The major portal of entry of koi herpesvirus in Cyprinus carpio is the skin

In vivo multiscale analyses of spring viremia of carp virus (SVCV) infection: From model organism to target species

Spring viremia of carp virus (SVCV) has a broad fish host spectrum and is responsible for a disease that generally affects juvenile fishes with a mortality rate of up to 90%. In the absence of treatments or vaccines against SVCV, the search for prophylactic or therapeutic solutions is thus relevant, particularly to identify solutions compatible with mass vaccination. In addition to being a threat to aquaculture and ecosystems, SVCV is a unique pathogen to study virus-host interactions in the zebrafish model. Establishing the first reverse genetics system for SVCV and the design of recombinant SVCV (rSVCV) expressing fluorescent or bioluminescent proteins adds a new dimension for the study of these interactions using innovative imaging techniques. The infection by bath immersion of zebrafish larvae with rSVCV expressing mCherry allows us to define the first SVCV replication sites and the host innate immune responses using different transgenic lines of zebrafish. The fins were found as the main initial sites of infection in both zebrafish and carp, its natural host. Hence, new insights into the physiopathology of SVCV infection have been described. We report that neutrophils are recruited at the sites of infection and persist up to the death of the animal leading to an uncontrolled inflammation correlated with the expression of the pro-inflammatory cytokine IL1β. Tissue damage was observed at the site of initial replication, a likely consequence of virus-induced injury or the pro-inflammatory response. Interestingly, SVCV infection by bath immersion triggers a persistent pro-inflammatory response rather than activation of the antiviral IFN signaling pathway as observed following intravenous injection, highlighting the importance of the route of infection on the progression of pathogenicity. Thus, this model of zebrafish larvae infection by rSVCV offers new perspectives to study in detail virus-host interactions and to discover new prophylactic or therapeutic solutions.

IFN-γ enhances protective efficacy against Nocardia seriolae infection in largemouth bass (Micropterus salmoides)

Introduction

Nocardia seriolae adversely impacts a diverse range of fish species, exhibiting significant pathogenic characteristics that substantially impede the progress of aquaculture. N. seriolae infects in fish has a long incubation period, and clinical symptoms are not obvious in the early stages. There is presently no viable and eco-friendly approach to combat the spread of the disease. According to reports, N. seriolae primarily targets macrophages in tissues after infecting fish and can proliferate massively, leading to the death of fish. Interferon-gamma (IFN-γ) is a crucial molecule that regulates macrophage activation, but little is known about its role in the N. seriolae prevention.

Methods

IFN-γ was first defined as largemouth bass (Micropterus salmoides, MsIFN-γ), which has a highly conserved IFN-γ characteristic sequence through homology analysis. The recombinant proteins (rMsIFN-γ) were obtained in Escherichia coli (E. coli) strain BL21 (DE3). The inflammatory response-inducing ability of rMsIFN-γ was assessed in vitro using monocytes/macrophages. Meanwhile, the protective effect of MsIFN-γ in vivo was evaluated by N. seriolae infection largemouth bass model.

Results

In the inflammatory response of the monocytes/macrophages activated by rMsIFN-γ, various cytokines were significantly increased. Interestingly, interleukin 1β (IL-1β) and tumor necrosis factor alpha (TNF-a) increased by 183- and 12-fold, respectively, after rMsIFN-γ stimulation. rMsIFN-γ improved survival by 42.1% compared with the control. The bacterial load in the liver, spleen and head kidney significantly decreased. rMsIFN-γ was also shown to better induce increased expression of IL-1β, TNF-α, hepcidin-1(Hep-1), major histocompatibility complex I (MHCI), and MHC II in head kidney, spleen and liver. The histopathological examination demonstrated the transformation of granuloma status from an early necrotic foci to fibrosis in the infection period. Unexpectedly, the development of granulomas was successfully slowed in the rMsIFN-γ group.

Discussion

This work paves the way for further research into IFN-γ of largemouth bass and identifies a potential therapeutic target for the prevention of N. seriolae.

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

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

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.

Susceptibility and Permissivity of Zebrafish (Danio rerio) Larvae to Cypriniviruses

The zebrafish (Danio rerio) represents an increasingly important model organism in virology. We evaluated its utility in the study of economically important viruses from the genus Cyprinivirus (anguillid herpesvirus 1, cyprinid herpesvirus 2 and cyprinid herpesvirus 3 (CyHV-3)). This revealed that zebrafish larvae were not susceptible to these viruses after immersion in contaminated water, but that infections could be established using artificial infection models in vitro (zebrafish cell lines) and in vivo (microinjection of larvae). However, infections were transient, with rapid viral clearance associated with apoptosis-like death of infected cells. Transcriptomic analysis of CyHV-3-infected larvae revealed upregulation of interferon-stimulated genes, in particular those encoding nucleic acid sensors, mediators of programmed cell death and related genes. It was notable that uncharacterized non-coding RNA genes and retrotransposons were also among those most upregulated. CRISPR/Cas9 knockout of the zebrafish gene encoding protein kinase R (PKR) and a related gene encoding a protein kinase containing Z-DNA binding domains (PKZ) had no impact on CyHV-3 clearance in larvae. Our study strongly supports the importance of innate immunity-virus interactions in the adaptation of cypriniviruses to their natural hosts. It also highlights the potential of the CyHV-3-zebrafish model, versus the CyHV-3-carp model, for study of these interactions.

