral replication in common carp brain (CCB) cells even after 22 days post-infection, entirely. Further, this study presents first data indicating an enormous potential using polysaccharides as an additive for aquacultures to lower or hinder the spread of the KHV and koi herpesvirus disease (KHVD) in future.

Comparison of PCR methods for the detection of genetic variants of carp edema virus

The infection of common carp and its ornamental variety, koi, with the carp edema virus (CEV) is often associated with the occurrence of a clinical disease called 'koi sleepy disease'. The disease may lead to high mortality in both koi and common carp populations. To prevent further spread of the infection and the disease, a reliable detection method for this virus is required. However, the high genetic variability of the CEV p4a gene used for PCR-based diagnostics could be a serious obstacle for successful and reliable detection of virus infection in field samples. By analysing 39 field samples from different geographical origins obtained from koi and farmed carp and from all 3 genogroups of CEV, using several recently available PCR protocols, we investigated which of the protocols would allow the detection of CEV from all known genogroups present in samples from Central European carp or koi populations. The comparison of 5 different PCR protocols showed that the PCR assays (both end-point and quantitative) developed in the Centre for Environment, Fisheries and Aquaculture Science exhibited the highest analytical inclusivity and diagnostic sensitivity. Currently, this makes them the most suitable protocols for detecting viruses from all known CEV genogroups.

Ultrastructural analysis of sequential cyprinid herpesvirus 3 morphogenesis in vitro

Cyprinid herpesvirus 3 (CyHV-3) is an alloherpesvirus, and it is the aetiological agent of koi herpesvirus disease. Although the complex morphogenic stages of the replication cycle of CyHV-3 were shown to resemble that of other members of the Herpesvirales, detailed analysis of the sequence and timing of these events was not definitively determined. This study describes these features through a time course using cyprinid cell cultures (KF-1 and CCB) infected with CyHV-3 (KHV isolate, H361) and analysed by transmission electron microscopy. Rapid viral entry was noted, with high levels of intracellular virus within 1-4 h post-infection (hpi). Intranuclear capsid assembly, paracrystalline array formation and primary envelopment of capsids occurred within 4 hpi. Between 1 and 3 days post-infection (dpi), intracytoplasmic secondary envelopment occurred, as well as budding of infectious virions at the plasma membrane. At 5-7 dpi, the cytoplasm contained cytopathic vacuoles, enveloped virions within vesicles, and abundant non-enveloped capsids; also there was frequent nuclear deformation. Several morphological features are suggestive of inefficient viral assembly, with production of non-infectious particles, particularly in KF-1 cells. The timing of this alloherpesvirus morphogenesis is similar to other members of the Herpesvirales, but there may be possible implications of using different cell lines for CyHV-3 propagation.

Investigation of latent infections caused by cyprinid herpesvirus 3 in koi ( Cyprinus carpio) in southern China

Although herpesviruses such as cyprinid herpesvirus 3 (CyHV-3) can establish lifelong latent infections, little is known about latency conditions in farmed koi populations in China. We used nested polymerase chain reaction targeting the TK gene and an indirect antibody ELISA to screen asymptomatic fish obtained from southern China for evidence of CyHV-3 infection. CyHV-3 DNA could be detected either in peripheral blood leukocytes or from gills of asymptomatic koi. Most koi sera did not contain anti-CyHV-3 antibodies; however, 5 samples were ELISA positive, providing evidence of prior CyHV-3 infections. These findings suggest that koi may survive CyHV-3 infections and become virus carriers.

