Piscine orthoreovirus: Distribution, characterization and experimental infections in salmonids

Abstract

Piscine orthoreovirus (PRV) belongs to the family Reoviridae and is most closely related to the genus Orthoreovirus. Piscine orthoreovirus are causative agents of emerging diseases for salmonid aquaculture worldwide. This viral species currently counts three different subtypes, each one with preferential host. PRV-1 is the causative agent of heart and skeletal muscle inflammation (HSMI) in Atlantic salmon (Salmo salar) and is associated with jaundice syndrome in farmed Chinook salmon (Oncorhynchus tshawytscha). PRV-2 causes erythrocytic inclusion body syndrome (EIBS) in Coho salmon (Oncorhynchus kisutch). PRV-3 causes heart pathology resembling HSMI in Rainbow trout (Onchorynchus mykiss). In Europe only PRV-1 and PRV-3 are present PRV-3 was firstly discovered in 2013 in Norway during disease outbreaks affecting farmed Rainbow trout. The first series of experimental trials were performed to assess its pathogenicity and pathogenesis in Rainbow trout and Atlantic salmon. The Norwegian PRV-3 isolate has been further characterized analyzing its genome and antigenic features. An experimental infection study with purified virus demonstrated that PRV-3 infection in Rainbow trout induces pathological heart lesions similar to HSMI, and thus fulfil causative relationship. Furthermore, the infection upregulates IFN production and induces a specific antibody response in later phases. In late 2017 the presence of PRV-3 was also reported in different countries in Europe including Scotland, Germany, France, Italy and Denmark. Interestingly, these viral isolates appear to be genetically distinct from the Norwegian isolate leading to the proposition of two separate clades within PRV-3 viral type (PRV-3a and PRV-3b). PRV-1 is prevalent in farmed Atlantic salmon in sea water in Europe. The prevalence of the virus has been investigated in wild salmon stocks to expand the knowledge of its epidemiology. PRV can induce a systemic antiviral immune response, which may affect the outcome of a secondary virus infection. Infectious haematopoietic necrosis virus (IHNV) causes major challenges to the Atlantic salmon industry in Canada, requiring the implementation of DNA vaccination to control the disease. IHNV is not present in Atlantic salmon farmed in Europe and feared as a major risk for the aquaculture industry, and therefore we have assessed the potential interaction of these two pathogens in Atlantic salmon in the co-challenge trial.