Virus-induced interference as a means for accelerating fitness-based selection of cyprinid herpesvirus 3 single-nucleotide variants in vitro and in vivo

Cyprinid herpesvirus 3 (CyHV-3) is the archetype of fish alloherpesviruses and is advantageous to research because, unlike many herpesviruses, it can be studied in the laboratory by infection of the natural host (common and koi carp). Previous studies have reported a negative correlation among CyHV-3 strains between viral growth in vitro (in cell culture) and virulence in vivo (in fish). This suggests the existence of genovariants conferring enhanced fitness in vitro but reduced fitness in vivo and vice versa. Here, we identified the syncytial plaque formation in vitro as a common trait of CyHV-3 strains adapted to cell culture. A comparison of the sequences of virion transmembrane protein genes in CyHV-3 strains, and the use of various recombinant viruses, demonstrated that this trait is linked to a single-nucleotide polymorphism (SNP) in the open reading frame (ORF) 131 coding sequence (C225791T mutation) that results in codon 183 encoding either an alanine (183A) or a threonine (183T) residue. In experiments involving infections with recombinant viruses differing only by this SNP, the 183A genovariant associated with syncytial plaque formation was the more fit in vitro but the less fit in vivo. In experiments involving coinfection with both viruses, the more fit genovariant contributed to the purifying selection of the less fit genovariant by outcompeting it. In addition, this process appeared to be accelerated by viral stimulation of interference at a cellular level and stimulation of resistance to superinfection at a host level. Collectively, this study illustrates how the fundamental biological properties of some viruses and their hosts may have a profound impact on the degree of diversity that arises within viral populations.

The influence of viral infection on cell line characteristics: Lessons learned from working with new cell lines from common carp

Several factors influence the susceptibility of cell lines to infection by different viruses. These can be related to tissue specificity of the viruses, physiological status of the cells, their differentiation level and their capacity to mount immune responses to combat viral infection. To study the influence of cell characteristics and immune responses on their susceptibility on virus infection, newly developed cell lines from common carp brain (CCAbre), fins (CCApin), gills (CCAgill), and heart (CCAcar) and the established common carp brain (CCB) cells were exposed to the carp infecting viruses cyprinid herpesvirus 3 (CyHV-3), carp oedema virus (CEV), and the yet not fully characterized common carp paramyxovirus (CCPV). The susceptibility of these cells to viral infection was measured by formation of a cytopathic effect (CPE), estimation of viral particles produced by the cells and presence of viral mRNA in the cells. Viral susceptibility of the cells was compared to cell characteristics, measured by mRNA expression of the epithelial cell markers cadherin 1, occludin, and cytokeratin 15 and the mesenchymal cell marker vimentin, as well as to the level of type I interferon (IFN) responses. All cell lines were susceptible to CyHV-3 and CCPV but not to CEV infection. The cell lines had different levels of type I IFN responses towards the viruses. Typically, CyHV-3 did not induce high type I IFN responses, while CCPV induced high responses in CCAbre, CCAcar, CCApin cells but no response in CCAgill cells. Consequently, the type I IFN response modulated cell susceptibility to CCPV but not to CyHV-3. Interestingly, when the three different passage levels of CCB cells were examined, the susceptibility of one passage was significantly lower for CyHV-3 and higher for CCPV infection. This coincided with a loss of epithelial markers and lower type I IFN responses. This study confirms an influence of cell characteristics and immune responses on the susceptibility of carp cell lines for virus infection. Depending on the vulnerability of the virus to type I IFN responses, cells with a lower IFN-response can be superior for replication of some viruses. Batches of CCB cells can differentiate and thus may have significantly different levels of susceptibility to certain viruses.

In vivo imaging reveals novel replication sites of a highly oncogenic avian herpesvirus in chickens

In vivo bioluminescence imaging facilitates the non-invasive visualization of biological processes in living animals. This system has been used to track virus infections mostly in mice and ferrets; however, until now this approach has not been applied to pathogens in avian species. To visualize the infection of an important avian pathogen, we generated Marek’s disease virus (MDV) recombinants expressing firefly luciferase during lytic replication. Upon characterization of the recombinant viruses in vitro, chickens were infected and the infection visualized in live animals over the course of 14 days. The luminescence signal was consistent with the known spatiotemporal kinetics of infection and the life cycle of MDV, and correlated well with the viral load measured by qPCR. Intriguingly, this in vivo bioimaging approach revealed two novel sites of MDV replication, the beak and the skin of the feet covered in scales. Feet skin infection was confirmed using a complementary fluorescence bioimaging approach with MDV recombinants expressing mRFP or GFP. Infection was detected in the intermediate epidermal layers of the feet skin that was also shown to produce infectious virus, regardless of the animals’ age at and the route of infection. Taken together, this study highlights the value of in vivo whole body bioimaging in avian species by identifying previously overlooked sites of replication and shedding of MDV in the chicken host.