Can water disinfection prevent the transmission of infectious koi herpesvirus to naïve carp? - a case report

Hygienic measures such as disinfection are important tools for the maintenance of fish health in aquaculture. While little information is available on the disinfection of water intended for fish containment, Huwa-San^® , a disinfectant used in food and water industries, was used for daily treatment at concentrations of approximately 60 ppm over a total period of 3 months (experiment 1) with a 3-week treatment-free interval after 2 months (experiment 2). During this period, koi herpesvirus (KHV) was added to the water of two aquaria, one used as a normal contact control, the other one receiving daily water disinfectant treatments that prevented KHV infection of carp. In the second experiment, Huwa-San^® treatment was interrupted and KHV infection was prevalent. However, when naïve fish were introduced to the same aquarium after re-application of disinfectant, KHV could not be detected in those naïve fish. Whilst KHV could not be detected in samples where disinfectant had been applied, it was present in samples of naïve fish cohabiting with infection contact control animals which had undergone no disinfectant treatment over experiments 1 and 2. The results presented here show that water treatment with a disinfectant may prevent transmission of infectious KHV to naïve carp cohabited with infected carp.

Genetic Variability of Koi Herpesvirus In vitro-A Natural Event?

Worldwide koi herpesvirus (KHV) causes high mortalities in Cyprinus carpio L. aquaculture. So far, it is unknown how the different variants of KHV have developed and how they spread in the fish, but also in the environmental water bodies. Therefore, a phylogenetic method based on variable number of tandem repeats (VNTR) was improved to gain deeper insights into the phylogeny of KHV and its possible worldwide distribution. Moreover, a VNTR-3 qPCR was designed which allows fast virus typing. This study presents a useful method for molecular tracing of diverse KHV types, variants, and lineages.

Detection of koi herpesvirus (KHV) using a monoclonal antibody against Cyprinus carpio IgM

Koi herpesvirus disease (KHVD) is associated with high mortality in both common carp and koi carp (Cyprinus carpio L.) worldwide. The indirect detection of fish viruses based on the identification of antibodies has emerged as a practical and reliable means of diagnosis. Thus, it is important to create monoclonal antibodies (MAbs) against carp IgM. By using hybridoma-monoclonal antibody technology, one hybridoma cell line secreting MAbs against IgM from carp was established. In western blot analysis, the secreted MAb from cell line A5-E10 recognized the heavy chain of IgM from common carp or koi but did not react with immunoglobulins from three different fish species: grass carp (Ctenopharyngodon idella), tilapia (Oreochromis mossambicus) and Mandarin fish (Siniperca chuatsi). These results demonstrated that this MAb is highly specific for the IgM of carp and suggested that it can be used for monitoring the immunity level of carp, for example for indirect KHV diagnosis by antibody ELISA. We therefore established an indirect ELISA, which was tested using 200 serum samples from koi from three farms. The final results showed that 147 (73.5%) samples were confirmed to be KHV antibody negative and 53 (26.5%) were definitely positive, containing antibodies against KHV.

Experimental infections of different carp strains with the carp edema virus (CEV) give insights into the infection biology of the virus and indicate possible solutions to problems caused by koi sleepy disease (KSD) in carp aquaculture

Outbreaks of koi sleepy disease (KSD) caused by carp edema virus (CEV) may seriously affect populations of farmed common carp, one of the most important fish species for global food production. The present study shows further evidence for the involvement of CEV in outbreaks of KSD among carp and koi populations: in a series of infection experiments, CEV from two different genogroups could be transmitted to several strains of naïve common carp via cohabitation with fish infected with CEV. In recipient fish, clinical signs of KSD were induced. The virus load and viral gene expression results confirm gills as the target organ for CEV replication. Gill explants also allowed for a limited virus replication in vitro. The in vivo infection experiments revealed differences in the virulence of the two CEV genogroups which were associated with infections in koi or in common carp, with higher virulence towards the same fish variety as the donor fish. When the susceptibility of different carp strains to a CEV infection and the development of KSD were experimentally investigated, Amur wild carp showed to be relatively more resistant to the infection and did not develop clinical signs for KSD. However, the resistance could not be related to a higher magnitude of type I IFN responses of affected tissues. Despite not having a mechanistic explanation for the resistance of Amur wild carp to KSD, we recommend using this carp strain in breeding programs to limit potential losses caused by CEV in aquaculture.

