Characterization of a new cell line from caudal fin of koi, Cyprinus carpio koi, and first isolation of cyprinid herpesvirus 3 in China

Development of a Loop-Mediated Isothermal Amplification Assay (LAMP) for rapid and visual detection of Anguillid herpesvirus 1

China has the largest aquaculture eel production in the world. High-density cultivation pattern often results in an outbreak of epidemic diseases. Since the 1990s, eel "mucus sloughing and hemorrhagic septicemia disease" was often broke out in China, and brought huge economic losses to eel breeders. Anguillid herpesvirus 1 (AngHV) was detected and isolated from the diseased eel, and proved to be the pathogen of the disease. In this study, a loop-mediated isothermal amplification (LAMP) assay was developed for rapid, sensitive, and specific detection of AngHV. A set of six primers targeting the ORF51 gene of AngHV was designed, which could effectively detect purified AngHV virions, AngHV-infected cells, or eel tissue samples. The suitable reaction temperature is 63℃, and the reaction time is 40min. There was no cross-reaction with eel and other fish viruses, including Infectious pancreatic necrosis virus (IPNV), Marine birnavirus (MABV), Rana grylio virus (RGV), Cyprinid herpesvirus 3 (CyHV-3), and Eel iridovirus (EIV). The lower detection limit of the AngHV LAMP assay is 10 copies of AngHV genome DNA, which is at least 100 times more sensitive than conventional PCR in detecting AngHV. The assay could effectively detect AngHV from collected samples with typical clinical symptoms of AngHV infection. It suggested that the LAMP assay could be used in specific detection of AngHV and has great potential for early diagnosis of AngHV infection in the farm.

Impact of Global Warming on the Severity of Viral Diseases: A Potentially Alarming Threat to Sustainable Aquaculture Worldwide

With an ever-increasing human population, food security remains a central issue for the coming years. The magnitude of the environmental impacts of food production has motivated the assessment of the environmental and health benefits of shifting diets, from meat to fish and seafood. One of the main concerns for the sustainable development of aquaculture is the emergence and spread of infectious animal diseases in a warming climate. We conducted a meta-analysis to investigate the influence of global warming on mortality due to viral infections in farmed aquatic animals. We found a positive trend between increasing temperature and increasing viral virulence, with an increase in water temperature of 1 °C resulting in an increase in mortality of 1.47-8.33% in OsHV-1 infected oysters, 2.55-6.98% in carps infected with CyHV-3 and 2.18-5.37% in fishes infected with NVVs. We suggest that global warming is going to pose a risk of viral disease outbreaks in aquaculture and could compromise global food security.

First report on the occurrence of cyprinid herpesvirus 3 in koi carp (Cyprinus carpio koi) in India

This study reports the occurrence of cyprinid herpesvirus 3 (CyHV-3) in koi carp (Cyprinus carpio koi) for the first time in India. The koi carp, with clinical signs of ulcer with haemorrhage on body surface, necrosis of fin and discolouration of gill associated with huge mortality, were observed in aquarium shops, rearing tanks and grow-out ponds located in Chennai, India. The PCR assay carried out on infected fish samples using different primer sets specific to CyHV-3 confirmed its presence in the infected fish. Sequence analysis of partial thymidine kinase gene revealed 100% similarity with the sequence of CyHV-3 available in GenBank. Cell lines of koi carp and catla were found to be susceptible to CyHV-3 and its replication was confirmed by viral-specific cytopathic effect, PCR and bioassay. The CyHV-3 infection was reproduced by intramuscular injection of inoculum prepared from CyHV-3-infected fish to satisfy Koch's postulates. Tissue tropism of CyHV-3 in infected fish by PCR assay revealed the presence of CyHV-3 in all vital organs with prominent band in gill and gut tissue.

Characterization of Ictalurid herpesvirus 1 Glycoprotein ORF59 and Its Potential Role on Virus Entry into the Host Cells

The channel catfish virus (CCV, Ictalurid herpesvirus 1) has caused sustained economic losses in the fish industry because of its strong infectivity and pathogenicity. Thus, it is necessary to determine the function of viral proteins in the CCV infection process. The present study aimed to characterize CCV glycoprotein ORF59 and explore its impact on virus infection in host cells. Firstly, its exclusive presence in the membrane fraction of the cell lysate and subcellular localization verified that CCV ORF59 is a viral membrane protein expressed at late-stage infection. A protein blocking assay using purified His6 tagged ORF59, expressed in sf9 insect cells using a baculovirus expression system, indicated a dose-dependent inhibitory effect of recombinant ORF59 protein on virus invasion. Knockdown of the ORF59 using a short hairpin (shRNA) showed that ORF59 silencing decreased the production of infectious virus particles in channel catfish ovary cells. The results of this study suggest that recombinant ORF59 protein might inhibit CCV entry into the host cells. These findings will promote future studies of the key functions of glycoprotein ORF59 during CCV infection.