In vivo bioluminescence imaging is a powerful tool to track virus infection in the whole body of living animals. This system has been successfully used in mice, ferrets, rats and even fishes, but until now never in birds. In this study, we performed the first in vivo imaging assessing the spread of an important avian pathogen, the highly oncogenic Marek’s disease virus (MDV). Using a recombinant virus expressing firefly luciferase, we visualized the course of MDV infection in chicks for 14 days. The bioluminescent signal was consistent with the known kinetics and sites of dissemination of MDV, notably in feathers. With this new approach, we also discovered two novels sites of early infection and replication that may contribute to persistent virus shedding. Both novel sites represent hard skin appendages like the feathers: the beak and the skin of the feet that are covered in scales. These results were confirmed with two recombinant viruses expressing fluorescent proteins. Fifty-five years after the discovery of MDV and thanks to in vivo imaging, we provide new insights in MDV life cycle in vivo, highlighting the importance of bioluminescence imaging of the entire body in living animals.

Intra vitam assays for detecting fish infected with cyprinid herpesvirus 3 (CyHV-3)

Koi carp is one of the most sensitive variants of common carp Cyprinus carpio to cyprinid herpesvirus 3, commonly known as koi herpesvirus (KHV). Given that this species is traded at high prices throughout the world, intra vitam assays for detecting KHV in targeted fish with a high detection efficiency are essential. In this study, 4 intra vitam assays were compared with regard to their efficiency of detecting KHV in koi carp on each day after viral exposure via experimental infection. The results indicated that PCR from the gills and scales sampled by biopsy using dissecting scissors and forceps, respectively, can detect KHV for apparently longer periods than the other assays. This study also suggests that a PCR detection assay for environmental samples could be developed as a convenient intra vitam assay to confirm the presence of virus in environments inhabited by virus-shedding fish.

Don't Let It Get Under Your Skin! - Vaccination Protects the Skin Barrier of Common Carp From Disruption Caused by Cyprinid Herpesvirus 3

Vaccination is the best form of protecting fish against viral diseases when the pathogen cannot be contained by biosecurity measures. Vaccines based on live attenuated viruses seem to be most effective for vaccination against challenging pathogens like Cyprinid herpesvirus 3. However, there are still knowledge gaps how these vaccines effectively protect fish from the deadly disease caused by the epitheliotropic CyHV-3, and which aspects of non-direct protection of skin or gill integrity and function are important in the aquatic environment. To elucidate some elements of protection, common carp were vaccinated against CyHV-3 using a double deletion vaccine virus KHV-T ΔDUT/TK in the absence or presence of a mix of common carp beta-defensins 1, 2 and 3 as adjuvants. Vaccination induced marginal clinical signs, low virus load and a minor upregulation of cd4, cd8 and igm gene expression in vaccinated fish, while neutralisation activity of blood serum rose from 14 days post vaccination (dpv). A challenge infection with CyHV-3 induced a severe disease with 80-100% mortality in non-vaccinated carp, while in vaccinated carp, no mortality was recorded and the virus load was >1,000-fold lower in the skin, gill and kidney. Histological analysis showed strongest pathological changes in the skin, with a complete destruction of the epidermis in non-vaccinated carp. In the skin of non-vaccinated fish, T and B cell responses were severely downregulated, inflammation and stress responses were increased upon challenge, whereas vaccinated fish had boosted neutrophil, T and B cell responses. A disruption of skin barrier elements (tight and adherence junction, desmosomes, mucins) led to an uncontrolled increase in skin bacteria load which most likely exacerbated the inflammation and the pathology. Using a live attenuated virus vaccine, we were able to show that increased neutrophil, T and B cell responses provide protection from CyHV-3 infection and lead to preservation of skin integrity, which supports successful protection against additional pathogens in the aquatic environment which foster disease development in non-vaccinated carp.

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.

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

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

Development 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.

Respiratory Tract Disorders in Fishes

"The piscine respiratory system is represented by gills. Gill diseases are extremely common and may be caused by a large variety of etiologic agents. The gills are in direct contact with water and reflect its quality, for example, pollution, and they also must face the presence of biotic agents, such as viruses, bacteria, fungi, and parasites. Evolution has established many defense mechanisms to combat these agents. Failure of these mechanisms is life-threatening for the fish, due to impaired respiration. Gills are relatively easily accessible for clinical examination and sampling, which facilitates intravital diagnosis."

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.

