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.

A Novel Rubi-Like Virus in the Pacific Electric Ray (Tetronarce californica) Reveals the Complex Evolutionary History of the Matonaviridae

Rubella virus (RuV) is the causative agent of rubella (“German measles”) and remains a global health concern. Until recently, RuV was the only known member of the genus Rubivirus and the only virus species classified within the Matonaviridae family of positive-sense RNA viruses. Recently, two new rubella-like matonaviruses, Rustrela virus and Ruhugu virus, have been identified in several mammalian species, along with more divergent viruses in fish and reptiles. To screen for the presence of additional novel rubella-like viruses, we mined published transcriptome data using genome sequences from Rubella, Rustrela, and Ruhugu viruses as baits. From this, we identified a novel rubella-like virus in a transcriptome of Tetronarce californica—order Torpediniformes (Pacific electric ray)—that is more closely related to mammalian Rustrela virus than to the divergent fish matonavirus and indicative of a complex pattern of cross-species virus transmission. Analysis of host reads confirmed that the sample analysed was indeed from a Pacific electric ray, and two other viruses identified in this animal, from the Arenaviridae and Reoviridae, grouped with other fish viruses. These findings indicate that the evolutionary history of the Matonaviridae is more complex than previously thought and highlights the vast number of viruses that remain undiscovered.

A novel rubi-like virus in the Pacific electric ray (Tetronarce californica) reveals the complex evolutionary history of the Matonaviridae

Rubella virus (RuV) is the causative agent of rubella (“German measles”) and remains a global health concern. Until recently, RuV was the only known member of the genus Rubivirus and the only virus species classified within the Matonaviridae family of positive-sense RNA viruses. Other matonaviruses, including two new rubella-like viruses, Rustrela virus and Ruhugu virus, have been identified in several mammalian species, along with more divergent viruses in fish and reptiles. To screen for the presence of additional novel rubella-like viruses we mined published transcriptome data using genome sequences from Rubella, Rustrela, and Ruhugu viruses as baits. From this, we identified a novel rubella-like virus in a transcriptome of Tetronarce californica (Pacific electric ray) that is more closely related to mammalian Rustrela virus than to the divergent fish matonavirus and indicative of a complex pattern of cross-species virus transmission. Analysis of host reads confirmed that the sample analysed was indeed from a Pacific electric ray, and two other viruses identified in this animal, from the Arenaviridae and Reoviridae, grouped with other fish viruses. These findings indicate that the evolutionary history of the Matonaviridae is more complex than previously thought and highlights the vast number of viruses still to be discovered.

Detection and Phylogenetic Analysis of Fish Viruses Inhabiting Freshwater Ornamental and Food Fish Species in India: A Survey-Based Study

Molecular screening of viruses through PCR is a fast detection method and several variants of it have been developed to meet the shortcomings of conventional method. Extensive surveillance is an essential tool to have an estimate of circulating potential viruses in Indian aquatic systems. The present study focuses on standardization of semi-quantitative PCR-based diagnostics for eight freshwater fish viral pathogens and documents number of positive incidences after screening 437 numbers of samples during 2017-2020 for suspected fifteen freshwater finfish viral pathogens. Positive samples of Cyprinid herpes virus-2, carp edema virus and viral nervous necrosis virus from different regions of eastern Indian states viz., Odisha, West Bengal and Assam have given a scope of comparative phylogenetic analysis of strain variability in detail. Nevertheless, the study highlights the importance of rendering protection to native cultured fish by preventing their introduction and further spread on a local or national scale to either known or unknown susceptible fish species available in the region, and emphasize upon strengthening surveillance system.

The Unexplored Virome of Two Atlantic Coast Fish: Contribution of Next-Generation Sequencing to Fish Virology

Much of the knowledge on viruses is focused on those that can be propagated using cell-cultures or that can cause disease in humans or in economically important animals and plants. However, this only reflects a small portion of the virosphere. Therefore, in this study, we explore by targeted next-generation sequencing, how the virome varies between Atlantic horse mackerels and gilthead seabreams from fisheries and aquaculture from the center and south regions of Portugal. Viral genomes potentially pathogenic to fish and crustaceans, as well as to humans, were identified, namely Astroviridae, Nodaviridae, Hepadnaviridae, Birnaviridae, Caliciviridae, and Picornaviridae families. Also bacteriophages sequences were identified corresponding to the majority of sequences detected, with Myoviridae, Podoviridae, and Siphoviridae, the most widespread families in both fish species. However, these findings can also be due to the presence of bacteria in fish tissues, or even to contamination. Overall, seabreams harbored viruses from a smaller number of families in comparison with mackerels. Therefore, the obtained data show that fish sold for consumption can harbor a high diversity of viruses, many of which are unknown, reflecting the overall uncharacterized virome of fish. While cross-species transmission of bonafide fish viruses to humans is unlikely, the finding of human pathogenic viruses in fish suggest that fish virome can be a potential threat regarding food safety.