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.

Characterization of a novel brain cell line from Jian carp (Cyprinus carpio var. Jian)

Jian carp (Cyprinus carpio var. Jian) is an economically important cultured fish in China. Currently, it is facing threats from infectious diseases including koi herpesvirus (KHV). Here, we established a new cell line, designated CCB-J, derived from the brain tissue of the Jian carp. CCB-J cells grew well in Leibovitz's L-15 medium containing 20% fetal bovine serum at 25 °C and have been subcultured for more than 60 passages. At the 30th passage, analysis showed that the number of chromosomes was 100, which is identical to that of other carp variants. Sequencing of the 18S ribosomal DNA confirmed that CCB-J originated from Jian carp. After transfection with the pEGFP-N1 plasmid, green fluorescence was observed in CCB-J. The replication of KHV in CCB-J cells was confirmed by RT-PCR and transmission electron microscopy. The viral titers of KHV in CCB-J cells and CCB cells, which have been widely used in the study of KHV, reached 10^3.9 and 10^1.8 median tissue culture infectious dose (TCID₅₀/mL), respectively, within 14 days. The result of TaqMan PCR revealed that CCB-J cells were more sensitive to KHV than CCB cells. Meanwhile, a cytopathic effect (CPE) was also observed in the CCB-J cells in a shorter time post-infection compared with CCB cells. In summary, the CCB-J cell line will be a useful tool in the study of viral pathogenesis and vaccine research.

Susceptibility of Koi, Koi×Red Common Carp, and Red Common Carp×Koi to Koi Herpesvirus (KHV)

The disease-causing koi herpes virus (KHV), also known as cyprinid herpesvirus-3 (CyHV-3), causes mass mortality of koi and carp. Koi (Cyprinus carpio) is a host for KHV, one of 12 virus species in the Alloherpesviridae family. We examined the effects of KHV disease koi (KK), and on koi×red common carp (KR) and red common carp×koi (RK) cross, using a virus challenge test. The infected fish had clinical signs that included gill necrosis and skin lesions. The RK and KR were highly more resistant (cumulative mortality: RK; 6% and KR; 8%) to KHV infection than KK fish (cumulative mortality: 28%). KHV DNA was confirmed in the tissues of all dead fish in groups by use of polymerase chain reaction (PCR), and the presence of the KHV protein in kidney was confirmed by immunohistochemistry. Histological analysis showed severe gill lesions and fusion of the lamellae in KK fish, but less severe damage in RK fish. In immunohistochemistry analysis, the KHV protein localized in the cytoplasm of infected kidney cells of KK, but the cross groups had lower levels of KHV antigen. Our data indicate that the cross groups had increased resistance to KHV disease.

Mass mortality associated with koi herpesvirus in common carp in Iraq

Koi herpesvirus disease is a serious disease affecting both wild and common carp species in different continents throughout the world. Based on pathological and molecular findings, we document the presence of koi herpesvirus disease in Iraq as a cause of mass mortality among the common carp of the Tigris river. On a macroscopic level, the fish exhibited variably sized skin ulcerations throughout the entire trunk. The gills showed variable degrees of discoloration with an increased amount of slimy mucus. Microscopically, degeneration and necrosis with infiltration of a heterogenous population of inflammatory cells characterized different organs, primarily the skin and gills, with occasional intranuclear inclusion bodies that are consistent with koi herpesvirus disease. A semi-nested PCR assay coupled with sequencing confirmed the pathological diagnosis. Genotyping and sequence analysis of the TK gene, ORF 136 and markers I and II identified the isolated CyHV-3 as variant A1 of the Asian genotype TUSMT1 (J strain) displaying the I⁺⁺II⁺ allele.

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.

Genetic Diversity of Cyprinid Herpesvirus 3 from Different Geographical Locations during 1999-2019 in the United States of America

