Genetic variation of Piscine orthoreovirus and the presence of HSMI in farmed Atlantic salmon from Arctic Norway
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Piscine orthoreovirus-1 (PRV1) is ubiquitous throughout the world and can cause heart- and skeletal muscle inflammation (HSMI) in farmed Atlantic salmon (Salmo salar). HSMI was first diagnosed in Norway in 1999. The virus is highly prevalent among farmed salmon in Norway, but the pathogen doesn’t always induce HSMI. PRV1 groups in two major clades, PRV1a and PRV1b, which is believed to vary in virulence and severity of disease. The putative low virulent PRV1a is dominating in North American Pacific Coast (NAPC) where only mild, or no lesions have been described from infected salmon. By looking at previously sequenced isolates from Norway, the putative high virulent PRV1b is dominating with one PRV1a isolate from 1988. Severe inflammation in heart- and skeletal muscle known as HSMI is frequently diagnosed in Norway. Virulence is previously linked to the viral proteins encoded by S1 and M2 segment, respectively. The aim of this study was to identify the genotypes of PRV1 associated with heart- and skeletal inflammation (HSMI) in farmed Atlantic Salmon in Northern Norway. Phylogenetic and sequence analyses, of segment S1 and M2, were performed on 58 PRV1 sequence isolates collected from the production sites in this study with the aim to further elucidate the linkage between the segments and the virulence of the genogroup. The results in the study confirms and further strengthen the classifications which differentiates PRV1 in two subgenotypes, PRV1a and PRV1b. Both clades were shown to be present in farmed salmon in the arctic region of Norway. PRV1b was shown to be the dominating clade. In sites where PRV1a was present its prevalence varies and was found to be 10.5 % at most. The actual importance of this in the field is unknown and needs to be explored further. The genetic variations of PRV1b were less significant with a few exceptions. Both clades are present in farmed Atlantic salmon in Arctic Norway. The prevalence of HSMI is overall high, regardless of which genotype is present at site. One site with PRV1b did not experience elevated mortality because of HSMI, which may indicate that other factors, as environmental factors are essential for development of HSMI.