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

Håvard Bjørgen,Katrine Bones Enger,Erling Olaf Koppang,Per Gunnar Fjelldal,Agnar Kvellestad,Tom Hansen,Håkon Rydland Sæbø,Espen Rimstad,Harald Sveier,Øystein Wessel,Eirik Monsen

doi:10.1186/s13567-015-0244-6

Håvard Bjørgen, Katrine Bones Enger + Show 9 more

Open Access

https://doi.org/10.1186/s13567-015-0244-6

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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.

Highlights

Farmed Atlantic salmon may develop melanised focal changes in the white muscle [1]
As red focal changes are much less frequent compared with melanised focal changes, an additional sampling was conducted where only muscle samples were collected on RNAlater and formalin (Group C)
Large discoloured focal changes in the white muscle, mainly in the cranio-dorsal and abdominal regions, were found in Groups A, B, C, D and E, but not in Groups F, G and H, including fish kept in in-house tanks and in wildcaught individuals (Figure 1 and Table 3)

Summary

Introduction

Farmed Atlantic salmon may develop melanised focal changes in the white muscle [1]. Changes typically appear as distinct areas within the white muscle, frequently located in the cranioventral and craniodorsal regions of the abdominal wall, but may be found elsewhere in the musculature [1,3]. This condition causes large economic losses as fillets with pigmentation disorders are downgraded. Melanised focal changes are common in farmed Atlantic salmon all along the Norwegian coast. There are no reports of melanised focal changes in wild-

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