Characterization of a novel virus causing a lethal disease in carp and koi

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.

Current knowledge and future prospects of vaccines against cyprinid herpesvirus 3 (CyHV-3)

Aquaculture is one of the world's most important and fastest growing food production sectors, with an average annual growth of 5.8% during the period 2001-2016. Common carp (Cyprinus carpio) is one of the main aquatic species produced for human consumption and is the world's third most produced finfish. Koi carp, on the other hand, are grown as a popular ornamental fish. In the late 1990s, both of these sectors were threatened by the emergence of a deadly disease caused by cyprinid herpesvirus 3 (CyHV-3; initially called koi herpesvirus or KHV). Since then, several research groups have focused their work on developing methods to fight this disease. Despite increasing knowledge about the pathobiology of this virus, there are currently no efficient and cost-effective therapeutic methods available to fight this disease. Facing the lack of efficient treatments, safe and efficacious prophylactic methods such as the use of vaccines represent the most promising approach to the control of this virus. The common carp production sector is not a heavily industrialized production sector and the fish produced have low individual value. Therefore, development of vaccine methods adapted to mass vaccination are more suitable. Multiple vaccine candidates against CyHV-3 have been developed and studied, including DNA, bacterial vector, inactivated, conventional attenuated and recombinant attenuated vaccines. However, there is currently only one vaccine commercially available in limited regions. The present review aims to summarize and evaluate the knowledge acquired from the study of these vaccines against CyHV-3 and provide discussion on future prospects.

Antiviral activities of Clinacanthus nutans (Burm.f.) Lindau extract against Cyprinid herpesvirus 3 in koi (Cyprinus carpio koi)

Cyprinid herpesvirus 3 (CyHV-3) or koi herpesvirus (KHV) is a virulent viral infection in common carp and koi. The disease has caused global epizootic and economic loss in fish aquaculture and in the wild. Clinacanthus nutans (Burm. f.) Lindau is a well-known medicinal plant used in Thai traditional medicine. Virucidal effects of the plant extract against human herpes simplex virus have been reported. In this study, C. nutans crude extract was tested for antiviral activities against CyHV-3 in koi carp. Results showed effective antiviral activity against CyHV-3 pre- and post-infection. The 50% lethal concentration (LC₅₀ ) of extract was higher than 5 mg/ml. The 50% effective dose (ED₅₀ ) was 0.99 mg/ml, 0.78 mg/ml, 0.75 mg/ml and 0.71 mg/ml at 1, 2, 3 and 4 hr pre-infection, respectively. The ED₅₀ from post-infection tests was 2.05 mg/ml and 2.34 mg/ml at 0 and 24 hr, respectively. These results demonstrated that crude extract expressed antiviral activity against CyHV-3 and can be applied as a therapeutic agent in common carp and koi aquaculture.

Proteomic and Functional Analyses of the Virion Transmembrane Proteome of Cyprinid Herpesvirus 3