Cyprinid herpesvirus 3, also known as koi herpesvirus (KHV), is an important pathogen in common and koi carp Cyprinus carpio, varieties. Two main genotypes of KHV have been reported worldwide that are associated with Asian and European origins. In the USA, outbreaks of KHV diseases have been reported in different states since the early 1990s; however, the diversity of KHV is unknown. In the current study, 67 DNA samples that were extracted from clinical cases of koi tissues that were submitted for diagnosis during KHV outbreaks from 10 different states in the USA from 1999 to 2019 were used to investigate their genetic diversity. The thymidine kinase gene was amplified, sequenced, and used for phylogenetic analysis. Our results showed that the KHV isolates that were collected from the different states were clustered in the two known KHV genogroups, where 31 isolates belonged to the Asian genotype branch and 36 to the European genotype branch. The spatiotemporal analysis demonstrated fluctuation of KHV genotypes in the USA, as the main KHV genotype that was detected in koi in the USA from 1999 to 2013 was the European genotype, whereas the Asian KHV genotype appeared to emerge in the USA in 2008, increasing in incidence until 2019. The current study provides evidence on the genetic diversity of KHV in the USA. Future studies that evaluate the virulence of these genetically diverse isolates is warranted to obtain a better understanding of the epidemiology of this re-emerging pathogen. This may provide an improved awareness of the current status of KHV and help to control the disease in the koi population in the USA.

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.

A novel cell line established from caudal fin tissue of Carassius auratus gibelio is susceptible to cyprinid herpesvirus 2 infection with the induction of apoptosis

Herpesviral haematopoietic necrosis induced by cyprinid herpesvirus 2 (CyHV-2) in Carassius auratus gibelio has caused huge economic losses in aquaculture. To better characterize this pathogen at molecular and cellular levels, it is necessary to study its pathogenesis in a sustainable cell line. In this study, a new cell line (GiCF) from the caudal fin of Carassius auratus gibelio (C. a. gibelio) was established and characterised. Typical cytopathic effect represented by characteristic morphological changes was induced in the cells at the 7th day post infection with CyHV-2, which was isolated from diseased C. a. gibeli. The susceptibility of GiCF to CyHV-2 was further demonstrated by the presence of viral assembling factory in the cytoplasm unveiled by transmission electron microscopy, and viral protein synthesis through immunofluorescence and Western blot assays. Sustainable propagation of CyHV-2 in GiCF had been demonstrated by virus infection and titration assays in a consecutive manner. The virus titer reached 10^4.9±0.22 TCID₅₀/ml, and infectious CyHV-2 was produced from the GiCF cells over 30 subcultures from the established cell lines. In addition, apoptosis was observed in the GiCF cells upon CyHV-2 infection by DAPI staining, TUNEL and Annexin V staining, and quantitative expression patterns of caspase genes suggested that caspase activation might involve in CyHV-2 induced apoptosis. In summary, a new cell line, GiCF, susceptible to CyHV-2 infection was established from caudal fin tissue of C. a. gibelio, and it should serve as an ideal infection platform to characterize CyHV-2 and its pathogenic effect including apoptosis.

Monitoring changing cellular characteristics during the development of a fin cell line from Cyprinus carpio

The establishment and in-depth characterization of a novel continuous cell line derived from fin tissue of common carp (Cyprinus carpio), CCApin, is reported. The cells of the cell line could be propagated in Leibovitz's L-15 medium containing 15% foetal calf serum and 0.5% carp serum for >150 passages during the last 24 months, with a stable fast growth. Furthermore, antibody staining indicated that cell types obtained in primary cultures, containing the epithelial stem-cell marker tumorprotein 63, were different from cells in long-term cell cultures, containing tight junction protein zona occludens 1 and cytokeratin 7. These observations suggest a switch of dominant cell types. Molecular analysis of gene expression profiles of caudal fin tissue and CCApin cells showed that genes relevant in epithelial cells but also in mesenchymal cells were expressed. However, during cultivation of CCApin a set of very steadily expressed, primarily mesenchymal genes like collagen 1 alpha 1, fibronectin or cadherin 2 was found. In summary, the long-term cell culture could be described as a stably growing epithelial population with some mesenchymal features. There are several application possibilities, especially for virus susceptibility studies, e.g. cyprinid herpesvirus-3 (CyHV-3). The study leads to a better understanding of molecular and physiological mechanisms of in vitro fish cell cultures.

Rapid detection and differentiation of carp oedema virus and cyprinid herpes virus-3 in koi and common carp

Carp oedema virus (CEV) and koi herpes virus (KHV) are of major concern to common carp breeders and koi enthusiasts worldwide. The viruses cause diseases that exhibit similar external signs; thus, it is difficult to distinguish between them clinically. In this study, we developed and optimized rapid and accurate single- and multiplex isothermal diagnostic tools, based on recombinase polymerase amplification (RPA), for detection and differentiation of CEV and KHV. The assays were combined with a lateral flow dipstick to enable visual detection of amplification products and simplify post-amplification analysis. Both CEV- and KHV-RPA assays were specific for their target virus. The lower detection limits of the assays were similar to those of established diagnostic PCR tests for the viruses. A sample preparation method was optimized to eliminate the need for total DNA extraction from fish tissues. The estimated time to perform these RPA assays, from receiving the sample to having a result, is 50 min, compared to 10 and 7 hr for CEV- and KHV-PCR tests, respectively. The assays can be performed in field situations to improve screening of fish and reduce spread of these viruses and thereby enhance the common carp and koi industries.