Seawater transmission and infection dynamics of pilchard orthomyxovirus (POMV) in Atlantic salmon (Salmo salar)

The Tasmanian salmon industry had remained relatively free of major viral diseases until the emergence of pilchard orthomyxovirus (POMV). Originally isolated from wild pilchards, POMV is of concern to the industry as it can cause high mortality in farmed salmon (Salmo salar). Field observations suggest the virus can spread from pen to pen and between farms, but evidence of passive transmission in sea water was unclear. Our aim was to establish whether direct contact between infected and naïve fish was required for transmission, and to examine viral infection dynamics. Atlantic salmon post-smolts were challenged with POMV by either direct exposure via cohabitation or indirect exposure via virus-contaminated sea water. POMV was transmissible in sea water and direct contact between fish was not required for infection. Head kidney and heart presented the highest viral loads in early stages of infection. POMV survivors presented low viral loads in most tissues, but these remained relatively high in gills. A consistent feature was the infiltration of viral-infected melanomacrophages in different tissues, suggesting an important role of these in the immune response to POMV. Understanding POMV transmission and host-pathogen interactions is key for the development of improved surveillance tools, transmission models and ultimately for disease prevention.

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.

Updates on Selected Emerging Infectious Diseases of Ornamental Fish

Emerging infectious diseases of ornamental fish are discussed with special focus on clinical relevance, detection, and treatment, where applicable. Important emerging infectious diseases of fish include goldfish herpesvirus, koi herpesvirus, carp edema virus, Erysipelothrix, Edwardsiella ictaluri, Edwardseilla piscicida, and Francisella. Some diseases are more species or genus specific, but many emerging diseases do not seem to have a species preference and affect a variety of species worldwide. Proper husbandry and biosecurity with a disease detection plan for ornamental fish is essential to monitor and prevent future outbreaks.

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.

Disease Progression in Lake Trout (Salvelinus namaycush) Experimentally Infected With Epizootic Epitheliotropic Disease Virus (Salmonid Herpesvirus-3)

Epizootic epitheliotropic disease virus (salmonid herpesvirus-3; EEDV) is responsible for the death of millions of hatchery-raised lake trout (Salvelinus namaycush) in the Laurentian Great Lakes Basin. However, little is known about its biology, pathology, tropism, and host interactions. In this study, the presence and disease progression of EEDV were evaluated following exposure of naïve juvenile lake trout to EEDV via bath immersion under controlled laboratory conditions (n = 84 infected; n = 44 control). Individual tissues (n = 10 per fish), collected over 6 weeks, were analyzed for viral load by quantitative polymerase chain reaction, gross and histopathologic changes, and virus cellular targets using in situ hybridization. Skin, fin, and ocular tissues were the earliest viral targets and yielded the highest viral loads throughout the course of infection. Early gross lesions included exophthalmia, ocular hemorrhage, fin congestion, and hyperemia of visceral blood vessels. Advanced disease was characterized by multifocal to coalescing erosions and ulcerations of the skin, and congestion of visceral organs. Microscopically, there was cellular degeneration and necrosis in the epidermis and spleen, and lymphohistiocytic perivasculitis of the dermis, omentum, and the epicardium. EEDV DNA was first detected by in situ hybridization in epithelial cells of the epidermis, with subsequent labeling in the epithelial lining of primary and secondary gill lamellae. During advanced disease, EEDV was detected in endothelial and dendritic cells as well as blood monocytes. This study characterized EEDV tissue tropism and associated pathologic features, to guide research aimed at understanding EEDV disease ecology and improving strategies for disease control.

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.

Biocontrol of the Common Carp (Cyprinus carpio) in Australia: A Review and Future Directions

Invasive pest species are recognized as one of the important drivers of reduced global biodiversity. In Australia, the 267 invasive plant, animal and microbial species, established since European colonization in the 1770s, have been unequivocally declared the most important threat to species diversity in this country. One invasive pest, the common carp (Cyprinus carpio), has been targeted in an integrated pest management plan that might include cyprinid herpesvirus 3 (CyHV-3) as a potential biocontrol agent. The species-specificity of the released virus (and of field variants that will inevitably arise) has been assessed, and the virus judged to be safe. It has also been hypothesised that, because the virulence of the CyHV-3 will likely decline following release, the virus should be used strategically: initially, the aim would be to markedly reduce numbers of carp in naive populations, and then some other, as yet uncertain, complementary broad-scale control measure would knock-down carp numbers even further. Brief results are included from recent studies on the modelling of release and spread of the virus, the ecological and social concerns associated with virus release, and the restoration benefits that might be expected following carp control. We conclude that, while further work is required (on the virus, the target species, environmental issues, and especially the identification of a suitable broad-scale complementary control measure), optimism must prevail in order to ensure an eventual solution to this important environmental problem.

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.

