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 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.
摘要：
在养殖的大西洋鲑鱼Salmosalar中，由于红色和黑化的病灶变化（RFC和MFC）引起的圆角变色很常见。在养殖的虹鳟鱼Oncorhynchusmykiss中，已经注意到类似的变化，但其患病率和组织学特征尚未得到研究。因此，我们进行了一项研究，包括来自挪威3个不同农场的1293条虹鳟鱼，都是在屠杀时检查的。对变化进行宏观和组织学评估。逆转录（RT）-qPCR分析和原位杂交（ISH）用于检测存在和位置，分别,潜在的病毒。在单个圆角中只检测到1个RFC，而MFCs的患病率在人群之间为1.46％至6.47％。这些变化主要位于圆角的颅腹区域。组织学检查揭示了坏死心肌细胞，纤维化，和肌细胞的再生。在受影响的区域和肌间隔脂肪组织中发现了黑素巨噬细胞。仅在1条鱼中观察到有组织的肉芽肿。值得注意的是,炎症细胞的存在，包括黑胶巨噬细胞,与以前在大西洋鲑鱼MFC中记录的相比，似乎更低。相反，纤维化和再生占主导地位。RT-qPCR和ISH揭示了骨骼肌中存在猪直道病毒1（PRV-1）和沙门氏菌甲病毒（SAV）。然而,这些病毒并不总是与病变区域相关，对比以前在大西洋鲑鱼中的发现。总之,虹鳟鱼发展出与养殖大西洋鲑鱼不同的特性的MFC，我们推测观察到的病理差异是否有助于减少养殖虹鳟鱼的发病率。