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.

Complete genome sequence and phylogenetic analysis of megalocytivirus RSIV-Ku: A natural recombination infectious spleen and kidney necrosis virus

Megalocytiviruses are classified into three genotypes, infectious spleen and kidney necrosis virus (ISKNV), red seabream virus (RSIV), and turbo reddish body iridovirus (TRBIV), based on the major capsid protein and ATPase genes. However, only a few complete genome sequences have been obtained. This paper reports the complete genome sequence and phylogenetic analysis of an RSIV-Ku strain megalocytivirus. The genome sequence comprises 111,154 bp, has 132 putative open reading frames, and is homologous mostly to ISKNV, except for the sequence in the region 58981-66830, which is more closely related to that of the RSIV genotype. The results imply that RSIV-Ku is actually a natural recombinant virus.

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.

First detection of koi herpesvirus from koi, Cyprinus carpio L. experiencing mass mortalities in Iran: clinical, histopathological and molecular study

Koi herpesvirus (KHV) is the aetiological agent of an emerging disease (KHVD) associated with mass mortalities in koi and common carp and reported from at least 30 countries. We report the first detection of KHV from koi in Iran using clinical, histopathological and molecular studies. KHV-infected fish showed reduced swimming activity, sunken eyes and increased mucus production on skin and fins. On post-mortem examination, gill necrosis was observed in the majority of fish. Histopathologically, the gill showed diffuse necrosis of the branchial epithelial cells. Margination of chromatin was detected in gills, kidney, heart, spleen, intestine and brain. In addition, sequence analyses of the TK gene, ORF 136 and marker I and II, demonstrates that Iranian KHV isolates were identical and classified as variant A1 of TUSMT1 (J strain) and displayed the I(++) II(+) allele of this Asian genotype.

Identification and expression analysis of cellular and viral microRNAs in CyHV3-infected KCF-1 cells

MicroRNAs (miRNAs) are small non-coding RNAs with approximately 22 nucleotides (nt) that are encoded by a diverse range of metazoan eukaryotes, plants and viruses. CyHV-3 (cyprinid herpesvirus-3) is a member of the Alloherpesviridae virus family and has caused severe economic losses for the common carp and koi carp fishery industries. In this study, a total of 15,987,652 clean reads were generated from a cDNA library of CyHV-3-infected KCF-1 (koi caudal fin) cells using high-throughput sequencing technology. Following annotation and secondary structure prediction, 28 miRNAs were identified as novel candidate miRNAs encoded by common carp (Cyprinus carpio), and seven miRNAs were shown to be encoded by CyHV-3. Next, 19 host miRNAs and seven viral miRNAs were validated by stem-loop real-time PCR. Northern blot analysis confirmed the presence of 14 host miRNAs and five CyHV-3-encoded novel miRNAs. The results of this study expand the knowledge of common carp and CyHV-3 microRNAs and provide a useful theoretical foundation for further study of CyHV-3.

Transcriptomic analysis of koi (Cyprinus carpio) spleen tissue upon cyprinid herpesvirus 3 (CyHV3) infection using next generation sequencing

Cyprinid Herpesvirus 3 (CyHV-3) can infect and specifically cause a huge economic loss in both common carp (Cyprinus carpio) and its ornamental koi variety. The molecular mechanisms underlying CyHV-3 infection are not well understood. In this study, koi spleen tissues of both mock and CyHV-3 infection groups were collected, and high-throughput sequencing technology was used to analyze the differentially expressed genes (DEGs) at the transcriptome level. A total of 105,356,188 clean reads from two libraries were obtained. After the de novo assembly of the transcripts, 129,314 unigenes were generated. Of these unigenes, 70,655 unigenes were matched to the known proteins in the database, while 2190 unigenes were predicted by ESTScan software. Comparing the infection group to the mock group, a total of 23,029 significantly differentially expressed unigenes were identified, including 10,493 up-regulated DEGs and 12,536 down-regulated DEGs. GO (Gene Ontology) annotation and functional enrichment analysis indicated that all of the DEGs were annotated into GO terms in three main GO categories: biological process, cellular component and molecular function. KEGG (Kyoto Encyclopedia of Genes and Genomes) analysis of the DEGs showed that a total of 12,002 DEG unigenes were annotated into 256 pathways classified into 6 main categories. Additionally, 20 differentially expressed genes were validated by quantitative real-time PCR. As the first report of a transcriptome analysis of koi carp with CyHV-3 infection, the data presented here provide knowledge of the innate immune response against CyHV-3 in koi carp and useful data for further research of the molecular mechanism of CyHV-3 infection.

