Major capsid protein gene sequence analysis of the Santee-Cooper ranaviruses DFV, GV6, and LMBV

Rapid and facile detection of largemouth bass ranavirus with CRISPR/Cas13a

Largemouth bass ranavirus (LMBV) is an epidemic disease that seriously jeopardizes the culture of largemouth bass（Micropterus salmoides）, and it has a very high incidence in largemouth bass. Once an outbreak occurs, it may directly lead to the failure of the culture, resulting in substantial economic losses, but there is no effective vaccine or special effective drug yet. Consequently, it is important to establish an accurate, sensitive, convenient and specific detection approach for preventing LMBV infection. The recombinant enzyme-assisted amplification (RAA) technology was used in combination with clustered regularly interspaced short palindromic repeats (CRISPR), and associated protein 13a (CRISPR/Cas13a) to detect LMBV. We designed RAA primers and CRISPR RNA (crRNA) that targeted the conserved region in the LMBV main capsid protein (MCP) gene, amplified sample nucleic acids using the RAA technology, performed CRISPR/Cas13a fluorescence detection and evaluated the sensitivity and specificity of the established method with qPCR as a control method. This technique was able to determine the results by collecting fluorescence signals, visualizing fluorescence by UV excitation and combining with lateral flow strips (LFS). The sensitivity and specificity of the established method were consistent with the qPCR method. Besides, it was performed at a constant temperature of 37 °C and the sensitivity of the reaction system was 3.1 × 101 copies/μL, with no cross-reactivity with other common aquatic pathogens. Further, the positive detection rate of the proposed method in 32 clinical samples was consistent with that of qPCR. In conclusion, our established RAA-CRISPR/Cas13 method for detecting LMBV is sensitive, simple and specific, which is applicable in the rapid on-site detection and epidemiological monitoring of LMBV.

Development of recombinant major capsid protein Vaccine and assessment of its efficacy against SRDV in similar damselfish (Pomacentrus similis)

Viral vaccines against emerging viral diseases are crucial for encouraging successful aquaculture production. In this research, an experimental recombinant major capsid protein vaccine of similar damselfish virus was prepared and examined for its efficacy in marine ornamental fish, similar damselfish (Pomacentrus similis). The MCP gene of the SRDV was amplified from the viral DNA by a specific primer set viz bamHI and XhoI- restriction sites and confirmed by agarose gel electrophoresis with a target size of 1416 bp. The gel-purified PCR product was double-digested with the said enzymes and incorporated into the pTriEx1.1 vector, which was subsequently transformed to E. coli DH5α. The plasmids of the two clones pTriEx-MCP-1416-1 and pTriEx-MCP-1416-3 were transformed to E. coli BL21 (DE-3) pLacI. A crude protein compound derived from a colony of E. coli BL21 (DE-3) with expressed MCP inserts was used to evaluate efficacy in similar damselfish by intra-peritoneal injection. After the challenge with SRDV, damselfish vaccinated with recombinant protein showed a lower protection level, while the fish vaccinated with recombinant protein supplemented Quil-A® adjuvant showed an RPS of 26%. According to RPS values recorded from the vaccinated and non-vaccinated damselfish group, the recombinant protein vaccine conferred only marginal protection against the SRDV challenge.

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.

Production and characterization of monoclonal antibodies against mandarinfish ranavirus and first identification of pyloric caecum as the major target tissue

Mandarinfish ranavirus (MRV), also known as a variant of largemouth bass virus (LMBV), is an emerging pathogen in mandarinfish aquaculture. In this study, monoclonal antibodies (mAbs) against MRV were produced and characterized, and 7 mAbs were obtained through Western blotting screening and all 7 mAbs specifically recognized MRV/LMBV but not several piscine iridoviruses as ISKNV, GIV and TFV. By LC MS/MS analysis, the recognized viral proteins by seven mAbs were identified as MRV-pORF47L, MRV-pORF55R, MRV-pORF57L, MRV-pORF77L and MRV-pORF78L, respectively, and all five viral proteins are late expression structural proteins by Western blotting. Based on mAb 1C4, immuno-histochemistry and immuno-histo-fluorescence were performed to re-assess the tissue tropism of MRV. The result showed that abundant reactive signals were observed in infected spleen, kidney as well as intestine and pyloric caecum. Real-time quantitative PCR also demonstrated that spleen as well as pyloric caecum and intestines are the major target tissue upon MRV infection. In infected intestines and pyloric caecum, numerous enlarged, multinucleated cells with intracytoplasmic inclusions were identified as the target cells of MRV, suggesting that MRV serves as a digestive tract pathogen to mandarinfish, which may explain why acute infection of MRV can cause the typical clinicopathology featured by severe ascites.

