Establishment of rock bream Oplegnathus fasciatus embryo (RoBE-4) cells with cytolytic infection of red seabream iridovirus (RSIV)

Evaluation of the diagnostic assays detecting red sea bream iridovirus infection at different severity levels

Red sea bream iridovirus (RSIV) is a highly contagious viral infection that affects various fish species and poses a significant threat to the global aquaculture industry. Thus, accurate and timely diagnosis is paramount for sustainable management of fish health. This study rigorously evaluated the diagnostic efficacy of various polymerase chain reaction (PCR) assays, focusing on those recommended by the World Organization for Animal Health (WOAH) and the assays newly proposed by WOAH's Aquatic Animals Health Standards Commission. Specifically, this study assessed conventional PCR, nested PCR, modified 1-F/1-R, and real-time PCR assays using a 95% limit of detection (LoD95%), as well as diagnostic sensitivity (DSe) and specificity (DSp) tests across different RSIV severity grades (G0-G4). In previous studies, the LoD95% for the 1-F/1-R and 4-F/4-R conventional assays were 225.81 and 328.7 copies/reaction, respectively. The modified 1-F/1-R exhibited a lower LoD95% of 51.32 copies/reaction. Notably, the nested PCR had an LoD95% of 11.23 copies/reaction, and the real-time PCR assay had an LoD95% of 12.02 copies/reaction. The DSe varied across RSIV severity grades, especially in the lower G0-G2 grades. The nested PCR and modified 1-F/1-R assays displayed the highest DSe, making them particularly useful for early-stage screening and detection of asymptomatic carriers. In addition, the PCR assays did not cross-react with any other aquatic pathogens except RSIV. Our findings significantly advanced the diagnostic capabilities of RSIVD by suggesting that nested PCR and modified 1-F/1-R assays are particularly promising for early detection. We propose their inclusion in future WOAH guidelines for a more comprehensive diagnostic framework.

Megalocytivirus and Other Members of the Family Iridoviridae in Finfish: A Review of the Etiology, Epidemiology, Diagnosis, Prevention and Control

Aquaculture has expanded to become the fastest growing food-producing sector in the world. However, its expansion has come under threat due to an increase in diseases caused by pathogens such as iridoviruses commonly found in aquatic environments used for fish farming. Of the seven members belonging to the family Iridoviridae, the three genera causing diseases in fish comprise ranaviruses, lymphocystiviruses and megalocytiviruses. These three genera are serious impediments to the expansion of global aquaculture because of their tropism for a wide range of farmed-fish species in which they cause high mortality. As economic losses caused by these iridoviruses in aquaculture continue to rise, the urgent need for effective control strategies increases. As a consequence, these viruses have attracted a lot of research interest in recent years. The functional role of some of the genes that form the structure of iridoviruses has not been elucidated. There is a lack of information on the predisposing factors leading to iridovirus infections in fish, an absence of information on the risk factors leading to disease outbreaks, and a lack of data on the chemical and physical properties of iridoviruses needed for the implementation of biosecurity control measures. Thus, the synopsis put forth herein provides an update of knowledge gathered from studies carried out so far aimed at addressing the aforesaid informational gaps. In summary, this review provides an update on the etiology of different iridoviruses infecting finfish and epidemiological factors leading to the occurrence of disease outbreaks. In addition, the review provides an update on the cell lines developed for virus isolation and culture, the diagnostic tools used for virus detection and characterization, the current advances in vaccine development and the use of biosecurity in the control of iridoviruses in aquaculture. Overall, we envision that the information put forth in this review will contribute to developing effective control strategies against iridovirus infections in aquaculture.

A New Cell Line Derived from the Caudal Fin of the Dwarf Gourami (Trichogaster lalius) and Its Susceptibility to Fish Viruses

The detection of megalocytiviruses, especially the infectious spleen and kidney necrosis virus (ISKNV), in ornamental fish has increased with the rapid growth of the ornamental fish industry. In this study, dwarf gourami fin (DGF) cells derived from the caudal fin of the dwarf gourami (Trichogaster lalius), which is highly susceptible to red sea bream iridovirus (RSIV) and ISKNV, were established and characterized. The DGF cells were grown at temperatures ranging from 25 °C to 30 °C in Leibovitz's L-15 medium supplemented with 15% fetal bovine serum and were subcultured for more than 100 passages, predominantly with epithelial-like cells. DGF cells had a diploid chromosome number of 2n = 44. Although the initial purpose of this study was to establish a cell line for the causative agents of red sea bream iridoviral disease (RSIV and ISKNV), DGF cells were also susceptible to rhabdoviruses (viral hemorrhagic septicemia virus, hirame rhabdovirus, and spring viraemia of carp virus), exhibiting a significant cytopathic effect characterized by cell rounding and lysis. Additionally, viral replication and virion morphology were confirmed using virus-specific conventional polymerase chain reaction and transmission electron microscopy. Furthermore, both RSIV and ISKNV were replicated at high concentrations in DGF cells compared to other cell lines. Notably, the DGF cells maintained a monolayer during ISKNV infection, indicating the possibility of persistent infection. Thus, DGF can be used for viral diagnosis and may play a critical role in advancing our understanding of ISKNV pathogenesis.