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.

Inhibiting cyprinid herpesvirus-3 replication with CRISPR/Cas9

OBJECTIVES

The potential of CRISPR/Cas9 gene editing to repress CyHV-3 was tested in vitro.

RESULTS

By targeting two basic target genes necessary for the early transcription of CyHV-3, we show that virus transcription and particle release were significantly decreased by CRISPR/Cas9, as measured by quantitative real-time PCR and virus titration experiments, respectively.

CONCLUSIONS

(A) The effectiveness is confirmed of the CRISPR/Cas9 system at repressing exogenous genes, including large viral genomic DNA, by introducing site-specific mutations in vitro. (B) The CyHV-3 virus replicates poorly in Cas9-positive cells. (C) The inhibition of thymidine kinase alone cannot block viral particle release.

Identification of a novel cyprinid herpesvirus 3 genotype detected in koi from the East Asian and South-East Asian Regions

Cyprinid herpesvirus 3 (CyHV-3) is a highly contagious virus that causes significant morbidity and mortality in common carp Cyprinus carpio L. and considered to be one of the most important pathogens of koi and common carp worldwide. Cyprinid herpesvirus 3 infected consignments imported from East Asian and South-East Asian regions were identified during quarantine period in Singapore, and virus from a 2005 consignment was successfully isolated in koi fin cells. A combination of sequence analyses and duplex PCR were used to characterize 15 CyHV-3 isolates detected in koi consignments between 2005 and 2011. Sequence analyses of the enlarged 9/5, SphI-5 and TK gene regions identified both the Asian 1 (n = 11) and European 4 (n = 4) genotypes. Duplex PCR analysis of two variable marker regions between ORF29 and ORF30 (marker I) as well as ORF133 and its upstream region (marker II) revealed viruses of genotypes J (I⁺⁺ II⁺ ), U/I (I^-- II^- ), an intermediate genotype (I⁺⁺ II^- ) and a novel genotype, I⁺⁺ II^+Δ , which was identified in viruses from seven different consignments. This novel genotype has a 13-bp deletion in marker II, while maintaining the I⁺⁺ allele of marker I. The I⁺⁺ II^+Δ genotype may have emerged from East Asian and South-East Asian regions in recent years.

Functional characterization of viral tumor necrosis factor receptors encoded by cyprinid herpesvirus 3 (CyHV3) genome

Cyprinid herpesvirus 3 (CyHV3) is a large double-stranded DNA virus of Alloherpesviridae family in the order Herpesvirales. It causes significant morbidity and mortality in common carp and its ornamental koi variety, and threatens the aquaculture industries worldwide. Mimicry of cytokines and cytokine receptors is a particular strategy for large DNA viruses in modulating the host immune response. Here, we report the identification and characterization of two novel viral homologues of tumor necrosis factor receptor (TNFR) encoded by CyHV3-ORF4 and -ORF12, respectively. CyHV3-ORF4 was identified as a homologue of HVEM and CyHV3-ORF12 as a homologue of TNFRSF1. Overexpression of ORF4 and ORF12 in zebrafish embryos results in embryonic lethality, morphological defects and increased apoptosis. Although we failed to identify any interaction between the two vTNFRs and their potential ligands in zebrafish TNF superfamily by yeast two-hybrid system, the expression of some genes in TNF superfamily or TNFR superfamily were mis-regulated in ORF4 or ORF12-overexpressing embryos, especially the death receptor zHDR and its cognate ligand DL1b. Further studies showed that the apoptosis induced by the both CyHV3 vTNFRs is mainly activated through the intrinsic apoptotic pathway and requires the crosstalk between the intrinsic and extrinsic apoptotic pathway. Additionally, using RT-qPCR and Western blot assays, the expression patterns of the both vTNFRs were also analyzed during CyHV3 productive infection. Collectively, this is the first functional study of two unique vTNFRs encoded by a herpesvirus infecting non-mammalian vertebrates, which may provide novel insights into viral immune regulation mechanism and the pathogenesis of CyHV3 infection.