Viruses Infecting the European Catfish (Silurus glanis)

In aquaculture, disease management and pathogen control are key for a successful fish farming industry. In past years, European catfish farming has been flourishing. However, devastating fish pathogens including limiting fish viruses are considered a big threat to further expanding of the industry. Even though mainly the ranavirus (Iridoviridea) and circovirus (Circoviridea) infections are considered well- described in European catfish, more other agents including herpes-, rhabdo or papillomaviruses are also observed in the tissues of catfish with or without any symptoms. The etiological role of these viruses has been unclear until now. Hence, there is a requisite for more detailed information about the latter and the development of preventive and therapeutic approaches to complete them. In this review, we summarize recent knowledge about viruses that affect the European catfish and describe their origin, distribution, molecular characterisation, and phylogenetic classification. We also highlight the knowledge gaps, which need more in-depth investigations in the future.

Partial validation of a TaqMan real-time quantitative PCR for the detection of ranaviruses

Ranaviruses are globally emerging pathogens negatively impacting wild and cultured fish, amphibians, and reptiles. Although conventional and diagnostic real-time PCR (qPCR) assays have been developed to detect ranaviruses, these assays often have not been tested against the known diversity of ranaviruses. Here we report the development and partial validation of a TaqMan real-time qPCR assay. The primers and TaqMan probe targeted a conserved region of the major capsid protein (MCP) gene. A series of experiments using a 10-fold dilution series of Frog virus 3 (FV3) MCP plasmid DNA revealed linearity over a range of 7 orders of magnitude (107-101), a mean correlation coefficient (R2) of >0.99, and a mean efficiency of 96%. The coefficient of variation of intra- and inter-assay variability ranged from <0.1-3.5% and from 1.1-2.3%, respectively. The analytical sensitivity was determined to be 10 plasmid copies of FV3 DNA. The qPCR assay detected a panel of 33 different ranaviral isolates originating from fish, amphibian, and reptile hosts from all continents excluding Africa and Antarctica, thereby representing the global diversity of ranaviruses. The assay did not amplify highly divergent ranaviruses, members of other iridovirus genera, or members of the alloherpesvirus genus Cyprinivirus. DNA from fish tissue homogenates previously determined to be positive or negative for the ranavirus Epizootic hematopoietic necrosis virus by virus isolation demonstrated a diagnostic sensitivity of 95% and a diagnostic specificity of 100%. The reported qPCR assay provides an improved expedient diagnostic tool and can be used to elucidate important aspects of ranaviral pathogenesis and epidemiology in clinically and sublinically affected fish, amphibians, and reptiles.

Characterization of a virulent ranavirus isolated from marine ornamental fish in India

A viral agent implicated in the mortality of marine ornamental "Similar Damselfish" (Pomacentrus similis Allen, 1991) was isolated and characterized. The virus grew well in marine and freshwater fish cell lines from seabass and snakehead. The virus was sensitive to chloroform, acidic pH (3.0) and heat treatment at 56 °C. Biochemical characterisation indicated that the virus had double stranded DNA genome. Transmission electron microscopic analysis of ultrathin sections of infected cell pellets showed iridovirus-like icosahedral virus particles of 120-130 nm. Purified virus had six structural protein bands that ranged from of 44 to 132 kDa. PCR analysis confirmed the presence of viral DNA in infected cell cultures and sequence analysis of the major capsid protein gene showed an identity of 99.82% to that of largemouth bass virus. Serum neutralization studies involving the viral agent and koi ranavirus (KIRV) indicated partial homogeneity between the two isolates. Experimental infection of seabass (Lates calcarifer) and similar damselfish (P. similis) fingerlings with the similar damselfish virus showed cumulative mortalities of 68.75 and 93.33%. The biophysical and biochemical properties of the viral agent isolated, serological characteristics, size of major capsid proteins and the sequence similarity of the MCP gene proved that the virus belongs to the genus Ranavirus of the family Iridoviridae. Ability of the virus to grow in marine and freshwater fish cell lines and its pathogenicity to one of the cultivable marine fish shows the wide host range of the virus. This is the first report of ranavirus induced mortality in marine fish in India.

First report of a cystic malformation on the upper jaw of hatchery-reared allis shad Alosa alosa

The anadromous allis shad Alosa alosa has suffered dramatic population declines throughout Europe and is currently considered as endangered throughout its entire distribution range. In order to reestablish allis shad in the River Rhine, which formerly housed one of the largest and most important populations, an EU-LIFE Project 'The re-introduction of allis shad in the Rhine system' was started in 2007. In course of the LIFE+ Projects, allis shad larvae bred from genitor fish of the Gironde-Garonne-Dordogne population in France were reared in a pilot ex situ stock plant pilot facility in Aßlar, Germany. At an age of 1-2 months, about 100% of these fish developed approximately 0.5- to 0.8-cm large, fluid-filled, transparent cysts in conjunction with the upper jaw. The performed microbiological, virological, parasitological and histological examinations did not detect any infectious agents. Possible causative agents are discussed with regard to environmental factors and the nutrition of larvae. In conclusion, the observed malformations are considered a sign for a severe health problem and therefore a risk for the successful breeding of allis shad in aquaculture.