Red Sea Bream Iridovirus Kinetics, Tissue Tropism, and Interspecies Horizontal Transmission in Flathead Grey Mullets (Mugil cephalus)

Red sea bream iridovirus (RSIV) causes significant economic losses in the aquaculture industry. We analyzed the pathogenicity of RSIV in flathead grey mullets (Mugil cephalus), the correlation of histopathological lesions, and interspecies horizontal transmission, through immersion infection and cohabitation challenges. Flathead grey mullets, which were challenged by immersion infection, exhibited mortality at 14 and 24 days after RSIV exposure. Viral shedding in seawater peaked 2-3 days before or after the observed mortality. Specific lesions of RSIV were observed in the spleen and kidney, and the correlation between histopathological grade and viral load was the highest in the spleen. In a cohabitation challenge, flathead grey mullets were the donors, and healthy rock bream, red sea bream, and flathead grey mullets were the recipients. Viral shedding in seawater was the highest in flathead grey mullet and rock bream at 25 °C, with 10^6.0 RSIV copies L/g at 14 dpi. No mortality was observed in any group challenged at 15 °C, and no RSIV was detected in seawater after 30 dpi. The virus shed from RSIV-infected flathead grey mullets caused horizontal transmission through seawater. These findings suggest that rapid decision-making is warranted when managing disease in fish farms.

Impact of Red Sea Bream Iridovirus Infection on Rock Bream (Oplegnathus fasciatus) and Other Fish Species: A Study of Horizontal Transmission

Red sea bream iridovirus (RSIV) causes significant economic losses in aquaculture. Here, we analyzed the pathogenicity, viral shedding, and transmission dynamics of RSIV in rock bream (Oplegnathus fasciatus) by employing immersion infection and cohabitation challenge models. Rock bream challenged by immersion exposure exhibited 100% mortality within 35 days post RSIV exposure, indicating that the viral shedding in seawater peaked after mortality. At 25 °C, a positive correlation between the viral loads within infected rock bream and virus shedding into the seawater was observed. Specific RSIV lesions were observed in the spleen and kidney of the infected rock bream, and the viral load in the spleen had the highest correlation with the histopathological grade. A cohabitation challenge mimicking the natural transmission conditions was performed to assess the virus transmission and determine the pathogenicity and viral load. The RSIV-infected rock breams (donors) were cohabited with uninfected rock bream, red sea bream (Pagrus major), and flathead grey mullet (Mugil cephalus) (recipients) at both 25 °C and 15 °C. In the cohabitation challenge group maintained at 15 °C, no mortality was observed across all experimental groups. However, RSIV was detected in both seawater and the recipient fish. Our results provide preliminary data for further epidemiological analyses and aid in the development of preventive measures and management of RSIVD in aquaculture.

Partial validation of a TaqMan quantitative polymerase chain reaction for the detection of the three genotypes of Infectious spleen and kidney necrosis virus

