DAO prepress abstract   -  DOI: https://doi.org/10.3354/dao03797

ABSTRACT: 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 from three different farm sites in Norway encompassing 1293 rainbow trout, all examined at the time of slaughter. Both macroscopic and histological assessments of the changes were performed. RT-qPCR analyses and in situ hybridization (ISH) were used to detect the presence and location, respectively, of potential viruses. Only one 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 one fish. Notably, the presence of inflammatory cells, including melano-macrophages, appeared lower compared to what has been previously documented in Atlantic salmon MFC. 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, the rainbow trout develops MFCs of a different character than farmed Atlantic salmon, and we speculate if the observed pathological differences are contributing to their reduced occurrence in farmed rainbow trout.