Abstract
Cardiomyopathy syndrome ( CMS ) in A tlantic salmon, S almo salar L ., is a severe cardiac disease characterized by a necrotizing myocarditis involving the atrium and the spongious part of the ventricle. The disease is caused by piscine myocarditis virus ( PMCV ), a double‐stranded RNA virus likely belonging to the family Totiviridae . The objective of this study was to evaluate the genetic variation in Norwegian PMCV isolates focusing on the putative structural proteins encoded by open reading frames ( ORF s) 1 and 3. The virus isolates were sampled from a total of 36 farms along the Norwegian coastline. This study represents the first investigation of PMCV genome variation and shows that Norwegian isolates are highly similar, with the most divergent isolates sharing 98.6% nucleotide identity. Interestingly, amino acid sequence diversity within ORF 3 is approximately threefold higher than for ORF 1. While phylogenetic analysis based on concatenated nucleotide data covering ORF 1 and ORF 3 revealed four main clusters, the maximum sequence variation of 1.4% at the nucleotide level suggests that all N orwegian isolates belong to a single genogroup. Substantial sequence variation within farms was also observed, which may complicate future molecular epidemiological investigations. The genetic homogeneity among the N orwegian isolates might facilitate development of both diagnostic tools and an efficient vaccine against CMS in the future.