Megalocytiviruses (MCVs) are double-stranded DNA viruses known to infect important freshwater and marine fish species in the aquaculture, food, and ornamental fish industries worldwide. Infectious spleen and kidney necrosis virus (ISKNV) is the type species within the genus Megalocytivirus that causes red seabream iridoviral disease (RSIVD) which is a reportable disease to the World Animal Health Organization (WOAH). To better control the transboundary spread of this virus and support WOAH reporting requirements, we developed and partially validated a TaqMan real-time qPCR assay (ISKNV104R) to detect all three genotypes of ISKNV, including the two genotypes that cause RSIVD. Parameters averaged across 48 experiments used a 10-fold dilution series of linearized plasmid DNA (107-101 copies), carrying a fragment of the three-spot gourami iridovirus (TSGIV) hypothetical protein revealed that the assay was linear over 7 orders of magnitude (107-101), a mean efficiency of 99.97 ± 2.92%, a mean correlation coefficient of 1.000 ± 0.001, and a limit of detection (analytical sensitivity) of ≤10 copies of TSGIV DNA. The diagnostic sensitivity and specificity for the ISKNV104R qPCR assay was evaluated and compared to other published assays using a panel of 397 samples from 21 source populations with different prevalence of ISKNV infection (0-100%). The diagnostic sensitivity and specificity for the ISKNV104R qPCR assay was 91.99% (87.28-95.6; 95% CI) and 89.8% (83.53-94.84). The latent class analysis showed that the ISKNV104R qPCR assay had similar diagnostic sensitivities and specificities with overlapping confidence limits compared to a second TaqMan qPCR assay and a SYBR green assay. This newly developed TaqMan assay represents a partially validated qPCR assay for the detection of the three genotypes of the species ISKNV. The ISKNV104R qPCR assay once fully validated, will serve as an improved diagnostic tool that can be used for ISKNV surveillance efforts and diagnosis in subclinical fish to prevent further spread of MCVs throughout the aquaculture and ornamental fish industries.

Characterization of three novel cell lines derived from the brain of spotted sea bass: Focusing on cell markers and susceptibility toward iridoviruses

Despite tens of cell lines originating from fish brain tissue have been constructed, little is known about the definite cell types they belong to. Whether fish cell lines derived from the brain shares similar characteristics is not well-answered yet. Here, we constructed three cell lines designated as LMB-S, LMB-M, LMB-L using brain tissue of spotted sea bass (Lateolabrax maculatus). Among them, LMB-L was identified as astroglia-like cells considering the high expression of GFAP, DCX, PTX, S100b, which are regarded as astrocyte-specific or astrocyte-associated cell markers. LMB-M exhibited smooth muscle-like features showing strong expression of LMOD1, SLAMP, M-cadherin, MGP, which are confirmed as muscle-restricted or myogenesis-involved cell markers. Although LMB-S was not definitely identified, it appeared an activation of WNT/β-catenin pathway. Besides the distinct expression profiles of cell markers, the three cell lines also presented differences in transfection efficiency and susceptibility to iridovirus infection. Relying on the established cell lines, a novel megalocytivirus, named LMIV (Lateolabrax maculatus iridovirus), was first isolated from diseased spotted sea bass. Genetic analysis of major capsid protein (MCP) and adenosine triphosphatase (ATPase) manifested that LMIV was clearly distinguishable from other representative teleost iridoviruses. Further investigations revealed that LMIV could replicate most efficiently in LMB-L cells obtaining the highest viral load (2.16 × 10¹⁰ copy/mL). By contrast, LMB-S cells gave rise to the highest viral load up to 3.86 × 10⁸ copy/mL, when the three cell lines were infected with MRV, a newly emerged ranavirus. Moreover, LMIV infection caused lots of cells to be detached from monolayers, generating adherent and non-adherent cells. An opposite expression profiling of type I IFN pathway-related genes (JAK1, STAT1, STAT2, IRF9, Mx1) was found between adherent and non-adherent cells. Combined with the analysis of MCP gene expression, it is speculated that inhibiting type I IFN pathway in non-adherent cells allowed the facilitation of virus duplication. Taken together, the present study broadens our understanding about the diversity of cell lines derived from fish brain tissue and screening cells more susceptible to virus is not only meaningful for the development of vaccine, but also provide clues for further clarification of cell-iridovirus interactions.

Red Sea Bream Iridovirus (RSIV) Kinetics in Rock Bream (Oplegnathus fasciatus) at Various Fish-Rearing Seawater Temperatures

Simple Summary

Red sea bream iridoviral disease (RSIVD) generates serious economic losses by causing mass mortality events of rock bream during the season with high water temperature in the Republic of Korea and other Asian countries. However, very few studies have investigated RSIV kinetics in rock bream under various rearing water temperatures. In this paper, we investigated the viral load shedding of RSIV into seawater after artificially infecting rock bream (Oplegnathus fasciatus) with the virus. Overall, our data suggest that the viral load shedding of RSIV into seawater varies depending on water temperature and virus inoculation concentration. Our results reveal the potential of non-invasive virus detection approaches, such as the utilization of environmental DNA in fish farms. In addition, we showed that the quantitative analysis of seawater viruses can indirectly improve our understanding of disease progression in fish, potentially contributing to enhanced disease control.

