Deciphering the pathogenesis of melanized focal changes in the white skeletal muscle of farmed Atlantic salmon (Salmo salar).

Melanized focal changes (MFCs) in the fillet of farmed Atlantic salmon is a major quality concern. The changes are thought to initially appear as acute red focal changes (RFCs) that progress into chronic MFCs. Recent findings have indicated that hypoxia may be important in their development, possibly leading to necrosis affecting not only myocytes but also adipocytes. Thus, the aim of this study was to investigate possible hypoxic conditions in RFCs and the subsequent inflammatory responses and lesions in the adipose tissue in RFCs and MFCs. A collection of RFCs, MFCs and control muscle samples from several groups of farmed salmon was studied. Using immunohistochemistry, we found induction of the hypoxia-inducible factor 1 pathway in RFCs. Histological investigations of RFCs and MFCs revealed different stages of fat necrosis, including necrotic adipocytes, a myospherulosis-like reaction and the formation of pseudocystic spaces. Accumulations of foamy macrophages were detected in MFCs, indicating degradation and phagocytosis of lipids. Using in situ hybridization, we showed the presence of tyrosinase- and tyrosinase-related protein-1-expressing amelanotic cells in RFCs, which in turn became melanized in MFCs. In conclusion, we propose a sequence of events leading to the formation of MFCs, highlighting the pivotal role of adiposity, hypoxia and fat necrosis.

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.

Red and melanized focal changes in white skeletal muscle in Atlantic salmon (Salmo salar): Comparative analysis of farmed, wild and hybrid fish reared under identical conditions.

Selective breeding plays a vital role in the production of farmed Atlantic salmon and has shown success in many aspects. Still, challenges related to fish health and welfare continue to result in significant economic losses. One such challenge is red and melanized focal changes (RFC/MFC), which result from acute and chronic inflammation, respectively, in the skeletal muscle. Importantly, RFC/MFC has not been observed in wild Atlantic salmon, suggesting that both external and genetic factors may contribute to the development of inflammation. To investigate the underlying cause of RFC/MFC, we conducted a study involving 1854 Atlantic salmon of farmed, wild and hybrid origin. All fish were reared under identical conditions to minimize the influence of external factors. Throughout the production cycle, the fish was monitored for growth parameters and examined for RFC/MFC using macroscopic and histological analysis. We found no association between the experimental groups and the presence of RFC/MFC. Histological investigations revealed melano-macrophages in the soft tissue in freshwater smolt, although no macroscopic discoloration was observed. MFC showed granulomas in various stages, suggesting a complex progression of the condition. In summary, we conclude that RFC/MFC is primarily caused by external factors found in the rearing facilities of farmed Atlantic salmon.

Anatomical and pathological characteristics of ribs in the Atlantic salmon (Salmo salar L.) and its relevance to soft tissue changes.

Studies on the anatomical and pathological characteristics of ribs in farmed Atlantic salmon (Salmo salar L.) are warranted due to their possible association with red and melanized focal changes (RFC and MFC) in the fillet, a major quality and animal welfare concern. In this work, we provide an anatomical description of ribs based on radiographical and histological analyses. We also address various pathological rib changes and their association to RFC and MFC. In total, 129 fish were investigated; captured wild (n = 10) and hatchery reared (n = 119) Atlantic salmon (3.5-6.1 kg). The fish were selected based on the macroscopic presence of RFC, MFC or no changes (controls). Radiographic results revealed costal abnormalities in all fish groups. By histological investigations of the variations herein, our results provide new insight into the anatomical characteristics including vascularization within the ribs; a potential site for haemorrhage following costal fractures. Costal fractures were detected by radiology in 40 of 129 samples (RFC: 38.4%, MFC: 47.2%, controls: 9.5 %). A statistically significant association was found between costal fractures and red (p = 0.007) and melanized changes (p = 0.000). However, red and melanized changes were also observed in samples with no costal fractures (n = 45), indicating that also other factors influence the development of RFC/MFC.