| Summary: | Piscine orthoreovirus-1 (PRV-1) is the causative agent of heart and skeletal muscle inflammation (HSMI) in farmed Atlantic salmon, first described in Norway in 1999. Not all infected fish develop HSMI, and the virus has been revived from archived samples dating back to 1988, leading to a hypothesis that viral evolution has generated new genotypes with increased virulence contributing to disease development. The objective was to identify PRV-1 isolates present in Norwegian farmed salmon in the years before and after 1999. Archived samples from Norwegian farmed salmon, comprising 204 samples from 1987-2008, were screened for PRV- 1. Coding sequences were obtained for segment S1, S4, M2, L1 and L2, putatively linked to virulence, from six isolates. Phylogenetic analysis and genogrouping was performed. Screening displayed that PRV-1 was common in farmed salmon predating the first described HSMI- outbreak, and sequence analysis demonstrated a mixture of assumed high and low virulent isolates following 1999. Coinfection with isolates belonging to different phylogenetic groups was demonstrated in two fish from 1998 and 2005. Co-infection is a prerequisite for reassortment, linked to the hypothesis of the origin of the high virulent strains through evolution. In the presumed receptor binding protein σ1, a new amino acid change was discovered (E254K). Piscine orthoreovirus-1 (PRV-1) er årsaken til hjerte- og skjelettmuskelbetennelse (HSMB) hos oppdrettslaks, først beskrevet i Norge i 1999. Alle PRV-1-infiserte fisk utvikler imidlertid ikke HSMB og viruset har blitt detektert helt tilbake til 1988, som har ført til en hypotese om at viral evolusjon har generert nye genotyper med økt virulens som har bidratt til sykdomsutvikling. Hovedmålet med studien var å kartlegge genomet til PRV-1-isolater før og etter første beskrevne HSMB-utbrudd. 204 arkiverte prøver fra norsk oppdrettslaks fra 1987-2008 ble screenet for PRV-1. Kodende sekvens ble sekvensert for segment S1, S4, M2, L1 og L2, antatt koblet til virulens, fra seks ...
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