Abstract

Red sea bream iridoviral disease (RSIVD) causes serious economic losses in the aquaculture industry. In this paper, we evaluated RSIV kinetics in rock bream under various rearing water temperatures and different RSIV inoculation concentrations. High viral copy numbers (approximately 10^3.7–10^6.7 RSIV genome copies/L/g) were observed during the period of active fish mortality after RSIV infection at all concentrations in the tanks (25 °C and 20 °C). In the group injected with 10⁴ RSIV genome copies/fish, RSIV was not detected at 21–30 days post-infection (dpi) in the rearing seawater. In rock bream infected at 15 °C and subjected to increasing water temperature (1 °C/d until 25 °C) 3 days later, the virus replication rate and number of viral copies shed into the rearing seawater increased. With the decrease in temperature (1 °C/d) from 25 to 15 °C after the infection, the virus replicated rapidly and was released at high loads on the initial 3–5 dpi, whereas the number of viral copies in the fish and seawater decreased after 14 dpi. These results indicate that the number of viral copies shed into the rearing seawater varies depending on the RSIV infection level in rock bream.

Establishment, characterization, and transfection potential of a new continuous fish cell line (CAM) derived from the muscle tissue of grass goldfish (Carassius auratus)

A new continuous fish cell line (CAM) has been successfully derived from the muscle tissues of grass goldfish, Carassius auratus. The primary cell cultures were initiated by incomplete trypsinization first and then explant culture in a Leibovitz-15 medium supplemented with 15% fetal bovine serum and 10% fish muscle extract. It was found that the CAM cells were very sensitive to trypsinization and needed to be sub-cultured at a low trypsin concentration of 0.0625% to be able to go through the crisis of spontaneous immortalization transformation, and afterward a total of five derivative cell strains were isolated from the original CAM cell line. This spontaneous immortalization transformation event was recorded successively at passages 44-47, beginning with a large-scale apoptosis and senescence and followed by mitosis arrest and re-activation, thus designated as cell strain CAM-44A, 44B, 45A, 44B, and 47A. Now both the CAM cell line and strains had been sub-cultured for more than 89 times and could be well cryopreserved in the growth medium containing 5% dimethylsulfoxide. Chromosome analysis and COI gene analysis had confirmed the grass goldfish origin of these CAM cells. Transfection potential analysis indicated that Lipofectamine LTX and Xfect were two suitable transfection reagents to be used in the gene delivery of CAM cells with a transfection efficiencies up to 11±6% and 8±3% in the CAM cell lines, respectively. Among the five cell strains, CAM-47A showed the highest transfection potential with a transfection efficiency up to 28 ± 5%. This work will provide a useful cell source for works on the cell-based artificial fish meat production and functional studies of fish myogenesis-related genes.

Application of Environmental DNA for Monitoring Red Sea Bream Iridovirus at a Fish Farm

Red sea bream iridoviral disease (RSIVD) causes high economic damage in mariculture in Asian countries. However, there is little information on the source of infection and viral dynamics in fish farms. In the present study, the dynamics of RSIV in a fish farm that mainly reared juveniles and broodstocks of red sea bream (Pagrus major) were monitored over 3 years (2016 to 2018) by targeting environmental DNA (eDNA) of seawater. Our monitoring demonstrated that red sea bream iridovirus (RSIV) was detected from the eDNA at least 5 days before an RSIVD outbreak in the juveniles. The viral loads of eDNA during the outbreak were highly associated with the numbers for daily mortality, and they reached a peak of 10⁶ copies/liter seawater in late July in 2017, when daily mortality exceeded 20,000 fish. In contrast, neither clinical signs nor mortality was observed in the broodstocks during the monitoring periods, whereas the broodstocks were confirmed to be virus carriers by an inspection in October 2017. Interestingly, the viral load of eDNA in the broodstock net pens (10⁵ copies/liter seawater) was higher than that in the juvenile net pens (10⁴ copies/liter seawater) just before the RSIVD outbreak in late June 2017. After elimination of all RSIV-infected surviving juveniles and 90% of broodstocks, few RSIV copies were detected in the eDNA in the fish farm from April 2018 onward (fewer than 10² copies/liter seawater). These results imply that the virus shed from the asymptomatically RSIV-infected broodstock was transmitted horizontally to the juveniles and caused further RSIVD outbreaks in the fish farm.

IMPORTANCE Environmental DNA (eDNA) could be applied in monitoring waterborne viruses of aquatic animals. However, there are few data for practical application of eDNA in fish farms for the control of disease outbreaks. The results of our field research over 3 years targeting eDNA in a red sea bream (Pagrus major) fish farm implied that red sea bream iridoviral disease (RSIVD) outbreaks in juveniles originated from virus shedding from asymptomatically virus-infected broodstocks. Our work identifies an infection source of RSIVD in a fish farm via eDNA monitoring, and it could be applied as a tool for application in aquaculture to control fish diseases.