Virion transmembrane proteins (VTPs) mediate key functions in the herpesvirus infectious cycle. Cyprinid herpesvirus 3 (CyHV-3) is the archetype of fish alloherpesviruses. The present study was devoted to CyHV-3 VTPs. Using mass spectrometry approaches, we identified 16 VTPs of the CyHV-3 FL strain. Mutagenesis experiments demonstrated that eight of these proteins are essential for viral growth in vitro (open reading frame 32 [ORF32], ORF59, ORF81, ORF83, ORF99, ORF106, ORF115, and ORF131), and eight are nonessential (ORF25, ORF64, ORF65, ORF108, ORF132, ORF136, ORF148, and ORF149). Among the nonessential proteins, deletion of ORF25, ORF132, ORF136, ORF148, or ORF149 affects viral replication in vitro, and deletion of ORF25, ORF64, ORF108, ORF132, or ORF149 impacts plaque size. Lack of ORF148 or ORF25 causes attenuation in vivo to a minor or major extent, respectively. The safety and efficacy of a virus lacking ORF25 were compared to those of a previously described vaccine candidate deleted for ORF56 and ORF57 (Δ56-57). Using quantitative PCR, we demonstrated that the ORF25 deleted virus infects fish through skin infection and then spreads to internal organs as reported previously for the wild-type parental virus and the Δ56-57 virus. However, compared to the parental wild-type virus, the replication of the ORF25-deleted virus was reduced in intensity and duration to levels similar to those observed for the Δ56-57 virus. Vaccination of fish with a virus lacking ORF25 was safe but had low efficacy at the doses tested. This characterization of the virion transmembrane proteome of CyHV-3 provides a firm basis for further research on alloherpesvirus VTPs.IMPORTANCE Virion transmembrane proteins play key roles in the biology of herpesviruses. Cyprinid herpesvirus 3 (CyHV-3) is the archetype of fish alloherpesviruses and the causative agent of major economic losses in common and koi carp worldwide. In this study of the virion transmembrane proteome of CyHV-3, the major findings were: (i) the FL strain encodes 16 virion transmembrane proteins; (ii) eight of these proteins are essential for viral growth in vitro; (iii) seven of the nonessential proteins affect viral growth in vitro, and two affect virulence in vivo; and (iv) a mutant lacking ORF25 is highly attenuated but induces moderate immune protection. This study represents a major breakthrough in understanding the biology of CyHV-3 and will contribute to the development of prophylactic methods. It also provides a firm basis for the further research on alloherpesvirus virion transmembrane proteins.

Cyprinid herpesvirus 3 as a potential biological control agent for carp (Cyprinus carpio) in Australia: susceptibility of non-target species

Carp (Cyprinus carpio L.) is a pest species in Australian waterways, and cyprinid herpesvirus 3 (CyHV-3) is being considered as a potential biological control (biocontrol) agent. An important consideration for any such agent is its target specificity. In this study, the susceptibility to CyHV-3 of a range of non-target species (NTS) was tested. The NTS were as follows: 13 native Australian, and one introduced, fish species; a lamprey species; a crustacean; two native amphibian species (tadpole and mature stages); two native reptilian species; chickens; and laboratory mice. Animals were exposed to 100-1000 times the approximate minimum amount of CyHV-3 required to cause disease in carp by intraperitoneal and/or bath challenge, and then examined clinically each day over the course of 28 days post-challenge. There were no clinical signs, mortalities or histological evidence consistent with a viral infection in a wide taxonomic range of NTS. Furthermore, there was no molecular evidence of infection with CyHV-3, and, in particular, all RT-PCRs for viral mRNA were negative. As a consequence, the results encourage further investigation of CyHV-3 as a potential biocontrol agent that is specific for carp.

Diagnostic validation of three test methods for detection of cyprinid herpesvirus 3 (CyHV-3)

Cyprinid herpesvirus 3 (CyHV-3) is the aetiological agent of koi herpesvirus disease in koi and common carp. The disease is notifiable to the World Organisation for Animal Health. Three tests-quantitative polymerase chain reaction (qPCR), conventional PCR (cPCR) and virus isolation by cell culture (VI)-were validated to assess their fitness as diagnostic tools for detection of CyHV-3. Test performance metrics of diagnostic accuracy were sensitivity (DSe) and specificity (DSp). Repeatability and reproducibility were measured to assess diagnostic precision. Estimates of test accuracy, in the absence of a gold standard reference test, were generated using latent class models. Test samples originated from wild common carp naturally exposed to CyHV-3 or domesticated koi either virus free or experimentally infected with the virus. Three laboratories in Canada participated in the precision study. Moderate to high repeatability (81 to 99%) and reproducibility (72 to 97%) were observed for the qPCR and cPCR tests. The lack of agreement observed between some of the PCR test pair results was attributed to cross-contamination of samples with CyHV-3 nucleic acid. Accuracy estimates for the PCR tests were 99% for DSe and 93% for DSp. Poor precision was observed for the VI test (4 to 95%). Accuracy estimates for VI/qPCR were 90% for DSe and 88% for DSp. Collectively, the results show that the CyHV-3 qPCR test is a suitable tool for surveillance, presumptive diagnosis and certification of individuals or populations as CyHV-3 free.

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.