Protective immunity against CyHV-3 infection via different prime-boost vaccination regimens using CyHV-3 ORF131-based DNA/protein subunit vaccines in carp Cyprinus carpio var. Jian

Cyprinid Herpesvirus 3 (CyHV-3), also known as Koi Herpesvirus (KHV), causes Koi Herpesvirus Disease (KHVD) which leads to serious economic losses worldwide. To exploit DNA/subunit vaccine candidates, CyHV-3 ORF131 gene and cDNA was cloned and analyzed in the present study. Major B cell epitopes of deduced CyHV-3 pORF131 was also predicted. Then the complete CDS of CyHV-3 ORF131 was inserted into pEGFP-N1 vector and a modified pYD1/EBY100 system to construct the DNA and subunit vaccine, respectively. Subsequently, carp were immunized with homologous and heterologous prime-boost regimens relying on the constructed DNA and oral subunit vaccines. Then the protective immunity generated from different vaccines and regimens as well as the capacity of yeast (Saccharomyces cerevisiae) as an oral vaccine vehicle was evaluated. Our study confirmed that CyHV-3 ORF131 gene consisted of 2 introns and 3 exons encoding a 428 amino acids peptide. Further analysis indicated that four fragments of CyHV-3 pORF131 contained the major B cell epitopes (Cys20~Val140, Ser169~Tyr245, Thr258~Pro390, Phe414~Gln428), which could be linked and expressed in E. coli (BL21) as a truncated pORF131. The expression of full-length CyHV-3 pORF131 by pEGFP-N1 and yeast surface display was verified by In vitro assays before vaccination. Immunization of carp with CyHV-3 ORF131 DNA and subunit vaccines could evoke the activation of immune-related genes such as CXCa, CXCR1, IL-1β, TNF-α, INF-a1, Mx-1, IgM, IgT1 and production of specific serum IgM measured by ELISA. RPS (relative percent of survival) ranging from 53.33% to 66.67% was acquired post challenge test. Moreover, flow cytometry analysis illustrated the delivery of surface-displayed CyHV-3 pORF131 to midgut after oral gavage. Thus, our findings suggest that CyHV-3 ORF131 can serve as DNA/subunit vaccines candidate and the yeast as an ideal oral vaccine vehicle.

First detection of koi herpesvirus and carp oedema virus in Iraq associated with a mass mortality in common carp (Cyprinus carpio)

At the end of October 2018, a mass fish mortality occurred in Iraq, involving thousands of tons of cultured and wild common carp (Cyprinus carpio) along Euphrates and Tigris rivers. Fish were found dead or moribund along rivers coasts, showing lethargy, dyspnoea and flared gills. At necropsy, discoloration patches were noticed on the gills. Wet preparations showed rare metacercariae and Dactylogyrus spp. Samples were subjected to bacteriological tests and virological investigation through real-time PCR and nested PCR. Both were positive for koi herpesvirus (KHV) and carp oedema virus. Results obtained were confirmed by the OIE reference laboratory of KHV disease (KHVD) at Cefas (UK) and by sequence analysis. This is the first report on the detection of both viruses in Iraq.

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.

Cyprinid viral diseases and vaccine development

In the past decades, global freshwater fish production has been rapidly growing, while cyprinid takes the largest portion. Along with the rapid rise of novel forms of intensive aquaculture, increased global aquatic animal movement and various anthropogenic stress to aquatic ecosystems during the past century, freshwater fish farming industry encounter the emergence and breakout of many diseases, especially viral diseases. Because of the ability to safely and effectively prevent aquaculture diseases, vaccines have become the mainstream technology for prevention and control of aquatic diseases in the world. In this review, authors summarized six major cyprinid viral diseases, including koi herpesvirus disease (KHVD), spring viraemia of carp (SVC), grass carp hemorrhagic disease (GCHD), koi sleepy disease (KSD), carp pox disease (CPD) and herpesviral haematopoietic necrosis (HPHN). The present review described the characteristics of these diseases from epidemiology, pathology, etiology and diagnostics. Furthermore, the development of specific vaccines respective to these diseases is stated according to preparation methods and immunization approaches. It is hoped that the review could contribute to aquaculture in prevention and controlling of cyprinid viral diseases, and serve the healthy and sustainable development of aquaculture industry.

Betanodavirus infection in bath-challenged Solea senegalensis juveniles: A comparative analysis of RGNNV, SJNNV and reassortant strains

Senegalese sole has been shown to be highly susceptible to betanodavirus infection, although virulence differences were observed between strains. To study the mechanisms involved in these differences, we have analysed the replication in brain tissue of three strains with different genotypes during 15 days after bath infection. In addition, possible portals of entry for betanodavirus into sole were investigated. The reassortant RGNNV/SJNNV and the SJNNV strain reached the brain after 1 and 2 days postinfection, respectively. Although no RGNNV replication was detected until day 3-4 postinfection, at the end of the experiment this strain yielded the highest viral load; this is in accordance with previous studies in which sole infected with the reassortant showed more acute signs and earlier mortality than the RGNNV and SJNNV strains. Differences between strains were also observed in the possible portals of entry. Thus, whereas the reassortant strain could infect sole mainly through the skin or the oral route, and, to a minor extent, through the gills, the SJNNV strain seems to enter fish only through the gills and the RGNNV strain could use all tissues indistinctly. Taken together, all these results support the hypothesis that reassortment has improved betanodavirus infectivity for sole.

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.