Isolation and characterisation of an ISKNV-genotype megalocytivirus from imported angelfish Pterophyllum scalare

Using cultures of the SKF-9 cell line, megalocytivirus AFIV-16 was isolated from imported angelfish Pterophyllum scalare held in quarantine at the Australian border. The cytopathic effect caused by isolate AFIV-16 presented as cell rounding and enlargement, but complete destruction of the infected cell cultures did not occur. The infected cells demonstrated immunocytochemical reactivity with monoclonal antibody M10, which is used for diagnosis of OIE-listed red sea bream iridoviral disease. Using electron microscopy, the virus particles, consisting of hexagonal nucleocapsids, were observed in the cytoplasm of SKF-9 cells. The replication of AFIV-16 in cultured SKF-9 cells was significantly greater at 28°C incubation than at 22 and 25°C incubation, whereas no difference in growth characteristics was observed for red sea bream iridovirus (RSIV) isolate KagYT-96 across this temperature range. Whole genome sequencing demonstrated that AFIV-16 has a 99.96% similarity to infectious spleen and kidney necrosis virus (ISKNV), the type species in the genus Megalocytivirus. AFIV-16 was classified into ISKNV genotype Clade 1 by phylogenetic analysis of the major capsid protein gene nucleotide sequence. This is the first report of whole genome sequencing of an ISKNV genotype megalocytivirus isolated from ornamental fish.

Development of primary cell culture from spleen of giant gourami Osphronemus goramy for propagation of giant gourami iridovirus (GGIV)

The severe mortality of fish due to the infection of megalocytivirus caused significant economic losses. Since 2011, megalocytivirus (giant gourami iridovirus (GGIV)) has become the main pathogen in giant gourami (Osphronemus goramy), particularly in West Java, Central Java and Bali. This study aimed to develop primary cell culture from spleen as the target organ for propagating megalocytivirus in vitro, which was developed by explant method with enzymatic dissociation. Optimization was carried out at incubation temperature, medium and serum concentrations. The origin of the primary cell, cell susceptibility and GGIV pathogenicity were observed. The results showed that the primary cell (GP cells) can grow well in 10% foetal bovine serum L-15 medium at 27°C, which was sufficient for cell growth. PCR and BLAST analyses showed the primary cell was originated from giant gourami. In infected GP cells, cell enlargement and cell rounding were observed. Virus propagated in GP cells was highly virulent when injecting giant gourami in an artificial infection experiment. Intraperitoneal injection of diluted virus supernatant showed 100% mortality in 7-11 days post-injection and 97% mortality in 21 days post-cohabitation, with abnormalities observed in spleen and kidney. In conclusion, GP cell was successfully subcultured for more than 30 passages and susceptible to GGIV.

Assessment of fish iridoviruses using a novel cell line GS-1, derived from the spleen of orange-spotted grouper Epinephelus coioides (Hamilton) and susceptible to ranavirus and megalocytivirus

A new cell line (GS-1) was developed from the spleen tissue of the orange-spotted grouper, Epinephelus coioides applied for viral infection studies of fish ranavirus and megalocytivirus. The cells proficiently multiplied in Leibovitz’s L-15 medium supplemented with 10% fetal bovine serum at temperatures between 20°C and 32°C. Morphologically, the cell line comprised fibroblast-like cells, and this was confirmed by immunostaining with vimentin, fibronectin, and desmin antibodies. The optimal temperature for grouper iridovirus (GIV) and infectious spleen and kidney necrosis virus (ISKNV) proliferation in GS-1 cells was 25°C, and the highest titer of GIV was 10^8.4 TCID₅₀/ml, and the highest titer of ISKNV was 10^5.2 TCID₅₀/ml. Electron micrographs showed that the mean diameter of GIV virions was 180−220 nm, which was larger than ISKNV virions (160−200 nm). Negatively stained GIV particles possessed an envelope structure that was assembled by the three-layered structure with an inner electron-dense core surrounded by a lighter coat (mean diameter, 27 ± 3 nm). The highest GIV-induced mortality of groupers occurred at 25°C, whereas the highest ISKNV-induced mortality occurred at 30°C. In summary, GS-1 cell line is a valuable tool for isolating and investigating fish ranavirus and megalocytivirus in the same host system.