Examination of the early infection stages of koi herpesvirus (KHV) in experimentally infected carp, Cyprinus carpio L. using in situ hybridization

Koi herpesvirus (KHV) causes a highly infectious disease afflicting common carp and koi, Cyprinus carpio L. Various molecular and antibody-based detection methods have been used to elucidate the rapid attachment and dissemination of the virus throughout carp tissues, facilitating ongoing development of effective diagnostic approaches. In situ hybridization (ISH) was used here to determine the target tissues of KHV during very early infection, after infecting carp with a highly virulent KHV isolate. Analysis of paraffin-embedded tissues (i.e. gills, skin, spleen, kidney, gut, liver and brain) during the first 8 h and following 10 days post-infection (hpi; dpi) revealed positive signals in skin mucus, gills and gut sections after only 1 hpi. Respiratory epithelial cells were positive as early as 2 hpi. Viral DNA was also detected within blood vessels of various tissues early in the infection. Notable increases in signal abundance were observed in the gills and kidney between 5 and 10 dpi, and viral DNA was detected in all tissues except brain. This study suggests that the gills and gut play an important role in the early pathogenesis of this Alloherpesvirus, in addition to skin, and demonstrates ISH as a useful diagnostic tool for confirmation of acutely infected carp.

Identification and Characterization of Cyprinid Herpesvirus-3 (CyHV-3) Encoded MicroRNAs

MicroRNAs (miRNAs) are a class of small non-coding RNAs involved in post-transcriptional gene regulation. Some viruses encode their own miRNAs and these are increasingly being recognized as important modulators of viral and host gene expression. Cyprinid herpesvirus 3 (CyHV-3) is a highly pathogenic agent that causes acute mass mortalities in carp (Cyprinus carpio carpio) and koi (Cyprinus carpio koi) worldwide. Here, bioinformatic analyses of the CyHV-3 genome suggested the presence of non-conserved precursor miRNA (pre-miRNA) genes. Deep sequencing of small RNA fractions prepared from in vitro CyHV-3 infections led to the identification of potential miRNAs and miRNA–offset RNAs (moRNAs) derived from some bioinformatically predicted pre-miRNAs. DNA microarray hybridization analysis, Northern blotting and stem-loop RT-qPCR were then used to definitively confirm that CyHV-3 expresses two pre-miRNAs during infection in vitro. The evidence also suggested the presence of an additional four high-probability and two putative viral pre-miRNAs. MiRNAs from the two confirmed pre-miRNAs were also detected in gill tissue from CyHV-3-infected carp. We also present evidence that one confirmed miRNA can regulate the expression of a putative CyHV-3-encoded dUTPase. Candidate homologues of some CyHV-3 pre-miRNAs were identified in CyHV-1 and CyHV-2. This is the first report of miRNA and moRNA genes encoded by members of the Alloherpesviridae family, a group distantly related to the Herpesviridae family. The discovery of these novel CyHV-3 genes may help further our understanding of the biology of this economically important virus and their encoded miRNAs may have potential as biomarkers for the diagnosis of latent 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.

Fish herpesvirus diseases: a short review of current knowledge

Fish herpesviruses can cause significant economic losses in aquaculture, and some of these viruses are oncogenic. The virion morphology and genome organization of fish herpesviruses are generally similar to those of higher vertebrates, but the phylogenetic connections between herpesvirus families are tenuous. In accordance with new taxonomy, fish herpesviruses belong to the family Alloherpesviridae in the order Herpesvirales. Fish herpesviruses can induce diseases ranging from mild, inapparent infections to serious ones that cause mass mortality. The aim of this work was to summarize the present knowledge about fish herpesvirus diseases.

