Abstract
Infectious hematopoietic necrosis virus (IHNV) is endemic in farmed rainbow trout in continental Europe and in various salmonid fish species at the Pacific coast of North America. IHN has never occurred in European Atlantic salmon (Salmo salar) farms, but is considered as a major threat for the European salmon industry. Another virus, Piscine orthoreovirus (PRV), is widespread in the sea phase of Atlantic salmon, and is identified as the causative agent of heart and skeletal muscle inflammation. The aim of this study was to investigate the interactions between a primary PRV infection and a secondary IHNV infection under experimental conditions. A PRV cohabitation challenge was performed with Atlantic salmon. At peak of PRV viremia the fish were challenged by immersion with an IHNV genogroup E isolate. Clinical signs and morbidity were monitored. Target organs were sampled at selected time points to assess viral loads of both pathogens. Antiviral immune response and presence of histopathological findings were also investigated. Whereas the PRV-negative/IHNV positive group suffered significant decrease in survival caused by IHNV, the PRV infected groups did not suffer any morbidity and showed negligible levels of IHNV infection. Antiviral response genes were induced, as measured in spleen samples, from PRV infected fish prior to IHNV challenge. In conclusion, PRV-infection protects Atlantic salmon against IHNV infection and morbidity, most likely by inducing a protective innate antiviral response.
Highlights
The experimental fish were screened for Infectious hematopoietic necrosis virus (IHNV), infectious pancreatic necrosis virus (IPNV) and viral haemorrhagic septicemia virus (VHSV) inoculating organ homogenate on cell culture according to a Council implementing decision 2015-1554 [17]
Protective effect of Piscine orthoreovirus (PRV) infection against subsequent IHNV challenge In the pathogenesis trial conducted in the 150 L tanks 2, 4, 5 and 6, onset of morbidity started as early as 5 days post IHNV challenge and reached 7% at the end of the trial
In the pathogenicity trial conducted in the 8 L bowls, morbidity was observed from 6 days post IHNV challenge, apart from one fish which succumbed on day 1 likely due to handling procedures during transfer
Summary
In net pens, farmed stocks can be exposed to pathogens through water at any given time. Knowledge about effects of co-infections and pathogen interactions is important for development and implementation of effective disease control strategies. Infectious hematopoietic necrosis virus (IHNV) is a member of the Rhabdoviridae family, genus Novirhabdovirus, which are bullet shaped viruses with non-segmented, negative single stranded RNA genome. Phylogenetic analyses of the genetically diverse G gene of IHNV define five major genogroups (U, M, L, E, J), which broadly refer to the geographical distribution of the genogroups [3, 4]. The presence of IHNV in Europe was first confirmed in 1987, with at least two different introductions in Italy and France, with viruses originating from the M genogroup
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