Genomic and biologic comparisons of cyprinid herpesvirus 3 strains

Cyprinid herpesvirus 3 (CyHV-3) is the archetypal fish alloherpesvirus and the etiologic agent of a lethal disease in common and koi carp. To date, the genome sequences of only four CyHV-3 isolates have been published, but no comparisons of the biologic properties of these strains have been reported. We have sequenced the genomes of a further seven strains from various geographical sources, and have compared their growth in vitro and virulence in vivo. The major findings were: (i) the existence of the two genetic lineages previously described as European and Asian was confirmed, but inconsistencies between the geographic origin and genotype of some strains were revealed; (ii) potential inter-lineage recombination was detected in one strain, which also suggested the existence of a third, as yet unidentified lineage; (iii) analysis of genetic disruptions led to the identification of non-essential genes and their potential role in virulence; (iv) comparison of the in vitro and in vivo properties of strains belonging to the two lineages revealed that inter-lineage polymorphisms do not contribute to the differences in viral fitness observed; and (v) a negative correlation was observed among strains between viral growth in vitro and virulence in vivo. This study illustrates the importance of coupling genomic and biologic comparisons of viral strains in order to enhance understanding of viral evolution and pathogenesis.

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.

Adaptive evolution of interferon regulatory factors is not correlated with body scale reduction or loss in schizothoracine fish

Body scales in teleost fish are the first line of defense in protecting the fish from invading pathogens. However, the relationship between the adaptive evolution of immune-related genes and changes in the body scale-covering of fish has not been previously studied. Schizothoracine fish, characterized by progressive reduction of body scales from the primitive to the highly specialized species, are a good group to investigate this relationship. We obtained 11 IRF genes (IRF1-11) from 14 of schizothoracine fish representing primitive, specialized, and highly specialized species, of which seven IRF genes (IRF2, IRF3, IRF5, IRF6, IRF7, IRF8 and IRF9) contained the complete CDS. Sequence analysis demonstrated the deletion or insertion of 4-7 amino acids in IRF2, IRF3, IRF6 and IRF9, which seems to be a common phenomenon in the schizothoracine fish. Selection pressure analysis supported the hypothesis that positive selection has been driving the rapid evolution of IRFs in specific lineages of the schizothoracine fish. This, however, is not correlated with body scale reduction or loss in the evolution of these IRFs. Remarkably, the deletion or insertion found in IRF protein sequences presented a regular pattern corresponding to the scale-covering changes in schizothoracine fish. Our study provides evidence for positive selection in the IRF family, contributing to a better understanding of the adaptive evolution of immune-related genes in schizothoracine fish in response to environmental changes of the Qinghai-Tibetan Plateau.

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.

A simple and non-invasive method for analyzing local immune responses in vivo using fish fin

Immunocompetence is an important parameter that reflects disease resistance in fish. Very few methods to examine immunocompetence in vivo have been developed, even for mammals. In the present study, we present a unique method for analyzing local immune responses using fish fin. We first demonstrated the migration of granulocytes to the site of zymosan injection in fin in a dose-dependent manner and that this could be easily observed macroscopically due to the fin membrane transparency. We also demonstrated phagocytic activity of accumulated leukocytes after zymosan administration and that almost all phagocytes were granulocytes. In addition, we succeeded to detect respiratory burst activity of granulocytes in vivo without any in vitro treatment of cells, indicating that our present method is suitable for the analysis of granulocyte phagocytic function in vivo. The method provides a unique tool applicable for fishes that possess transparent fins and may lead to better understanding of the mechanisms of local immune responses in fish.

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.

Identification of an essential virulence gene of cyprinid herpesvirus 3

The genus Cyprinivirus consists of a growing list of phylogenetically related viruses, some of which cause severe economic losses to the aquaculture industry. The archetypal member, cyprinid herpesvirus 3 (CyHV-3) causes mass mortalities worldwide in koi and common carp. A CyHV-3 mutant was described previously that is attenuated in vivo by a deletion affecting two genes (ORF56 and ORF57). The relative contributions of ORF56 and ORF57 to the safety and efficacy profile of this vaccine candidate have now been assessed by analysing viruses individually deleted for ORF56 or ORF57. Inoculation of these viruses into carp demonstrated that the absence of ORF56 did not affect virulence, whereas the absence of ORF57 led to an attenuation comparable to, though slightly less than, that of the doubly deleted virus. To demonstrate further the role of ORF57 as a key virulence factor, a mutant retaining the ORF57 region but unable to express the ORF57 protein was produced by inserting multiple in-frame stop codons into the coding region. Analysis of this virus in vivo revealed a safety and efficacy profile comparable to that of the doubly deleted virus. These findings show that ORF57 encodes an essential CyHV-3 virulence factor. They also indicate that ORF57 orthologues in other cypriniviruses may offer promising targets for the rational design of attenuated recombinant vaccines.

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Cyprinid herpesvirus 3 (CyHV-3) causes a lethal disease in common and koi carp and is the archetypal fish alloherpesvirus.

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CyHV-3 ORF57 encodes an essential virulence factor and ORF57 deleted viruses represent attenuated vaccine candidates.

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ORF57 orthologues in other alloherpesviruses may offer promising targets for the design of attenuated recombinant vaccines.

