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

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.

Occurrence of herpesvirus in fish

Introduction

Herpesviruses are common agents in animals of the aquatic environment. They infect many species of fish but only lead to disease in one or two species. Nevertheless, infected fish without clinical symptoms can actively transfer infectious agents to disease-susceptible species. The aim of the study was to identify and prove the natural presence of different herpesviruses.

Material and Methods

Koi, Nile tilapia, grass carp, goldfish and crucian carp were infected with a herpesvirus isolate 99% identical to goldfish herpesvirus (GHV) or cyprinid herpesvirus 2 (CyHV-2) obtained from crucian carp. Before and after infection, samples were collected non-lethally at different time points from all five fish species to identify and evaluate the replication of viruses naturally infecting the fish as well as the CyHV-2 experimentally infecting them. Gill swabs and separated leukocytes were subjected to PCR and the results compared.

Results

These samples yielded DNA of koi herpesvirus (KHV, also referred to as CyHV-3), GHV and a new herpesvirus. While Asian-lineage CyHV-3 DNA was detected in samples from crucian carp and goldfish, CyHV-2 DNA was found in samples from koi and tilapia. A new, hitherto unknown herpesvirus was identified in samples from grass carp, and was confirmed by nested PCR and sequence analysis. The survival rates were 5% for grass carp, 30% for tilapia, 55% for crucian carp, 70% for koi and 100% for goldfish at 20 days post infection. Evolutionary analyses were conducted and five clusters were visible: CyHV-1 (carp pox virus), CyHV-2 with sequences from koi and tilapia, CyHV-3 with sequences from crucian carp and goldfish, probable CyHV-4 from sichel and a newly discovered herpesvirus - CyHV-5 - from grass carp.

Conclusion

The results obtained with the molecular tools as well as from the animal experiment demonstrated the pluripotency of aquatic herpesviruses to infect different fish species with and without visible clinical signs or mortality.

Development of a droplet digital PCR assay for the sensitive detection of iridovirus in Andrias davidianus

Chinese giant salamander iridovirus (GSIV) is the first known and causative viral pathogen in Andrias davidianus. Developing a sensitive, accurate and specific assay to detect GSIV in samples is essential to prevent the further spread of the pathogen. In this study, we established a droplet digital PCR (ddPCR) assay that targeted the mcp gene of GSIV, enabling rapid and quantitative detection of the virus. We determined that the optimal annealing temperature, primer concentration and probe concentration were 57.1°C, 50 nM and 500 nM, respectively. We analysed the specificity and sensitivity of the ddPCR assay and found that five common aquatic animal viruses, including Cyprinid herpesvirus 2 (CyHV-2), infectious spleen and kidney necrosis virus (ISKNV), Koi herpesvirus (KHV) and Carp Edema Virus (CEV) displayed negative results based on this GSIV ddPCR assay. The assay can detect GSIV with the lowest detection limit of 3.7 copies per reaction. To evaluate the sensitivity and accuracy of the ddPCR assay, we tested different infected tissue samples with both the ddPCR and TaqMan real-time PCR assays. Our results showed that the ddPCR assay detected GSIV in all samples with 100% positivity, while the TaqMan real-time PCR assay detected GSIV in only 82.1% of samples. The established ddPCR method provided several advantages in detecting GISV, including high sensitivity, high precision and absolute quantification, making it a powerful tool for detection of possible and potential GSIV infection, even in samples with low viral load.

Autophagy induced by Cyprinid herpesvirus 3 (CyHV-3) facilitated intracellular viral replication and extracellular viral yields in common carp brain cells

Autophagy is a conservative and important process that exists in all eukaryotic cells in nature. Cyprinid herpesvirus 3 (CyHV-3), also known as KHV (Koi Herpesvirus), is a pathogen that mainly infecting common carp and koi. In the present study, we identified the CcLC3B gene, with a length of 379 bp and displaying a close evolutionary relationship with other sixteen different species, the tissue distribution and expression pattern of CcLC3 were also identified. We found that CyHV-3 infection could promote autophagy in CCB cells at the early stage but inhibit autophagy at the late stage by using confocal fluorescence microscopy, transmission electron microscopy and western blotting. And we measured the protein levels associated with the Akt/mTOR signalling pathway, intracellular replication of CyHV-3 at the mRNA and protein levels as well as viral titters. Collectively, the results taken together suggested that CyHV-3 infection could promote autophagy in CCB cells at the early stage but inhibit autophagy at the late stage via mTOR and that promoting autophagy could facilitate CyHV-3 intracellular replication and extracellular viral yields in CCB cells. These findings revealed the relationship between CyHV-3 and autophagy and provided a novel treatment strategy targeting the autophagy signalling pathway against CyHV-3 infection.