Comparison of the exomes of common carp (Cyprinus carpio) and zebrafish (Danio rerio)

Research on common carp, Cyprinus carpio, is beneficial for zebrafish research because of resources available owing to its large body size, such as the availability of sufficient organ material for transcriptomics, proteomics, and metabolomics. Here we describe the shot gun sequencing of a clonal double-haploid common carp line. The assembly consists of 511891 scaffolds with an N50 of 17 kb, predicting a total genome size of 1.4-1.5 Gb. A detailed analysis of the ten largest scaffolds indicates that the carp genome has a considerably lower repeat coverage than zebrafish, whilst the average intron size is significantly smaller, making it comparable to the fugu genome. The quality of the scaffolding was confirmed by comparisons with RNA deep sequencing data sets and a manual analysis for synteny with the zebrafish, especially the Hox gene clusters. In the ten largest scaffolds analyzed, the synteny of genes is almost complete. Comparisons of predicted exons of common carp with those of the zebrafish revealed only few genes specific for either zebrafish or carp, most of these being of unknown function. This supports the hypothesis of an additional genome duplication event in the carp evolutionary history, which--due to a higher degree of compactness--did not result in a genome larger than that of zebrafish.

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.

Koi herpesvirus encodes and expresses a functional interleukin-10

Koi herpesvirus (KHV) (species Cyprinid herpesvirus 3) ORF134 was shown to transcribe a spliced transcript encoding a 179-amino-acid (aa) interleukin-10 (IL-10) homolog (khvIL-10) in koi fin (KF-1) cells. Pairwise sequence alignment indicated that the expressed product shares 25% identity with carp IL-10, 22 to 24% identity with mammalian (including primate) IL-10s, and 19.1% identity with European eel herpesvirus IL-10 (ahvIL-10). In phylogenetic analyses, khvIL-10 fell in a divergent position from all host IL-10 sequences, indicating extensive structural divergence following capture from the host. In KHV-infected fish, khvIL-10 transcripts were observed to be highly expressed during the acute and reactivation phases but to be expressed at very low levels during low-temperature-induced persistence. Similarly, KHV early (helicase [Hel] and DNA polymerase [DNAP]) and late (intercapsomeric triplex protein [ITP] and major capsid protein [MCP]) genes were also expressed at high levels during the acute and reactivation phases, but only low-level expression of the ITP gene was detected during the persistent phase. Injection of khvIL-10 mRNA into zebrafish (Danio rerio) embryos increased the number of lysozyme-positive cells to a similar degree as zebrafish IL-10. Downregulation of the IL-10 receptor long chain (IL-10R1) using a specific morpholino abrogated the response to both khvIL-10 and zebrafish IL-10 transcripts, indicating that, despite the structural divergence, khvIL-10 functions via this receptor. This is the first report describing the characteristics of a functional viral IL-10 gene in the Alloherpesviridae.

Herpesviruses that infect fish

Herpesviruses are host specific pathogens that are widespread among vertebrates. Genome sequence data demonstrate that most herpesviruses of fish and amphibians are grouped together (family Alloherpesviridae) and are distantly related to herpesviruses of reptiles, birds and mammals (family Herpesviridae). Yet, many of the biological processes of members of the order Herpesvirales are similar. Among the conserved characteristics are the virion structure, replication process, the ability to establish long term latency and the manipulation of the host immune response. Many of the similar processes may be due to convergent evolution. This overview of identified herpesviruses of fish discusses the diseases that alloherpesviruses cause, the biology of these viruses and the host-pathogen interactions. Much of our knowledge on the biology of Alloherpesvirdae is derived from research with two species: Ictalurid herpesvirus 1 (channel catfish virus) and Cyprinid herpesvirus 3 (koi herpesvirus).

