Genetic diversity of piscine myocarditis virus in Atlantic salmon Salmo salar L. in Ireland.

Piscine myocarditis virus (PMCV) is a double-stranded RNA virus which has been linked to cardiomyopathy syndrome (CMS) in Atlantic salmon (Salmo salar L.). The first recorded outbreak of CMS in Ireland occurred in 2012. Heart tissue samples were collected in the current study from farmed Atlantic salmon from various marine sites around Ireland, and the open reading frames (ORFs) 1 and 3 were amplified and sequenced in order to examine the genetic diversity of PMCV. Results showed PMCV to be largely homogenous in Irish samples, showing little genetic diversity. However, several amino acid positions within both ORF1 and ORF3 showed consistent variations unique to the Irish PMCV strains when compared with previously published Norwegian strains. The phylogeny generated in the present study suggests that PMCV may have been introduced into Ireland in two waves, both coming from the southern part of PMCV's range in Norway. In addition, over three-quarters of the PMCV strains which were sequenced came from fish not exhibiting any clinical signs of CMS, suggesting that either PMCV is evolving to become less virulent in Ireland or Irish Atlantic salmon are developing immunity to the disease.