Disease ecology and host range of Cyprinid herpesvirus 3 (CyHV-3) in CyHV-3 endemic lakes of North America

Cyprinid herpesvirus-3 (CyHV-3) is an important pathogen of common carp (Cyprinus carpio, carp) causing significant economic and ecological impacts worldwide. The recent emergence of CyHV-3 in the Upper Midwest region of the United States has raised questions related to the disease ecology and host specificity of CyHV-3 in wild carp populations. To determine the prevalence of CyHV-3 in wild populations of fishes in Minnesota, we surveyed five lakes in 2019 in which the virus was known to have caused mass mortality events in carp from 2017 to 2018. A total of 28 species (n = 756 total fish) of native fishes and 730 carp were screened for the presence of CyHV-3 DNA using specific qPCR. None of the native fish tissues tested positive for CyHV-3 although the prevalence of CyHV-3 in carp was 10%-50% in the five lakes. A single lake (Lake Elysian) with a 50% DNA detection rate and evidence of ongoing transmission and CyHV-3-associated mortality was surveyed again in 2020 from April to September. During this period, none of the tissues from 24 species (n = 607 total fish) tested positive for CyHV-3 though CyHV-3 DNA and mRNA (indicating viral replication) was detected in carp tissues during the sampling period. CyHV-3 DNA was detected most often in brain samples without evidence of replication, potentially indicating that brain tissue is a site for CyHV-3 latency. Paired qPCR and ELISA testing for Lake Elysian in 2019-2020 identified young carp (especially males) to be the primary group impacted by CyHV-3-associated mortality and acute infections, but with no positive detections in juvenile carp. Seroprevalence of carp from Lake Elysian was 57% in 2019, 92% in April of 2020 and 97% in September 2020. These results further corroborate the host specificity of CyHV-3 to carp in mixed wild populations of fishes in Minnesota and provide additional insights into the ecological niche of CyHV-3 in shallow lake populations of carp in North America.

Protective and Therapeutic Capacities of Lactic Acid Bacteria Postmetabolites against Koi Herpesvirus Infection In Vitro

Background: The accumulation of data on beneficial biological effects of probiotics and their metabolic products favors their potential use in the prevention and treatment of various malaises. Methods: Nine postmetabolites from Lactic acid bacteria (LAB) of human or dairy origin and their antiviral activity were studied using the cytopathic effect inhibition test. The virucidal capacity, their influence on the adsorption stage of Koi herpes virus (KHV) and their preventive role against subsequent viral challenge on intact Common carp brain (CCB) cells were also determined by titration assay. Residual viral infectivity in postmetabolites-treated samples was compared to mock-treated controls and Δlgs were calculated. Results: When administered during KHV replication, the microbial products isolated from Lactiplantibacillus plantarum showed remarkable activity with a selectivity index (SI) between 26.5 and 221.4, as those effects were dependent on the sample-virus incubation time. Postmetabolites from Lactobacillus gasseri and Lactiplantibacillus plantarum also demonstrated significant inhibition of KHV replication with SI of 24 and 16, respectively. The bioactive metabolites isolated from Limosilactobacillus fermentum had a minor effect on the viral replicative cycle. Compounds, produced during the fermentation by lactobacilli, grown on different nutritive media and collected at different time points, significantly inhibited extracellular KHV virions. All investigated postmetabolites remarkably blocked KHV attachment to the host cell (CCB), leading to a drop in viral titers by Δlg = 4.25–5.25, and exerted protective effects on CCB cells before they were subjected to viral infection. Conclusions: Our results open new horizons and promote LAB and their postbiotic products to be used in the prophylaxis and therapy of viral infections.