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

A new continuous cell line (KCF-1) from caudal fin of koi, Cyprinus carpio koi, was developed and sub-cultured more than 100 passages since the present study was initiated in March 2006. KCF-1 predominantly consisted of short fibroblast-like cells and grew well in Dulbecco's modified Eagle medium (DMEM) supplemented with 10% fetal bovine serum (FBS). Chromosome analysis revealed that 56% of the KCF-1 cells maintained normal diploid chromosome number (2n=100) at Passage 82. Using the KCF-1 cell line, a strain of cyprinid herpesvirus 3 (designated as CyHV-3-QY08) was isolated from the diseased koi. CyHV-3-QY08 continuously propagated in the KCF-1 cells, as confirmed by immunofluorescence assay (IFA) and transmission electron microscopy (TEM). KCF-1 cells infected with CyHV-3-QY08 produced typical cytopathic effects characterized by severe vacuolation, deformation of nuclei, and marginalization of the nuclear chromatin, which are consistent with those of previous reports. CyHV-3-QY08 was purified and subsequently analyzed by SDS-PAGE and TEM. The results showed that the purified virions contained two types of morphologies and were composed of more than 30 obvious viral polypeptides. An infectivity experiment revealed that CyHV-3-QY08 could cause 100% mortality in the infected koi. Based on the genome sequence of CyHV-3-I/U, the CyHV-3(I/U)-ORF136 homologue in CyHV-3-QY08 was cloned and sequenced. Multiple sequence alignments of CyHV-3-I/U-ORF136 homologues showed that CyHV-3-QY08 belonged to the typical Asian genotype. The CyHV-3(I/U)-ORF136 homologue seems to be a novel molecule marker, which can be used to distinguish Asia isolates from Europe-America strains.

Koi herpesvirus epizootic in cultured carp and koi, Cyprinus carpio L., in Taiwan

Koi herpesvirus (KHV) poses a significant threat to cultured koi and common carp, both Cyprinus carpio L. Since the first reported case in Israel in 1998, KHV has rapidly spread worldwide. This study investigates the spread of KHV to Taiwan by collecting 49 cases of suspected common carp and koi infections from 2003 to 2005 for analysis. Clinical signs included lethargy, anorexia, increased respiratory movements and uncoordinated swimming. Hyperaemia, haemorrhage on body surface and necrotic gill filaments were recorded. Gill epithelial hyperplasia, necrosis and eosinophilic intranuclear inclusion bodies were observed by histological examination, while virions were detected using transmission electron microscopy. By detecting the presence of the KHV thymidine kinase (TK) gene and the KHV 9/5 gene using polymerase chain reaction (PCR), 37 cases were identified as KHV-positive, and the cumulative mortality of infected fish was 70-100%. Positive cases showed identical sequences for the genes analysed, implying that they were of the same origin. For the KHV 9/5 gene sequence, these cases exhibited 100% identity with the Japanese strain (TUMST1, accession number AP008984) and 99% identity with the Israeli (KHV-I, DQ177346) and US (KHV-U, DQ657948) strains. Additionally, a loop-mediated isothermal amplification (LAMP) assay was performed and found to be more sensitive than PCR tests, suggesting its potential use as a rapid diagnostic method for KHV. This is the first epidemiological study of KHV infection in cultured common carp and koi in Taiwan.

Quantification of cyprinid herpesvirus 3 in environmental water by using an external standard virus

Cyprinid herpesvirus 3 (CyHV-3), a lethal DNA virus that spreads in natural lakes and rivers, infects common carp and koi. We established a quantification method for CyHV-3 that includes a viral concentration method and quantitative PCR combined with an external standard virus. Viral concentration methods were compared using the cation-coated filter and ultrafiltration methods. The recovery of virus-like particles was similar for the two methods (cation-coated filter method, 44%+/-19%, n=3; ultrafiltration method, 50%+/-3%, n=3); however, the former method was faster and more suitable for routine determinations. The recovery of seeded CyHV-3 based on the cation-coated filter method varied by more than 3 orders of magnitude among the water samples. The recovery yield of CyHV-3 was significantly correlated with that of the seeded lambda phage, and the average ratio of lambda to the CyHV-3 recovery yield was 1.4, indicating that lambda is useful as an external standard virus for determining the recovery yield of CyHV-3. Therefore, to quantify CyHV-3 in environmental water, a known amount of lambda was added as an external standard virus to each water sample. Using this method, CyHV-3 DNA was detected in 6 of the 10 (60%) types of environmental water tested; the highest concentration of CyHV-3 DNA was 2x10(5) copies liter(-1). The lowest recovery limit of CyHV-3 DNA was 60 copies liter(-1). This method is practical for monitoring CyHV-3 abundance in environmental water.

