In vivo study of viral haemorrhagic septicaemia virus and infectious pancreatic necrosis virus coexistence in Senegalese sole (Solea senegalensis)

Viral Coinfections

In nature, viral coinfection is as widespread as viral infection alone. Viral coinfections often cause altered viral pathogenicity, disrupted host defense, and mixed-up clinical symptoms, all of which result in more difficult diagnosis and treatment of a disease. There are three major virus-virus interactions in coinfection cases: viral interference, viral synergy, and viral noninterference. We analyzed virus-virus interactions in both aspects of viruses and hosts and elucidated their possible mechanisms. Finally, we summarized the protocol of viral coinfection studies and key points in the process of virus separation and purification.

Early or Simultaneous Infection with Infectious Pancreatic Necrosis Virus Inhibits Infectious Hematopoietic Necrosis Virus Replication and Induces a Stronger Antiviral Response during Co-infection in Rainbow Trout (Oncorhynchus mykiss)

Infectious hematopoietic necrosis (IHN) and infectious pancreatic necrosis (IPN) are the most common viral diseases of salmon in aquaculture worldwide. The co-infection of rainbow trout (Oncorhynchus mykiss) with IHN virus (IHNV) and IPN virus (IPNV) is known to occur. To determine the influence of IPNV on IHNV in co-infection, rainbow trout were intraperitoneally (i.p.) injected with IPNV at different time intervals prior to, simultaneously to, or after IHNV infection. The replication of IHNV in the brain, gill, heart, liver, spleen, and head kidney was detected by real-time quantitative polymerase chain reaction (qRT-PCR). The results showed that when rainbow trout were i.p. injected with IPNV prior to, simultaneously to, or after IHNV on 2 day (d), IHNV replication was inhibited (p < 0.05) in all collected tissues. Nevertheless, when rainbow trout were i.p. injected with IPNV after IHNV on 7 d (H7P), IHNV replication was only inhibited (p < 0.05) in the liver 14 d post-IHNV infection. Moreover, stronger antiviral responses occurred in all challenge groups. Our results suggest that IPNV can inhibit IHNV replication before or simultaneously with IHNV infection, and induce a stronger antiviral response, and that this inhibition is most sensitive in the liver. Early i.p. injection of IPNV can significantly reduce the mortality of rainbow trout, compared with the group only injected with IHNV.

Fish Innate Immune Response to Viral Infection-An Overview of Five Major Antiviral Genes

Fish viral diseases represent a constant threat to aquaculture production. Thus, a better understanding of the cellular mechanisms involved in establishing an antiviral state associated with protection against virus replication and pathogenesis is paramount for a sustainable aquaculture industry. This review summarizes the current state of knowledge on five selected host innate immune-related genes in response to the most relevant viral pathogens in fish farming. Viruses have been classified as ssRNA, dsRNA, and dsDNA according to their genomes, in order to shed light on what those viruses may share in common and what response may be virus-specific, both in vitro (cell culture) as well as in vivo. Special emphasis has been put on trying to identify markers of resistance to viral pathogenesis. That is, those genes more often associated with protection against viral disease, a key issue bearing in mind potential applications into the aquaculture industry.

Comparison of Selected Immune Parameters in a Single Infection and Co-Infection with Infectious Pancreatic Necrosis Virus with Other Viruses in Rainbow Trout

Infectious pancreatic necrosis virus (IPNV) often occurs in an aquatic environment in co-infection with other viruses. In this study, we wanted to investigate the effect of this virus on the course of co-infection with other viruses in rainbow trout. For co-infection we used viral hemorrhagic septicemia virus (VHSV), infectious hematopoietic necrosis virus (IHNV) and salmonid alphavirus (SAV) field strains and infected rainbow trout divided into eight groups; I; IPNV, II; IHNV, III; VHSV, I; SAV, V; IPNV+IHNV, VI; IPNV+VHSV, VII; IPNV+SAV, and the control group. We assessed apoptosis in white blood cells and used a real time RT-PCR to analyze RNA obtained from the internal organs of the fish. During single infection and co-infection the level of expression of immune genes such as interferon and toll-like receptor 3 (TLR-3) was assessed. The highest mortality during the experiment was observed in group III infected by VHSV. The average percentage of apoptotic cells was higher in groups without co-infection, especially in groups II and III. Interferon expression was higher in singly infected groups, the highest being in the heart in group III, while expression of the TLR-3 gene was generally raised in all tested organs in all groups. We found that co-infection with IPNV had a positive impact on the course of infection with the viruses listed because it lowered mortality, reduced apoptosis in co-infected cells, and positively affected fish health.

Co-infection of rainbow trout (Oncorhynchus mykiss) with infectious hematopoietic necrosis virus and Flavobacterium psychrophilum

Co-infection of rainbow trout with infections haematopoietic necrosis virus (IHNV) and Flavobacterium psychrophilum is known to occur, and it has been speculated that a combined infection can result in dramatic losses. Both pathogens can persist in fish in an asymptomatic carrier state, but the impact of co-infection has not been well characterized or documented. In this study, it was hypothesized that fish co-infected with F. psychrophilum and IHNV would exhibit greater mortality than fish infected with either pathogen alone. To test this, juvenile rainbow trout were co-infected with low doses of either IHNV or F. psychrophilum, and at 2 days post-initial challenge, they were given a low dose of the reciprocal pathogen. This combined infection caused high mortality (76.2%-100%), while mortality from a single pathogen infection with the same respective dose was low (5%-20%). The onset of mortality was earlier in the co-infected group (3-4 days) when compared with fish infected with F. psychrophilum alone (6 days) or IHNV (5 days), confirming the synergistic interaction between both pathogens. Co-infection led to a significant increase in the number of F. psychrophilum colony-forming units and IHNV plaque-forming units within tissues. This finding confirms that when present together in co-infected fish, both pathogens are more efficiently recovered from tissues. Furthermore, pathogen genes were significantly increased in co-infected groups, which parallel the findings of increased systemic pathogen load. Extensive tissue necrosis and abundant pathogen present intracellularly and extracellularly in haematopoietic tissue. This was pronounced in co-infected fish and likely contributed to the exacerbated clinical signs and higher mortality. This study provides novel insight into host-pathogen interactions related to co-infection by aquatic bacterial and viral pathogens and supports our hypothesis. Such findings confirm that mortality in fish exposed to both pathogens is greatly elevated compared to a single pathogen infection.

Virological and Immunological Outcomes of Coinfections

Coinfections involving viruses are being recognized to influence the disease pattern that occurs relative to that with single infection. Classically, we usually think of a clinical syndrome as the consequence of infection by a single virus that is isolated from clinical specimens. However, this biased laboratory approach omits detection of additional agents that could be contributing to the clinical outcome, including novel agents not usually considered pathogens. The presence of an additional agent may also interfere with the targeted isolation of a known virus. Viral interference, a phenomenon where one virus competitively suppresses replication of other coinfecting viruses, is the most common outcome of viral coinfections. In addition, coinfections can modulate virus virulence and cell death, thereby altering disease severity and epidemiology. Immunity to primary virus infection can also modulate immune responses to subsequent secondary infections. In this review, various virological mechanisms that determine viral persistence/exclusion during coinfections are discussed, and insights into the isolation/detection of multiple viruses are provided. We also discuss features of heterologous infections that impact the pattern of immune responsiveness that develops.