Analysing codon usage bias of cyprinid herpesvirus 3 and adaptation of this virus to the hosts

The codon usage patterns of open reading frames (ORFs) in cyprinid herpesvirus 3 (CyHV-3) have been investigated in this study. The high correlation between GC12 % and GC3 % suggests that mutational pressure rather than natural selection is the main factor that determines the codon usage and base component in the CyHV-3, while mutational pressure effect results from the high correlation between GC3 % and the first principal axis of principle component analysis (Axis 1) on the relative synonymous codon usage (RSCU) value of the viral functional genes. However, the interaction between the absolute codon usage bias and GC3 % suggests that other selections take part in the formation of codon usage, except for the mutational pressure. It is noted that the similarity degree of codon usage between the CyHV-3 and goldfish, Carassius auratus (L.), is higher than that between the virus and common carp, Cyprinus carpio L., suggesting that the goldfish plays a more important role than the common carp in codon usage pattern of the CyHV-3. The study of codon usage in CyHV-3 can provide some evidence about the molecular evolution of the virus. It can also enrich our understanding about the relationship between the CyHV-3 and its hosts by analysing their codon usage patterns.

Establishment of a new cell line susceptible to Cyprinid herpesvirus 3 (CyHV-3) and possible latency of CyHV-3 by temperature shift in the cells

A new cell line named CCF-K104 predominantly consisting of fibroblastic cells showed optimal growth at temperatures from 25 °C to 30 °C. Serial morphological changes in the cells induced by Cyprinid herpesvirus 3 (CyHV-3) included cytoplasmic vacuolar formation, cell rounding and detachment. Mature virions were purified from CyHV-3-infected CCF-K104 cells by sucrose gradient ultracentrifugation and had a typical herpesvirus structure on electron microscopy. Infectious CyHV-3 was produced stably in CCF-K104 cells over 30 viral passages. Our findings showed that CCF-K104 is a useful cell line for isolation and productive replication of CyHV-3. A temperature shift from 25 °C to 15 °C or 35 °C did not allow serial morphological changes as observed at 25 °C for 14 days. Under the same conditions, real-time PCR showed that CyHV-3 was present with low viral DNA loads, suggesting that CyHV-3 may establish latent infection in CCF-K104 cells. Amplification of the left and right terminal repeat sequences of the CyHV-3 genome arranged in a head-to-tail manner was detected by nested PCR following an upshift in temperature from 25 °C to 35 °C. The PCR results suggested that the circular genome may represent a latent form of CyHV-3.

Cyprinid Herpesvirus 3: An Archetype of Fish Alloherpesviruses

The order Herpesvirales encompasses viruses that share structural, genetic, and biological properties. However, members of this order infect hosts ranging from molluscs to humans. It is currently divided into three phylogenetically related families. The Alloherpesviridae family contains viruses infecting fish and amphibians. There are 12 alloherpesviruses described to date, 10 of which infect fish. Over the last decade, cyprinid herpesvirus 3 (CyHV-3) infecting common and koi carp has emerged as the archetype of fish alloherpesviruses. Since its first description in the late 1990s, this virus has induced important economic losses in common and koi carp worldwide. It has also had negative environmental implications by affecting wild carp populations. These negative impacts and the importance of the host species have stimulated studies aimed at developing diagnostic and prophylactic tools. Unexpectedly, the data generated by these applied studies have stimulated interest in CyHV-3 as a model for fundamental research. This review intends to provide a complete overview of the knowledge currently available on CyHV-3.

Whole-genome sequence of a novel Chinese cyprinid herpesvirus 3 isolate reveals the existence of a distinct European genotype in East Asia

Cyprinid herpesvirus 3 (CyHV3), also known as koi herpesvirus (KHV), can be subdivided primarily into European and Asian genotypes, which are represented by CyHV3-U or CyHV3-I and CyHV3-J, respectively. In this study, the whole genome sequence of a novel Chinese CyHV3 isolate (GZ11) was determined and annotated. CyHV3-GZ11 genome was found to contain 295,119 nucleotides with 52.9% G/C content, which is highly similar to those of published CyHV3-U, CyHV3-I, and CyHV3-J strains. With reference to CyHV3-U, CyHV3-I, and CyHV3-J, CyHV3-GZ11 was also classified into 164 open reading frames (ORF), which include eight repeated ORFs. On the basis of the 12 alloherpeviruses core genes, results from phylogenetic analysis showed that CyHV3-GZ11 had closer evolutionary relationships with CyHV3-U and CyHV3-I than with CyHV3/KHV-J, which were also supported by genome wide-based single nucleotide substitution analysis and the use of a series of developed molecular markers. This study was the first to reveal the presence of a distinct European CyHV3 genotype in East and Southeast Asia at a whole genome level, which will evoke new insights on exploring the origin, evolution, and epidemiology of the virus.