Detection of cyprinid herpesvirus 3 DNA in river water during and after an outbreak

The disease caused by cyprinid herpesvirus 3 (CyHV-3) brings catastrophic damages to cultivated carp and koi and to natural carp populations; however, the dynamics of the virus in environmental waters are unclear. In July 2007, CyHV-3 DNA was detected in a dead common carp collected from the Yura River in Kyoto Prefecture, Japan, and this was followed by mass mortality. We collected water samples at eight sites along the Yura River for 3 months immediately after confirmation of the disease outbreak and attempted to detect and quantify CyHV-3 DNA in the water samples using molecular biological methods. The virus concentration was carried out by the cation-coated filter method, while the purification of DNA from the samples was achieved using phenol-chloroform extraction and a commercial DNA extraction kit. CyHV-3 was detected by PCR using six sets of conditions, three sets of primers (SphI-5, AP, and B22Rh exon 1), and two volumes of template DNA, and was quantified using real-time PCR. Our results indicate broader distribution of CyHV-3, even though dead fish were found only in a limited area; moreover, the virus was present at high levels in the river not only during the mass mortality caused by the disease but also for at least 3 months after the end of mass mortality. Our results suggest the possibility of infection by CyHV-3 via environmental water. The sequences of CyHV-3 collected from the Yura River matched perfectly with that of the CyHV-3 Japanese strain, suggesting that they share the same origin.

Cloning of the koi herpesvirus genome as an infectious bacterial artificial chromosome demonstrates that disruption of the thymidine kinase locus induces partial attenuation in Cyprinus carpio koi

Koi herpesvirus (KHV) is the causative agent of a lethal disease in koi and common carp. In the present study, we describe the cloning of the KHV genome as a stable and infectious bacterial artificial chromosome (BAC) clone that can be used to produce KHV recombinant strains. This goal was achieved by the insertion of a loxP-flanked BAC cassette into the thymidine kinase (TK) locus. This insertion led to a BAC plasmid that was stably maintained in bacteria and was able to regenerate virions when permissive cells were transfected with the plasmid. Reconstituted virions free of the BAC cassette but carrying a disrupted TK locus (the FL BAC-excised strain) were produced by the transfection of Cre recombinase-expressing cells with the BAC. Similarly, virions with a wild-type revertant TK sequence (the FL BAC revertant strain) were produced by the cotransfection of cells with the BAC and a DNA fragment encoding the wild-type TK sequence. Reconstituted recombinant viruses were compared to the wild-type parental virus in vitro and in vivo. The FL BAC revertant strain and the FL BAC-excised strain replicated comparably to the parental FL strain. The FL BAC revertant strain induced KHV infection in koi carp that was indistinguishable from that induced by the parental strain, while the FL BAC-excised strain exhibited a partially attenuated phenotype. Finally, the usefulness of the KHV BAC for recombination studies was demonstrated by the production of an ORF16-deleted strain by using prokaryotic recombination technology. The availability of the KHV BAC is an important advance that will allow the study of viral genes involved in KHV pathogenesis, as well as the production of attenuated recombinant candidate vaccines.

Morphogenesis of koi herpesvirus observed by electron microscopy

Koi herpesvirus (KHV or cyprinid herpesvirus 3) was inoculated onto three fish cell lines derived from carp, Cyprinus carpio L., and the process of virion formation observed with electron microscopy. Essentially, similar features of virus particles were observed in all three cell lines. The nucleus of infected cells was characterized by margination of chromatin and often contained many capsids of about 110 nm in diameter with varying morphology. The morphological variation of the capsids was very similar to that of mammalian herpesviruses. Some capsids protruded from the inner nuclear membrane, and others, with envelopes, were located in the perinuclear space between the inner and outer nuclear membrane, suggesting budding of capsids at the inner nuclear membrane. Naked capsids and envelopment of capsids by budding into vesicles were also observed in the cytoplasm. Mature, enveloped virions 170-200 nm in diameter were seen in cytoplasmic vesicles or outside the cell. These observations suggest KHV virions mature through a complex morphological pathway including two distinct envelopments, which have been found only in herpesviruses. These observations support the inclusion of KHV in the family Herpesviridae.

