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Piscine orthoreovirus (PRV) in red and melanised foci in white muscle of Atlantic salmon (Salmo salar).

Bjørgen H, Wessel Ø, Fjelldal PG, Hansen T, Sveier H, Sæbø HR, Enger KB, Monsen E, Kvellestad A, Rimstad E, Koppang EO - Vet. Res. (2015)

Bottom Line: Red focal changes contained significantly higher levels of PRV RNA than apparently non-affected areas in white muscle of the same individuals.Some changes displayed a transient form between a red and melanised pathotype, indicating a progression from an acute to a chronic manifestation.We conclude that PRV is associated with the focal pathological changes in the white muscle of farmed Atlantic salmon and is a premise for the development of focal melanised changes.

View Article: PubMed Central - PubMed

Affiliation: Institute of Basic Science and Aquatic Medicine, School of Veterinary Medicine, Norwegian University of Life Sciences, Oslo, Norway. havard.bjorgen@nmbu.no.

ABSTRACT
Melanised focal changes (black spots) are common findings in the white skeletal muscle of seawater-farmed Atlantic salmon (Salmo salar). Fillets with melanised focal changes are considered as lower quality and cause large economic losses. It has been suggested that red focal changes (red spots) precede the melanised focal changes. In the present work, we examined different populations of captive and wild salmon for the occurrence of both types of changes, which were investigated for the presence of different viruses by immunohistochemistry and RT-qPCR. The occurrence of red or melanised foci varied significantly between the populations, from none in wild fish control group, low prevalence of small foci in fish kept in in-house tanks, to high prevalence of large foci in farm-raised salmon. Large amounts of Piscine orthoreovirus (PRV) antigen were detected in all foci. No other viruses were detected. Red focal changes contained significantly higher levels of PRV RNA than apparently non-affected areas in white muscle of the same individuals. Some changes displayed a transient form between a red and melanised pathotype, indicating a progression from an acute to a chronic manifestation. We conclude that PRV is associated with the focal pathological changes in the white muscle of farmed Atlantic salmon and is a premise for the development of focal melanised changes.

No MeSH data available.


Related in: MedlinePlus

Immunohistochemical staining for PRV antigens of red (A-B) and melanised (C-H) focal changes. (A) Abundant amounts of PRV-positive cells (brown) and erythrocytes in the necrotic muscle tissue. (B) Transverse section of necrotic myocytes with intracellular PRV-positive macrophage-like cells and erythrocytes (brown). Distinct granular staining is present in the cytoplasm of the macrophage-like cells. (C) A well-organised granuloma is present in the center of the image, heavily positive for PRV (red), and with vast amounts of elongated melano-macrophages (black). (D) A single granuloma with PRV-positive cells (red) surrounded by heavily pigmented melano-macrophages. (E) A focus with necrotic tissue and infiltrates of melano-macrophages (black) and macrophage-like cells positive for PRV (red). (F) A close-up of E where the distinct reaction in the cytoplasm is evident (red). (G) PRV antigen (red) in melano-macrophages (black) in a necrotic myocyte. Higher resolution image in the upper right corner (100 x). (H) A single necrotic myocyte undergoing phagocytosis and containing abundant PRV-positive macrophage-like cells (red). (A, C) scale bar = 200 μm, (E) scale bar = 100 μm, (B, D, F, G, H) scale bar = 50 μm
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Fig6: Immunohistochemical staining for PRV antigens of red (A-B) and melanised (C-H) focal changes. (A) Abundant amounts of PRV-positive cells (brown) and erythrocytes in the necrotic muscle tissue. (B) Transverse section of necrotic myocytes with intracellular PRV-positive macrophage-like cells and erythrocytes (brown). Distinct granular staining is present in the cytoplasm of the macrophage-like cells. (C) A well-organised granuloma is present in the center of the image, heavily positive for PRV (red), and with vast amounts of elongated melano-macrophages (black). (D) A single granuloma with PRV-positive cells (red) surrounded by heavily pigmented melano-macrophages. (E) A focus with necrotic tissue and infiltrates of melano-macrophages (black) and macrophage-like cells positive for PRV (red). (F) A close-up of E where the distinct reaction in the cytoplasm is evident (red). (G) PRV antigen (red) in melano-macrophages (black) in a necrotic myocyte. Higher resolution image in the upper right corner (100 x). (H) A single necrotic myocyte undergoing phagocytosis and containing abundant PRV-positive macrophage-like cells (red). (A, C) scale bar = 200 μm, (E) scale bar = 100 μm, (B, D, F, G, H) scale bar = 50 μm

Mentions: Large amounts of PRV antigens were found in macrophage-like cells and in erythrocytes in the haemorrhagic necrotic tissue (Figure 6A). PRV-antigen positive cells were observed both outside and within necrotic myocytes. Myophagocytosis was highly prevalent, with substantial number of PRV-antigen positive macrophage-like cells within myocytes. Staining was cytoplasmic even in most PRV-antigen positive cells, though cells also displayed polar granular staining (Figure 6B). Such cytoplasmic PRV inclusions have previously been identified with confocal microscopy and immunofluorescent staining as viral factories [20].Figure 6


Piscine orthoreovirus (PRV) in red and melanised foci in white muscle of Atlantic salmon (Salmo salar).

