Cardiomyopathy syndrome (CMS) caused by piscine myocarditis virus (PMCV) has emerged in aquaculture of Atlantic salmon (Salmo salar). The lack of cell culture has hampered studies of the infection. In this study samples from naturally PMCV infected Atlantic salmon from commercial farms were collected and used.
In situ hybridization revealed intense staining for PMCV RNA in myocard in the spongiform layer of the heart, but almost no staining in the compact layer. In kidneys only sporadic staining was seen. Viral RNA was present in all organs, with the highest loads in the heart, kidney, and spleen.
The high viral PMCV RNA loads in the heart were caused by extensive viral mRNA transcription. The high ratio of viral mRNA to viral genomic dsRNA, indicated active transcription but limited production of new viral particles. This suggests that the histopathological changes of the heart are caused by viral mRNA and corresponding viral proteins.
The production of full-length transcripts is regulated, with a reduction in the relative amount of ORF3-containing transcripts at high transcription rates. Efforts to identify alternative hosts, such as fungi, were inconclusive, as fungal sequences were found inconsistently in salmon tissue samples. The results reinforce the need for further research to fully understand PMCV’s life cycle and potential alternative hosts and its whereabouts when it is not infecting the heart of the At-lantic salmon.