Susceptibility of cyprinid cultured cells to cyprinid herpesvirus 3

Cyprinid herpesvirus 3 is a highly contagious and lethal virus that affects ornamental koi and common carp worldwide. However, it is not yet known whether other cyprinids are infected and/or harbor the virus. Here, we report that cultured cells derived from common carp, koi, silver carp and goldfish allow CyHV-3 propagation, while cyprinid cells derived from fathead minnow and non-cyprinid cells derived from the channel catfish ovary are resistant to CyHV-3 infection. Interestingly, the epithelioma papulosum Cyprini cells derived from Cyprinus carpio are restrictive to the virus. These results indicate that CyHV-3 is not restricted to common carp and koi, but other cyprinids are also vulnerable to the virus.

Genome sequences of three koi herpesvirus isolates representing the expanding distribution of an emerging disease threatening koi and common carp worldwide

Since the mid-1990s, lethal infections of koi herpesvirus (KHV) have been spreading, threatening the worldwide production of common carp and koi (both Cyprinus carpio). The complete genome sequences of three KHV strains from Japan, the United States, and Israel revealed a 295-kbp genome containing a 22-kbp terminal direct repeat. The finding that 15 KHV genes have clear homologs in the distantly related channel catfish virus (ictalurid herpesvirus 1) confirms the proposed place of KHV in the family Herpesviridae, specifically in the branch with fish and amphibian hosts. KHV thus has the largest genome reported to date for this family. The three strains were interpreted as having arisen from a wild-type parent encoding 156 unique protein-coding genes, 8 of which are duplicated in the terminal repeat. In each strain, four to seven genes from among a set of nine are fragmented by frameshifts likely to render the encoded proteins nonfunctional. Six of the affected genes encode predicted membrane glycoproteins. Frameshifts or other mutations close to the 3' ends of coding sequences were identified in a further six genes. The conclusion that at least some of these mutations occurred in vivo prompts the hypothesis that loss of gene functions might be associated with emergence of the disease and provides a basis for further investigations into the molecular epidemiology of the virus.

Persistence of cyprinid herpesvirus 3 in infected cultured carp cells

Cyprinid herpesvirus 3 (CyHV-3), previously designated carp interstitial nephritis and gill necrosis virus or koi herpesvirus, is the cause of a worldwide mortal disease of koi and carp. Morphologically, the virus resembles herpesviruses, yet it bears a genome of 277 to 295 kbp, which is divergent from most of the genomic sequences available in GenBank. The disease afflicts fish in the transient seasons, when the water temperature is 18 to 28 degrees C, conditions which permit virus propagation in cultured cells. Here we report that infectious virus is preserved in cultured cells maintained for 30 days at 30 degrees C. CyHV-3-infected vacuolated cells with deformed morphology converted to normal, and plaques disappeared following shifting up of the temperature and reappeared after transfer to the permissive temperature. Viral propagation and viral gene transcription were turned off by shifting cells to the nonpermissive temperature. Upon return of the cells to the permissive temperature, transcription of viral genes was reactivated in a sequence distinguished from that occurring in naïve cells following infection. Our results show that CyHV-3 persists in cultured cells maintained at the nonpermissive temperature and suggest that viruses could persist for long periods in the fish body, enabling a new burst of infection upon a shift to a permissive temperature.

Cyprinid herpes virus-3 (CyHV-3) bears genes of genetically distant large DNA viruses

A large DNA virus, designated koi herpes virus (KHV), carp interstitial nephritis gill necrosis virus (CNGV) and Cyprinid herpes virus-3 (CyHV-3), causes massive mortality of carp. Morphologically, the virus resembles herpes viruses, but it contains a genome of ca 295 kbp, larger than that of any Herpesviridae member. Interestingly, three CyHV-3 genes, thymidylate monophosphate kinase (TmpK), ribonucleotide reductase and thymidine kinase, which are involved in deoxynucleotide tri-phosphate synthesis, resemble those of pox viruses. In addition to the TmpK gene, which is nonexistent in the genome of herpes viruses, CyHV-3 contains a B22R-like gene, exclusively expressed by pox viruses. These results raise questions on the phylogenic origin of CyHV-3.