Development of an Ussuri catfish Pseudobagrus ussuriensis skin cell line displaying differential cytopathic effects to three aquatic animal viruses

An Ussuri catfish Pseudobagrus ussuriensis skin (UCS) cell line was developed and subcultured for more than 60 passages. UCS cells consisted of mostly epithelial-like cells and multiplied well in TC199 medium supplemented with 10% fetal bovine serum at 25°C. Chromosome analysis revealed that most UCS cells had a normal diploid karyotype with 2n=52. UCS cells showed differential cytopathic effects (CPEs) after inoculation of spring viremia of carp virus (SVCV, a negative-strand RNA virus), grass carp reovirus (GCRV, a multi-segmented double-stranded RNA virus) and Rana grylio virus (RGV, a large double-stranded DNA virus), and were indicative of high sensitivities to these three aquatic animal viruses by a virus titration study. The CPE caused by SVCV appeared as rounded and granular cells, grape-like clusters and small lytic plaques. Characteristic CPE containing plaque-like syncytia was induced by GCRV. RGV-infected cells produced typical CPE characterized by cells shrinkage and aggregation, formation of clear plaques and cell sheet detachment. Furthermore, significant fluorescent signals were observed after UCS cells were transfected with green fluorescent protein reporter plasmids, and the development of CPE induced by a recombinant RGV, ΔTK-RGV, in UCS cells was illustrated using a combination of light and fluorescence microscopy. The data from this study suggested that UCS cell line can potentially serve as a useful tool for the comparison study of different aquatic animal viruses and the isolation of some newly emerging viruses in Ussuri catfish farming.

Cloning of a new fibroblast cell line from an early primary culture from mandarin fish (Siniperca chuatsi) fry for efficient proliferation of megalocytiviruses

Megalocytiviruses are important emerging pathogens in both freshwater and marine finfish aquaculture. However, a limited number of piscine cell lines are persistently susceptible to these viruses, which greatly limits the study of megalocytiviruses. In this study, a new fibroblast-like cell line was established from an early primary culture from mandarin fish fry by a single cell cloning and was designated as MFF-8C1. The MFF-8C1 cells grow well in Dulbecco's modified Eagle's medium supplemented with 10 % fetal bovine serum and had been subcultured more than 60 passages since the initial recovery culture in October 2009. Chromosomal analysis revealed that 91 % of the MFF-8C1 cells maintained a normal diploid chromosome number (2n = 48) in the 46th passage. Infection experiments showed that both freshwater-borne and marine-borne megalocytiviruses induce severe cytopathic effects in infected MFF-8C1 cells characterized by the rounding and enlargement of cells, which are highly consistent with the previous description of the infection in other susceptible cells with megalocytivirus. Megalocytivirus infections were further confirmed by a transmission electron microscopy. Furthermore, the MFF-8C1-cultured megalocytiviral suspension was highly virulent to infected mandarin fish. In summary, a new fibroblast cell line from mandarin fish fry that was highly permissive to megalocytiviruses was established. The MFF-8C1 cell line is a promising cellular substrate candidate for cell-cultured vaccine production of megalocytivirus.

Cyprinid herpesvirus 3: an interesting virus for applied and fundamental research

Cyprinid herpesvirus 3 (CyHV-3), a member of the family Alloherpesviridae is the causative agent of a lethal, highly contagious and notifiable disease in common and koi carp. The economic importance of common and koi carp industries together with the rapid spread of CyHV-3 worldwide, explain why this virus became soon after its isolation in the 1990s a subject of applied research. In addition to its economic importance, an increasing number of fundamental studies demonstrated that CyHV-3 is an original and interesting subject for fundamental research. In this review, we summarized recent advances in CyHV-3 research with a special interest for studies related to host-virus interactions.

Antibody screening identifies 78 putative host proteins involved in Cyprinid herpesvirus 3 infection or propagation in common carp, Cyprinus carpio L

Cyprinid herpesvirus 3 (CyHV-3) is the aetiological agent of a serious and notifiable disease afflicting common and koi carp, Cyprinus carpio L., termed koi herpesvirus disease (KHVD). Significant progress has been achieved in the last 15 years, since the initial reports surfaced from Germany, USA and Israel of the CyHV-3 virus, in terms of pathology and detection. However, relatively few studies have been carried out in understanding viral replication and propagation. Antibody-based affinity has been used for detection of CyHV-3 in enzyme-linked immunosorbent assay and PCR-based techniques, and immunohistological assays have been used to describe a CyHV-3 membrane protein, termed ORF81. In this study, monoclonal antibodies linked to N-hydroxysuccinimide (NHS)-activated spin columns were used to purify CyHV-3 and host proteins from tissue samples originating in either CyHV-3 symptomatic or asymptomatic fish. The samples were next analysed either by polyacrylamide gel electrophoresis (PAGE) and subsequently by electrospray ionization coupled to mass spectrometry (ESI-MS) or by ESI-MS analysis directly after purification. A total of 78 host proteins and five CyHV-3 proteins were identified in the two analyses. These data can be used to develop novel control methods for CyHV-3, based on pathways or proteins identified in this study.