Grass Carp Reovirus (GCRV) Giving Its All to Suppress IFN Production by Countering MAVS-TBK1 Activation

As a crucial signaling pathway for interferon (IFN) production, the RIG-I-like receptor (RLR) axis is usually the host target of viruses to enhance viral infection. To date, though immune evasion methods to contrapose IFN production have been characterized for a series of terrestrial viruses, the strategies employed by fish viruses remain unclear. Here, we report that all grass carp reovirus (GCRV) proteins encoded by segments S1 to S11 interact with fish RLR factors, specifically for mitochondrial antiviral signaling protein-TANK-binding kinase 1 (MAVS-TBK1) signaling transduction, leading to decreased IFN expression. First, the GCRV viral proteins blunted the MAVS-induced expression of IFN but had little effect on TBK1-induced IFN expression. Subsequently, interestingly, co-immunoprecipitation experiments demonstrated that all GCRV viral proteins interacted with several RLR cascades, especially with TBK1. To further illustrate the mechanisms of these interactions between GCRV viral proteins and host RLRs, two of the viral proteins, NS79 (S4) and VP3 (S3), were selected as representative proteins for the study. The obtained data demonstrated that NS79 did not affect the stability of the host RLR protein, but was phosphorylated by gcTBK1, leading to the reduction of host substrate gcIRF3/7 phosphorylation. On the other hand, VP3 degraded gcMAVS and the degradation was significantly reversed by 3-MA. The biological effects of both NS79 and VP3 were consistently found to be related to the suppression of IFN expression and the promotion of viral evasion. Our findings shed light on the special evasion mechanism utilized by fish virus through IFN regulation, which might differ between fish and mammals.Author summaryThe RLR signaling pathway is crucial for IFN induction when host cells are infected with virus and RLR factors are targeted by virus. To date, the evasion mechanisms of fish viruses remain mysterious. In this study, we reveal that all 11 GCRV proteins interact with fish RLR factors and suppress the activation of MAVS-TBK1 signaling transduction, leading to the reduction of IFN expression. Two viral proteins were employed as examples to investigate the different evasion mechanisms of GCRV. These findings reveal the novel countermeasures used by fish virus to avoid the host IFN response.

Zebrafish RPZ5 Degrades Phosphorylated IRF7 To Repress Interferon Production

ABSTRACT Interferon (IFN) production activated by phosphorylated interferon regulatory factor 7 (IRF7) is a pivotal process during host antiviral infection. For viruses, suppressing the host IFN response is beneficial for viral proliferation; in such cases, evoking host-derived IFN negative regulators would be very useful for viruses. Here, we report that the zebrafish rapunzel 5 (RPZ5) protein which activated by virus degraded phosphorylated IRF7 is activated by TANK-binding kinase 1 (TBK1), leading to a reduction in IFN production. Upon viral infection, zebrafish rpz5 was significantly upregulated, as was ifn, in response to the stimulation. Overexpression of RPZ5 blunted the IFN expression induced by both viral and retinoic acid-inducible gene I (RIG-I) like-receptor (RLR) factors. Subsequently, RPZ5 interacted with RLRs but did not affect the stabilization of the proteins in the normal state. Interestingly, RPZ5 degraded the phosphorylated IRF7 under TBK1 activation through K48-linked ubiquitination. Finally, the overexpression of RPZ5 remarkably reduced the host cell antiviral capacity. These findings suggest that zebrafish RPZ5 is a negative regulator of phosphorylated IRF7 and attenuates IFN expression during viral infection, providing insight into the IFN balance mechanism in fish. IMPORTANCE The phosphorylation of IRF7 is helpful for host IFN production to defend against viral infection; thus, it is a potential target for viruses to mitigate the antiviral response. We report that the fish RPZ5 is an IFN negative regulator induced by fish viruses and degrades the phosphorylated IRF7 activated by TBK1, leading to IFN suppression and promotion of viral proliferation. These findings reveal a novel mechanism for interactions between the host cell and viruses in the lower vertebrate.