Assessment of listing and categorisation of animal diseases within the framework of the Animal Health Law (Regulation (EU) No 2016/429): Koi herpes virus disease (KHV)

Koi herpes virus (KHV) disease has been assessed according to the criteria of the Animal Health Law (AHL), in particular criteria of Article 7 on disease profile and impacts, Article 5 on the eligibility of KHV disease to be listed, Article 9 for the categorisation of KHV disease according to disease prevention and control rules as in Annex IV and Article 8 on the list of animal species related to KHV disease. The assessment has been performed following a methodology composed of information collection and compilation, expert judgement on each criterion at individual and, if no consensus was reached before, also at collective level. The output is composed of the categorical answer, and for the questions where no consensus was reached, the different supporting views are reported. Details on the methodology used for this assessment are explained in a separate opinion. According to the assessment performed, it is inconclusive whether KHV disease can be considered eligible to be listed for Union intervention as laid down in Article 5(3) of the AHL because there was no full consensus on the criterion 5 A(v). Consequently, the assessment on compliance of KHV disease with the criteria as in Annex IV of the AHL, for the application of the disease prevention and control rules referred to in Article 9(1) is also inconclusive, as well as which animal species can be considered to be listed for KHV disease according to Article 8(3) of the AHL.

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.

Cyprinid herpesvirus 3 and its evolutionary future as a biological control agent for carp in Australia

Biological invasions are a major threat to global biodiversity. Australia has experienced many invasive species, with the common carp (Cyprinus carpio L.) a prominent example. Cyprinid herpesvirus 3 (CyHV-3) has been proposed as a biological control (biocontrol) agent for invasive carp in Australia. Safety and efficacy are critical factors in assessing the suitability of biocontrol agents, and extensive host-specificity testing suggests that CyHV-3 is safe. Efficacy depends on the relationship between virus transmissibility and virulence. Based on observations from natural outbreaks, as well as the biology of virus-host interactions, we hypothesize that (i) close contact between carp provides the most efficient transmission of virus, (ii) transmission occurs at regular aggregations of carp that favour recrudescence of latent virus, and (iii) the initially high virulence of CyHV-3 will decline following its release in Australia. We also suggest that the evolution of carp resistance to CyHV-3 will likely necessitate the future release of progressively more virulent strains of CyHV-3, and/or an additional broad-scale measure(s) to complement the effect of the virus. If the release of CyHV-3 does go ahead, longitudinal studies are required to track the evolution of a virus-host relationship from its inception, and particularly the complex interplay between transmission, virulence and host resistance.

The zebrafish galectins Drgal1-L2 and Drgal3-L1 bind in vitro to the infectious hematopoietic necrosis virus (IHNV) glycoprotein and reduce viral adhesion to fish epithelial cells

The infectious hematopoietic necrosis virus (IHNV; Rhabdoviridae, Novirhabdovirus) infects teleost fish, such as salmon and trout, and is responsible for significant losses in the aquaculture industry and in wild fish populations. Although IHNV enters the host through the skin at the base of the fins, the viral adhesion and entry mechanisms are not fully understood. In recent years, evidence has accumulated in support of the key roles played by protein-carbohydrate interactions between host lectins secreted to the extracellular space and virion envelope glycoproteins in modulating viral adhesion and infectivity. In this study, we assessed in vitro the potential role(s) of zebrafish (Danio rerio) proto type galectin-1 (Drgal1-L2) and a chimera galectin-3 (Drgal3-L1) in IHNV adhesion to epithelial cells. Our results suggest that the extracellular Drgal1-L2 and Drgal3-L1 interact directly and in a carbohydrate-dependent manner with the IHNV glycosylated envelope and glycans on the epithelial cell surface, significantly reducing viral adhesion.

Infectious salmon anaemia virus (ISAV) mucosal infection in Atlantic salmon

All viruses infecting fish must cross the surface mucosal barrier to successfully enter a host. Infectious salmon anaemia virus (ISAV), the causative agent of the economically important infectious salmon anaemia (ISA) in Atlantic salmon, Salmo salar L., has been shown to use the gills as its entry point. However, other entry ports have not been investigated despite the expression of virus receptors on the surface of epithelial cells in the skin, the gastrointestinal (GI) tract and the conjunctiva. Here we investigate the ISAV mucosal infection in Atlantic salmon after experimental immersion (bath) challenge and in farmed fish collected from a confirmed outbreak of ISA in Norway. We show for the first time evidence of early replication in several mucosal surfaces in addition to the gills, including the pectoral fin, skin and GI tract suggesting several potential entry points for the virus. Initially, the infection is localized and primarily infecting epithelial cells, however at later stages it becomes systemic, infecting the endothelial cells lining the circulatory system. Viruses of low and high virulence used in the challenge revealed possible variation in virus progression during infection at the mucosal surfaces.