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

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

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

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

Investigation of Cyprinid Herpesvirus 3 (CyHV-3) Disease Periods and Factors Influencing CyHV-3 Transmission in A Low Stocking Density Infection Trial

Simple Summary

Pathogens are the primary limitation to aquaculture production of fish and a major issue in consideration of the interface between cultured and wild populations of fishes worldwide. While rapid spread of fish pathogens between populations (wild or farmed) is generally anthropogenic and the result of trade, the mechanisms of transmission once a pathogen has been introduced to a fish population are not well understood. The most widespread pathogen impacting both aquaculture and wild populations of common carp (Cyprinus carpio, carp) is Cyprinid herpesvirus 3 (CyHV-3). To understand how CyHV-3 is transmitted in a population we conducted a series of infection trials, designed to determine the kinetics CyHV-3 infections, identify the contributions of direct and indirect forms of CyHV-3 transmission, and to determine the contributions of contact rate, viral load, pathogenicity, and contact type. We found that direct contact between fish was the primary mechanism of CyHV-3 transmission rather than transmission through contaminated water. Additionally, CyHV-3 transmission occurred primarily during the incubation period of CyHV-3, prior to the appearance of disease signs and disease-associated reduction in contact rate.

Abstract

Cyprinid herpesvirus 3 (CyHV-3) is the etiological agent of koi herpesvirus disease (KHVD) and important pathogen of aquaculture and wild populations of common carp worldwide. Understanding the relative contributions of direct and indirect transmission of CyHV-3 as well as the factors that drive CyHV-3 transmission can clarify the importance of environmental disease vectors and is valuable for informing disease modeling efforts. To study the mechanisms and factors driving CyHV-3 transmission we conducted infection trials that determined the kinetics of KHVD and the contributions of direct and indirect forms of CyHV-3 transmission, as well as the contributions of contact rate, viral load, pathogenicity and contact type. The incubation period of KHVD was 5.88 + 1.75 days and the symptomatic period was 5.31 + 0.87 days. Direct transmission was determined to be the primary mechanism of CyHV-3 transmission (OR = 25.08, 95%CI = 10.73–99.99, p = 4.29 × 10⁻¹⁸) and transmission primarily occurred during the incubation period of KHVD. Direct transmission decreased in the symptomatic period of disease. Transmissibility of CyHV-3 and indirect transmission increased during the symptomatic period of disease, correlating with increased viral loads. Additionally, potential virulence-transmission tradeoffs and disease avoidance behaviors relevant to CyHV-3 transmission were identified.

Inhibitory properties of crude microalgal extracts on the in vitro replication of cyprinid herpesvirus 3

Microalgae are possible sources of antiviral substances, e.g. against cyprinid herpesvirus 3 (CyHV-3). Although this virus leads to high mortalities in aquacultures, there is no treatment available yet. Hence, ethanolic extracts produced with accelerated solvent extraction from six microalgal species (Arthrospira platensis, Chlamydomonas reinhardtii, Chlorella kessleri, Haematococcus pluvialis, Nostoc punctiforme and Scenedesmus obliquus) were examined in this study. An inhibition of the in vitro replication of CyHV-3 could be confirmed for all six species, with the greatest effect for the C. reinhardtii and H. pluvialis crude extracts. At still non-cytotoxic concentrations, viral DNA replication was reduced by over 3 orders of magnitude each compared to the untreated replication controls, while the virus titers were even below the limit of detection (reduction of 4 orders of magnitude). When pre-incubating both cells and virus with C. reinhardtii and H. pluvialis extracts before inoculation, the reduction of viral DNA was even stronger (> 4 orders of magnitude) and no infectious viral particles were detected. Thus, the results of this study indicate that microalgae and cyanobacteria are a promising source of natural bioactive substances against CyHV-3. However, further studies regarding the isolation and identification of the active components of the extracts are needed.

