| Summary: | Piscine orthoreovirus (PRV) is a virus of salmonid fish. PRV has been associated with different diseases in various salmonid fish species, including heart and skeletal muscle inflammation (HSMI) in Atlantic salmon and more recently a disease resembling HSMI in rainbow trout. In general, PRV appears to be widely distributed, with detection in diseased as well as non-diseased fish. For some of the diseases, a causative relationship has been established, confirming PRV as the etiological agent of the disease, whereas for others the disease association is questioned. The combination of multiple salmonid species and discovery of multiple PRV variants, compose a complex landscape to study disease association. This calls for basic studies revealing the characteristics of the virus, combined with targeted and controlled experiments to settle disease association. The present thesis focuses on PRV-1 infection of Atlantic salmon and PRV-3 in Rainbow trout. The work encompasses the basic characterization of PRV and provides important information on the pathogenesis and disease association. The first study addressed potential virulence differences between PRV-1 strains infecting Atlantic salmon. A full genome sequence analysis of different PRV-1 strains was conducted. The analysis of HSMI-associated PRV-1 strains and low virulent North American pacific coast (NAPC) isolates revealed connection foremost with genomic segments S1 and M2 and the HSMI trait. Reassortment of these two genomic segments and/or possibly the accumulation of mutations have contributed to the evolution of the virulent strains. Some of the PRV-1 isolates showed segment reassortments, indicating that this mechanism contributes to PRV evolution. PRV strains from archived samples, revived by in vivo propagation and full-genome sequenced, confirmed the presence of different variants of PRV during the pre-HSMI period in Norway. The HSMI associated genotypes are adapted to farmed Atlantic salmon, as indicated by the sequences of S1 and M2 segments being stable ...
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