The Genome of a Tortoise Herpesvirus (Testudinid Herpesvirus 3) Has a Novel Structure and Contains a Large Region That Is Not Required for Replication In Vitro or Virulence In Vivo

Testudinid herpesvirus 3 (TeHV-3) is the causative agent of a lethal disease affecting several tortoise species. The threat that this virus poses to endangered animals is focusing efforts on characterizing its properties, in order to enable the development of prophylactic methods. We have sequenced the genomes of the two most studied TeHV-3 strains (1976 and 4295). TeHV-3 strain 1976 has a novel genome structure and is most closely related to a turtle herpesvirus, thus supporting its classification into genus Scutavirus, subfamily Alphaherpesvirinae, family Herpesviridae. The sequence of strain 1976 also revealed viral counterparts of cellular interleukin-10 and semaphorin, which have not been described previously in members of subfamily Alphaherpesvirinae. TeHV-3 strain 4295 is a mixture of three forms (m1, m2, and M), in which, in comparison to strain 1976, the genomes exhibit large, partially overlapping deletions of 12.5 to 22.4 kb. Viral subclones representing these forms were isolated by limiting dilution assays, and each replicated in cell culture comparably to strain 1976. With the goal of testing the potential of the three forms as attenuated vaccine candidates, strain 4295 was inoculated intranasally into Hermann's tortoises (Testudo hermanni). All inoculated subjects died, and PCR analyses demonstrated the ability of the m2 and M forms to spread and invade the brain. In contrast, the m1 form was detected in none of the organs tested, suggesting its potential as the basis of an attenuated vaccine candidate. Our findings represent a major step toward characterizing TeHV-3 and developing prophylactic methods against it.

IMPORTANCE

Testudinid herpesvirus 3 (TeHV-3) causes a lethal disease in tortoises, several species of which are endangered. We have characterized the viral genome and used this information to take steps toward developing an attenuated vaccine. We have sequenced the genomes of two strains (1976 and 4295), compared their growth in vitro, and investigated the pathogenesis of strain 4295, which consists of three deletion mutants. The major findings are that (i) TeHV-3 has a novel genome structure, (ii) its closest relative is a turtle herpesvirus, (iii) it contains interleukin-10 and semaphorin genes (the first time these have been reported in an alphaherpesvirus), (iv) a sizeable region of the genome is not required for viral replication in vitro or virulence in vivo, and (v) one of the components of strain 4295, which has a deletion of 22.4 kb, exhibits properties indicating that it may serve as the starting point for an attenuated vaccine.

Pathogenesis of acute and chronic diseases caused by cyprinid herpesvirus-3

The pathogenesis of cyprinid herpesvirus-3 (CyHV-3) was studied using different lineages of carp/koi. After exposure to the virus, infected cells were first found in the skin by histopathology and by in situ hybridization. The epidermis of the skin was most severely damaged and often sloughed off in the fish sampled on days 5 through 8, and the fish that were highly sensitive to the virus died within 8 or 10 days after infection. Serum osmolality of the infected fish, particularly just before death, was significantly lower, suggesting that the osmotic shock consequent on the damage to the skin was the direct cause of the acute deaths. On the other hand, clinical and histopathological observations indicate that the carp of a less sensitive lineage most probably died of viral encephalitis around 3 weeks after infection. For these fish, the largest number of infected cells was found in the central nervous system (CNS) sampled on day 12. A substantial amount of viral genome was found in the CNS of carp surviving more than 1 year after the infection. Thus, the CNS is probably a major target for CyHV-3, and the virus can persistently infect the CNS, presumably establishing latency.

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.

Recombinant lactobacillus expressing G protein of spring viremia of carp virus (SVCV) combined with ORF81 protein of koi herpesvirus (KHV): A promising way to induce protective immunity against SVCV and KHV infection in cyprinid fish via oral vaccination

Spring viremia of carp virus (SVCV) and koi herpesvirus (KHV) are highly contagious and pathogenic to cyprinid fish, causing enormous economic losses in aquaculture. Although DNA vaccines reported in recent years could induce protective immune responses in carps against these viruses via injection, there are a number of consequences and uncertainties related to DNA vaccination. Therefore, more effective and practical method to induce protective immunity such as oral administration would be highly desirable. In this study, we investigated the utilities of a genetically engineered Lactobacillus plantarum (L. plantarum) coexpressing glycoprotein (G) of SVCV and ORF81 protein of KHV as oral vaccine to induce protective immunity in carps via oral vaccination. The surface-displayed recombinant plasmid pYG-G-ORF81 was electroporated into L. plantarum, giving rise to LP/pYG-G-ORF81, where expression and localization of G-ORF81 fusion protein from the LP/pYG-G-ORF81 was identified by SDS-PAGE, Western blotting and immunofluorescence assay. Bait feed particles containing the LP/pYG-G-ORF81 were used as vaccine to immunize carps via gastrointestinal route. Compared to control groups, the carps orally immunized with the LP/pYG-G-ORF81 were induced significant levels of immunoglobulin M (IgM), and its immunogenicity was confirmed by viral loads reduction detected by PCR assay after virus challenge followed by an effective protection rate 71% in vaccinated carps and 53% in vaccinated koi until at days 65 post challenge, respectively. Our study here demonstrates, for the first time, the ability of recombinant L. plantarum as oral vaccine against SVCV and KHV infection in carps, suggesting a practical multivalent strategy for the control of spring viremia of carp and koi herpesvirus disease.

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.