Lactococcus lactis Expressing Type I Interferon From Atlantic Salmon Enhances the Innate Antiviral Immune Response In Vivo and In Vitro

In salmon farming, viruses are responsible for outbreaks that produce significant economic losses for which there is a lack of control tools other than vaccines. Type I interferon has been successfully used for treating some chronic viral infections in humans. However, its application in salmonids depends on the proper design of a vehicle that allows its massive administration, ideally orally. In mammals, administration of recombinant probiotics capable of expressing cytokines has shown local and systemic therapeutic effects. In this work, we evaluate the use of Lactococcus lactis as a type I Interferon expression system in Atlantic salmon, and we analyze its ability to stimulate the antiviral immune response against IPNV, in vivo and in vitro. The interferon expressed in L. lactis, even though it was located mainly in the bacterial cytoplasm, was functional, stimulating Mx and PKR expression in CHSE-214 cells, and reducing the IPNV viral load in SHK-1 cells. In vivo, the oral administration of this L. lactis producer of Interferon I increases Mx and PKR expression, mainly in the spleen, and to a lesser extent, in the head kidney. The oral administration of this strain also reduces the IPNV viral load in Atlantic salmon specimens challenged with this pathogen. Our results show that oral administration of L. lactis producing Interferon I induces systemic effects in Atlantic salmon, allowing to stimulate the antiviral immune response. This probiotic could have effects against a wide variety of viruses that infect Atlantic salmon and also be effective in other salmonids due to the high identity among their type I interferons.

Emodin resists to Cyprinid herpesvirus 3 replication via the pathways of Nrf2/Keap1-ARE and NF-κB in the ornamental koi carp (Cyprinus carpio haematopterus)

Cyprinid herpesvirus 3 (CyHV-3) causes high mortality in carp. Emodin has been shown of the effects of antioxidant, anti-inflammatory and antiviral. In present study, we investigated the preventive effects and mechanism of emodin on CyHV-3 infection. The ornamental koi carp (Cyprinus carpio haematopterus) were intraperitoneally injected with emodin (10 mg/kg, 20 mg/kg, or 40 mg/kg). 72 h later, an intraperitoneal injection of CyHV-3 was administered, and collected the samples one week later to detect the antioxidant parameters, antioxidant genes, inflammatory genes and to perform histopathology assays. The results showed that emodin significantly suppressed CyHV-3 replication (P < 0.05), improved the koi survival rate and slowed the damage caused by CyHV-3. Emodin treatment increased the antioxidant activity and decreased the lipid peroxidation level of the koi. Compared to the CyHV-3 group, emodin treatment resulted in the same antioxidant parameters after CyHV-3 infection. Emodin treatment activated the Nuclear factorery throid 2-related factor 2/Kelch-like ECH-associated protein 1-antioxidatant response element (Nrf2/Keap1-ARE) pathway and upregulated the expression of heme oxygenase 1 (HO-1), superoxide dismutase (SOD), and catalase (CAT) in the hepatopancreas after CyHV-3 infection. Emodin activated the nuclear factor kappa-B (NF-κB) pathway and decreased the expression of interleukin-6 (IL-6), interleukin-8 (IL-8), and tumour necrosis factor-α (TNF-α) in the koi induced by CyHV-3. In conclusion, emodin treatment can suppress CyHV-3 replication and reduce the mortality of koi caused by CyHV-3. Emodin improves antioxidant function, relieves oxidative stress and inflammation cytokines via Nrf2/Keap1-ARE and NF-κB pathways, and protects against the adverse effects induced by CyHV-3.

Oral Probiotic Vaccine Expressing Koi Herpesvirus (KHV) ORF81 Protein Delivered by Chitosan-Alginate Capsules Is a Promising Strategy for Mass Oral Vaccination of Carps against KHV Infection