CyHV-3: the third cyprinid herpesvirus

Common carp (including ornamental koi carp) Cyprinus carpio L. are ecologically and economically important freshwater fish in Europe and Asia. C. carpio have recently been endangered by a third cyprinid herpesvirus, known as cyprinid herpesvirus-3 (CyHV-3), the etiological agent of koi herpesvirus disease (KHVD), which causes significant morbidity and mortality in koi and common carp. Clinical and pathological signs include epidermal abrasions, excess mucus production, necrosis of gill and internal organs, and lethargy. KHVD has decimated major carp populations in Israel, Indonesia, Taiwan, Japan, Germany, Canada, and the USA, and has been listed as a notifiable disease in Germany since 2005, and by the World Organisation for Animal Health since 2007. KHVD is exacerbated in aquaculture because of the relatively high host stocking density, and CyHV-3 may be concentrated by filter-feeding aquatic organisms. CyHV-3 is taxonomically grouped within the family Alloherpesviridae, can be propagated in a number of cell lines, and is active at a temperature range of 15 to 28°C. Three isolates originating from Japan (KHV-J), USA (KHV-U), and Israel (KHV-I) have been sequenced. CyHV-3 has a 295 kb genome with 156 unique open reading frames and replicates in the cell nucleus, and mature viral particles are 170 to 200 nm in diameter. CyHV-3 can be detected by multiple PCR-based methods and by enzyme-linked immunosorbent assay. Several modes of immunization have been developed for KHVD; however, fish immunized with either vaccine or wild-type virus may become carriers for CyHV-3. There is no current treatment for KHVD.

Characterization of a novel cell line from the caudal fin of koi carp Cyprinus carpio

A continuous cell line (KF-101) derived from the caudal fin of the koi carp Cyprinus carpio was established and characterized. The KF-101 cell line multiplied abundantly in Leibovitz's L-15 medium containing 10% foetal bovine serum at 25° C, and was subcultured for >90 passages over a period of 3 years. Immunocytochemistry revealed that the KF-101 cells contain keratin, junction proteins connexin-43 and occludin, and ectodermal stem-cell marker Pax-6, but not vimentin. Furthermore, the KF-101 cells reacted with anti-human DARPP-32 and anti-human GATA-4 antibodies, and the labelling was regulated according to the cell cycle. The labels of the DARPP-32 and GATA-4 antibodies in the KF-101 cells were the suggested phosphatase-1 inhibitor-1 and GATA-3, respectively. In addition, the KF-101 cells were susceptible to koi herpesvirus but were resistant to eel herpesvirus, iridovirus, grouper nodavirus and chum salmon (Oncorhynchus keta) virus. The results indicate that the KF-101 cells are suitable materials for investigating biological and virological development.

Koi herpes virus: a review and risk assessment of Indian aquaculture

Common carp (Cyprinus carpio) is a widely cultivated freshwater fish for human consumption, while koi carp, is a farmed colored sub species of common carp used for ornamental purposes. Since 1998, both common carp and koi carp are severely affected by a viral disease called as Koi herpes virus disease (KHVD). This disease is caused by Koi herpes virus (KHV), also known as cyprinid herpes virus-3. The virus causes interstitial nephritis and gill necrosis in carps, so it is also termed as carp interstitial nephritis and gill necrosis virus. KHV is a double stranded icosahedral DNA virus belonging to family Alloherpesviridae, with a genome size of 295 kbp, larger than any member of Herpesviridae. The viral genome encodes 156 potential protein coding open reading frames. Each virion consists of forty structural proteins, which are classified as capsid (3), envelope (13), tegument (2) and unclassified (22) structural proteins. Diagnosis of KHVD is mainly based on detection of viral DNA by polymerase chain reaction amplification using specific primers or loop mediated isothermal amplification. Temperature dependent latent infection is unique to KHV; and carrier fish are often not detected, thereby possibly resulting in spread of this pathogen to newer areas. The disease is now known to occur in, or has been recorded from at least 26 different countries of the world. Fortunately, KHVD has not been reported from India or from Indian major carps. To monitor the disease status of the country, a total of 254 fish samples collected from different parts of India were screened by PCR for the presence of KHV. None of the tested samples were found to be positive for KHV. These results demonstrate that tested samples from different parts of India were apparently free from KHV. Preliminary risk assessment of KHV suggest that in the event of unrestricted importation of koi carps into our country, there is a higher probability of risk to aquaculture as compared to natural waters. So there is strong need to develop diagnostic capabilities and launch surveillance programmes for KHV in India.