Red and melanized focal changes in the white skeletal muscle of farmed rainbow trout Oncorhynchus mykiss.

Fillet discoloration by red and melanized focal changes (RFCs and MFCs) is common in farmed Atlantic salmon Salmo salar. In farmed rainbow trout Oncorhynchus mykiss, similar changes have been noted, but their prevalence and histological characteristics have not been investigated. Thus, we conducted a study encompassing 1293 rainbow trout from 3 different farm sites in Norway, all examined at the time of slaughter. Both macroscopic and histological assessments of the changes were performed. Reverse transcription (RT)-qPCR analyses and in situ hybridization (ISH) were used to detect the presence and location, respectively, of potential viruses. Only 1 RFC was detected in a single fillet, while the prevalence of MFCs ranged from 1.46 to 6.47% between populations. The changes were predominantly localized in the cranioventral region of the fillet. Histological examinations unveiled necrotic myocytes, fibrosis, and regeneration of myocytes. Melano-macrophages were found in the affected areas and in myoseptal adipose tissue. Organized granulomas were observed in only 1 fish. Notably, the presence of inflammatory cells, including melano-macrophages, appeared lower compared to what has been previously documented in Atlantic salmon MFCs. Instead, fibrosis and regeneration dominated. RT-qPCR and ISH revealed the presence of piscine orthoreovirus 1 (PRV-1) and salmonid alphavirus (SAV) in skeletal muscle. However, these viruses were not consistently associated with lesioned areas, contrasting previous findings in Atlantic salmon. In conclusion, rainbow trout develop MFCs of a different character than farmed Atlantic salmon, and we speculate whether the observed pathological differences are contributing to their reduced occurrence in farmed rainbow trout.