First report of a cystic malformation on the upper jaw of hatchery-reared allis shad Alosa alosa

The anadromous allis shad Alosa alosa has suffered dramatic population declines throughout Europe and is currently considered as endangered throughout its entire distribution range. In order to reestablish allis shad in the River Rhine, which formerly housed one of the largest and most important populations, an EU-LIFE Project 'The re-introduction of allis shad in the Rhine system' was started in 2007. In course of the LIFE+ Projects, allis shad larvae bred from genitor fish of the Gironde-Garonne-Dordogne population in France were reared in a pilot ex situ stock plant pilot facility in Aßlar, Germany. At an age of 1-2 months, about 100% of these fish developed approximately 0.5- to 0.8-cm large, fluid-filled, transparent cysts in conjunction with the upper jaw. The performed microbiological, virological, parasitological and histological examinations did not detect any infectious agents. Possible causative agents are discussed with regard to environmental factors and the nutrition of larvae. In conclusion, the observed malformations are considered a sign for a severe health problem and therefore a risk for the successful breeding of allis shad in aquaculture.

The amino-terminal domain of ORF149 of koi herpesvirus is preferentially targeted by IgM from carp populations surviving infection

Recombinantly expressed fragments of the protein encoded by ORF149 (pORF149), a structural protein from the common- and koi-carp-infecting cyprinid herpesvirus-3 (CyHV-3) that was previously shown to be antigenic, were used to obtain evidence that its amino-terminal part contains immunodominant epitopes in fish populations that survived the infection. To obtain such evidence, nonspecific binding of carp serum tetrameric IgM had to be overcome by a novel ELISA protocol (rec2-ELISA). Rec2-ELISA involved pre-adsorption of carp sera with a heterologous recombinant fragment before incubation with pORF149 fragments and detection with anti-carp IgM monoclonal antibodies. Only in this way was it possible to distinguish between sera from uninfected and survivor carp populations. Although IgM from survivors recognised pORF149 fragments to a lesser degree than whole virus, specificity was confirmed by correlation of rec2- and CyHV-3-ELISAs, inhibition of rec2-ELISA by an excess of frgIIORF149, ELISA using IgM-capture, Western blotting, and reduction of reactivity in CyHV-3-ELISA by pre-adsorption of sera with frgIIORF149. The similarity of IgM-binding profiles between frgIORF149 (amino acid residues 42-629) and frgIIORF149 (42-159) and their reactivities with previously described anti-CyHV-3 monoclonal antibodies confirmed that most pORF149 epitopes were localised in its amino-terminal part.

Expression of immunogenic structural proteins of cyprinid herpesvirus 3 in vitro assessed using immunofluorescence

Cyprinid herpesvirus 3 (CyHV-3), also called koi herpesvirus (KHV), is the aetiological agent of a fatal disease in carp and koi (Cyprinus carpio L.), referred to as koi herpesvirus disease. The virus contains at least 40 structural proteins, of which few have been characterised with respect to their immunogenicity. Indirect immunofluorescence assays (IFAs) using two epitope-specific monoclonal antibodies (MAbs) were used to examine the expression kinetics of two potentially immunogenic and diagnostically relevant viral antigens, an envelope glycoprotein and a capsid-associated protein. The rate of expression of these antigens was determined following a time-course of infection in two CyHV-3 susceptible cell lines. The results were quantified using an IFA, performed in microtitre plates, and image analysis was used to analyse confocal micrographs, enabling measurement of differential virus-associated fluorescence and nucleus-associated fluorescence from stacks of captured scans. An 8-tenfold increase in capsid-associated protein expression was observed during the first 5 days post-infection compared to a ≤ 2-fold increase in glycoprotein expression. A dominant protein of ~100 kDa reacted with the capsid-associated MAb (20F10) in western blot analysis. This band was also recognised by sera obtained from carp infected with CyHV-3, indicating that this capsid-associated protein is produced in abundance during infection in vitro and is immunogenic to carp. Mass spectrometry carried out on this protein identified it as a previously uncharacterised product of open reading frame 84. This abundantly expressed and immunogenic capsid-associated antigen may be a useful candidate for KHV serological diagnostics.

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.