Aquatic viruses induce host cell death pathways and its application

Virus infections of mammalian and animal cells consist of a series of events. As intracellular parasites, viruses rely on the use of host cellular machinery. Through the use of cell culture and molecular approaches over the past decade, our knowledge of the biology of aquatic viruses has grown exponentially. The increase in aquaculture operations worldwide has provided new approaches for the transmission of aquatic viruses that include RNA and DNA viruses. Therefore, the struggle between the virus and the host for control of the cell's death machinery is crucial for survival. Viruses are obligatory intracellular parasites and, as such, must modulate apoptotic pathways to control the lifespan of their host to complete their replication cycle. This paper updates the discussion on the detailed mechanisms of action that various aquatic viruses use to induce cell death pathways in the host, such as Bad-mediated, mitochondria-mediated, ROS-mediated and Fas-mediated cell death circuits. Understanding how viruses exploit the apoptotic pathways of their hosts may provide great opportunities for the development of future potential therapeutic strategies and pathogenic insights into different aquatic viral diseases.

Isolation and characterization of a ranavirus from koi, Cyprinus carpio L., experiencing mass mortalities in India

We investigated mass mortalities of koi, Cyprinus carpio Linnaeus, 1758, experienced in South Indian fish farms by virus isolation, electron microscopy, PCR detection, sequencing of capsid protein gene and transmission studies. Samples of moribund koi brought to the laboratory suffered continuous mortality exhibiting swimming abnormalities, intermittent surfacing and skin darkening. Irido-like virus was isolated from the infected fish in the indigenous snakehead kidney cell line (SNKD2a). Icosahedral virus particles of 100 to 120 nm were observed in the infected cell cultures, budding from the cell membrane. Virus transmission and pathogenicity studies revealed that horizontal transmission occurred associated with mortality. PCR analysis of infected fish and cell cultures confirmed the presence of Ranavirus capsid protein sequences. Sequence analysis of the major capsid protein gene showed an identity of 99.9% to that of largemouth bass virus isolated from North America. Detection and successful isolation of this viral agent becomes the first record of isolation of a virus resembling Santee-Cooper Ranavirus from a koi and from India. We propose the name koi ranavirus to this agent.

Involvement of the PI3K and ERK signaling pathways in largemouth bass virus-induced apoptosis and viral replication

Increased reports demonstrated that largemouth Bass, Micropterus salmoides in natural and artificial environments were always suffered from an emerging iridovirus disease, largemouth Bass virus (LMBV). However, the underlying mechanism of LMBV pathogenesis remained largely unknown. Here, we investigated the cell signaling events involved in virus induced cell death and viral replication in vitro. We found that LMBV infection in epithelioma papulosum cyprini (EPC) cells induced typical apoptosis, evidenced by the appearance of apoptotic bodies, cytochrome c release, mitochondrial membrane permeabilization (MMP) destruction and reactive oxygen species (ROS) generation. Two initiators of apoptosis, caspase-8 and caspase-9, and the executioner of apoptosis, caspase-3, were all significantly activated with the infection time, suggested that not only mitochondrion-mediated, but also death receptor-mediated apoptosis were involved in LMBV infection. Reporter gene assay showed that the promoter activity of transcription factors including p53, NF-κB, AP-1 and cAMP response element-binding protein (CREB) were decreased during LMBV infection. After treatment with different signaling pathway inhibitors, virus production were significantly suppressed by the inhibition of phosphatidylinositol 3-kinase (PI3K) pathway and extracellular-signal-regulated kinases (ERK) signaling pathway. Furthermore, LMBV infection induced apoptosis was enhanced by PI3K inhibitor LY294002, but decreased by addition of ERK inhibitor UO126. Therefore, we speculated that apoptosis was sophisticatedly regulated by a series of cell signaling events for efficient virus propagation. Taken together, our results provided new insights into the molecular mechanism of ranavirus infection.

Expression analysis of immune response genes in fish epithelial cells following ranavirus infection

Ranaviruses (family Iridoviridae) are a growing threat to fish and amphibian populations worldwide. The immune response to ranavirus infection has been studied in amphibians, but little is known about the responses elicited in piscine hosts. In this study, the immune response and apoptosis induced by ranaviruses were investigated in fish epithelial cells. Epithelioma papulosum cyprini (EPC) cells were infected with four different viral isolates: epizootic haematopoietic necrosis virus (EHNV), frog virus 3 (FV3), European catfish virus (ECV) and doctor fish virus (DFV). Quantitative real-time PCR (qPCR) assays were developed to measure the mRNA expression of immune response genes during ranavirus infection. The target genes included tumour necrosis factor α (TNF-α), interleukin-1β (IL-1β), β2-microglobulin (β2M), interleukin-10 (IL-10) and transforming growth factor β (TGF-β). All ranaviruses elicited changes in immune gene expression. EHNV and FV3 caused a strong pro-inflammatory response with an increase in the expression of both IL-1β and TNF-α, whereas ECV and DFV evoked transient up-regulation of regulatory cytokine TGF-β. Additionally, all viral isolates induced increased β2M expression as well as apoptosis in the EPC cells. Our results indicate that epithelial cells can serve as an in vitro model for studying the mechanisms of immune response in the piscine host in the first stages of ranavirus infection.