Koi herpesvirus (KHV) is highly contagious and lethal to cyprinid fish, causing significant economic losses to the carp aquaculture industry, particularly to koi carp breeders. Vaccines delivered through intramuscular needle injection or gene gun are not suitable for mass vaccination of carp. So, the development of cost-effective oral vaccines that are easily applicable at a farm level is highly desirable. In this study, we utilized chitosan-alginate capsules as an oral delivery system for a live probiotic (Lactobacillus rhamnosus) vaccine, pYG-KHV-ORF81/LR CIQ249, expressing KHV ORF81 protein. The tolerance of the encapsulated recombinant Lactobacillus to various digestive environments and the ability of the probiotic strain to colonize the intestine of carp was tested. The immunogenicity and the protective efficacy of the encapsulated probiotic vaccine was evaluated by determining IgM levels, lymphocyte proliferation, expression of immune-related genes, and viral challenge to vaccinated fish. It was clear that the chitosan-alginate capsules protected the probiotic vaccine effectively against extreme digestive environments, and a significant level (P < 0.01) of antigen-specific IgM with KHV-neutralizing activity was detected, which provided a protection rate of ca. 85% for koi carp against KHV challenge. The strategy of using chitosan-alginate capsules to deliver probiotic vaccines is easily applicable for mass oral vaccination of fish.IMPORTANCE An oral probiotic vaccine, pYG-KHV-ORF81/LR CIQ249, encapsulated by chitosan-alginate capsules as an oral delivery system was developed for koi carp against koi herpesvirus (KHV) infection. This encapsulated probiotic vaccine can be protected from various digestive environments and maintain effectively high viability, showing a good tolerance to digestive environments. This encapsulated probiotic vaccine has a good immunogenicity in koi carp via oral vaccination, and a significant level of antigen-specific IgM was effectively induced after oral vaccination, displaying effective KHV-neutralizing activity. This encapsulated probiotic vaccine can provide effective protection for koi carp against KHV challenge, which is handling-stress free for the fish, cost effective, and suitable for the mass oral vaccination of koi carp at a farm level, suggesting a promising vaccine strategy for fish.

Susceptibility of Pimephales promelas and Carassius auratus to a strain of koi herpesvirus isolated from wild Cyprinus carpio in North America

Cyprinid herpesvirus-3 (CyHV-3, syn. koi herpesvirus) is an important pathogen worldwide and a common cause of mass mortality events of wild common carp (Cyprinus carpio) in North America, however, reference strains and genomes obtained from wild carp are not available. Additionally, it is unclear if fishes in North America are susceptible to CyHV-3 infection due to incomplete susceptibility testing. Here we present the first North American type strain and whole-genome sequence of CyHV-3 isolated from wild carp collected from a lake with a history and recent incidence of carp mortality. Additionally, the strain was used in an in-vivo infection model to test the susceptibility of a common native minnow (Pimephales promelas) and goldfish (Carrasius auratus) which is invasive in North America. Detection of CyHV-3 DNA was confirmed in the tissues of a single fathead minnow but the same tissues were negative for CyHV-3 mRNA and samples from exposed fathead minnows were negative on cell culture. There was no detection of CyHV-3 DNA or mRNA in goldfish throughout the experiment. CyHV-3 DNA in carp tissues was reproducibly accompanied by the detection of CyHV-3 mRNA and isolation on cell culture. Additionally, environmental CyHV-3 DNA was detected on all tank filters during the study. These findings suggest that fathead minnows and goldfish are not susceptible to CyHV-3 infection and that detection of CyHV-3 DNA alone in host susceptibility trials should be interpreted with caution.

Characterization of the monoclonal antibody specific to the ORF72 protein of koi herpesvirus and cellular distribution analysis of the viral protein

Koi herpesvirus (KHV) is an emerging pathogen of koi and common carp that causes a severe disease and mass mortality of infected fish. The KHV ORF72 protein is an important capsid protein that has been suggested to be a candidate for the development of diagnostic reagents and KHV vaccines. The purpose of this study was to clone and express the KHV ORF72 gene for further preparation of a specific monoclonal antibody (mAb) and to analyse cellular distribution of the viral protein. The mAb 3E1 could specifically recognize the expressed ORF72 protein of transfected cells by indirect immunofluorescence, and the antigenic site recognized by the mAb 3E1 was mapped to the region of N-terminal 124 residues of KHV ORF72. This mAb was further demonstrated to specifically detect the KHV-infected fish tissue by immunohistochemistry, thereby suggesting its high diagnostic potential. In addition, the cellular distribution analysis of the KHV ORF72 protein revealed that the region of amino acid residues 125-247 was related to mitochondrial localization and proliferation. Furthermore, a putative nuclear export signal (NES) of ORF72 at the residues 201-212 was confirmed on the basis of its function associated with NES activity.