Bjørgen H, Wessel Ø, Fjelldal PG, Hansen T, Sveier H, Sæbø HR, Enger KB, Monsen E, Kvellestad A, Rimstad E, Koppang EO - Vet. Res. (2015)

Immunohistochemical staining for PRV antigens of red (A-B) and melanised (C-H) focal changes. (A) Abundant amounts of PRV-positive cells (brown) and erythrocytes in the necrotic muscle tissue. (B) Transverse section of necrotic myocytes with intracellular PRV-positive macrophage-like cells and erythrocytes (brown). Distinct granular staining is present in the cytoplasm of the macrophage-like cells. (C) A well-organised granuloma is present in the center of the image, heavily positive for PRV (red), and with vast amounts of elongated melano-macrophages (black). (D) A single granuloma with PRV-positive cells (red) surrounded by heavily pigmented melano-macrophages. (E) A focus with necrotic tissue and infiltrates of melano-macrophages (black) and macrophage-like cells positive for PRV (red). (F) A close-up of E where the distinct reaction in the cytoplasm is evident (red). (G) PRV antigen (red) in melano-macrophages (black) in a necrotic myocyte. Higher resolution image in the upper right corner (100 x). (H) A single necrotic myocyte undergoing phagocytosis and containing abundant PRV-positive macrophage-like cells (red). (A, C) scale bar = 200 μm, (E) scale bar = 100 μm, (B, D, F, G, H) scale bar = 50 μm
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Fig6: Immunohistochemical staining for PRV antigens of red (A-B) and melanised (C-H) focal changes. (A) Abundant amounts of PRV-positive cells (brown) and erythrocytes in the necrotic muscle tissue. (B) Transverse section of necrotic myocytes with intracellular PRV-positive macrophage-like cells and erythrocytes (brown). Distinct granular staining is present in the cytoplasm of the macrophage-like cells. (C) A well-organised granuloma is present in the center of the image, heavily positive for PRV (red), and with vast amounts of elongated melano-macrophages (black). (D) A single granuloma with PRV-positive cells (red) surrounded by heavily pigmented melano-macrophages. (E) A focus with necrotic tissue and infiltrates of melano-macrophages (black) and macrophage-like cells positive for PRV (red). (F) A close-up of E where the distinct reaction in the cytoplasm is evident (red). (G) PRV antigen (red) in melano-macrophages (black) in a necrotic myocyte. Higher resolution image in the upper right corner (100 x). (H) A single necrotic myocyte undergoing phagocytosis and containing abundant PRV-positive macrophage-like cells (red). (A, C) scale bar = 200 μm, (E) scale bar = 100 μm, (B, D, F, G, H) scale bar = 50 μm
Mentions: Large amounts of PRV antigens were found in macrophage-like cells and in erythrocytes in the haemorrhagic necrotic tissue (Figure 6A). PRV-antigen positive cells were observed both outside and within necrotic myocytes. Myophagocytosis was highly prevalent, with substantial number of PRV-antigen positive macrophage-like cells within myocytes. Staining was cytoplasmic even in most PRV-antigen positive cells, though cells also displayed polar granular staining (Figure 6B). Such cytoplasmic PRV inclusions have previously been identified with confocal microscopy and immunofluorescent staining as viral factories [20].Figure 6

Bottom Line: Red focal changes contained significantly higher levels of PRV RNA than apparently non-affected areas in white muscle of the same individuals.Some changes displayed a transient form between a red and melanised pathotype, indicating a progression from an acute to a chronic manifestation.We conclude that PRV is associated with the focal pathological changes in the white muscle of farmed Atlantic salmon and is a premise for the development of focal melanised changes.

View Article: PubMed Central - PubMed

Affiliation: Institute of Basic Science and Aquatic Medicine, School of Veterinary Medicine, Norwegian University of Life Sciences, Oslo, Norway. havard.bjorgen@nmbu.no.

ABSTRACT
Melanised focal changes (black spots) are common findings in the white skeletal muscle of seawater-farmed Atlantic salmon (Salmo salar). Fillets with melanised focal changes are considered as lower quality and cause large economic losses. It has been suggested that red focal changes (red spots) precede the melanised focal changes. In the present work, we examined different populations of captive and wild salmon for the occurrence of both types of changes, which were investigated for the presence of different viruses by immunohistochemistry and RT-qPCR. The occurrence of red or melanised foci varied significantly between the populations, from none in wild fish control group, low prevalence of small foci in fish kept in in-house tanks, to high prevalence of large foci in farm-raised salmon. Large amounts of Piscine orthoreovirus (PRV) antigen were detected in all foci. No other viruses were detected. Red focal changes contained significantly higher levels of PRV RNA than apparently non-affected areas in white muscle of the same individuals. Some changes displayed a transient form between a red and melanised pathotype, indicating a progression from an acute to a chronic manifestation. We conclude that PRV is associated with the focal pathological changes in the white muscle of farmed Atlantic salmon and is a premise for the development of focal melanised changes.

No MeSH data available.


Related in: MedlinePlus