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

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.

The ontogeny of complement component C3 in the spotted wolffish (Anarhichas minor Olafsen).

Eggs and larvae from the spotted wolffish (Anarhichas minor Olafsen) were analysed to study the ontogeny of the complement component C3. By Western blotting experiments, the results revealed that C3 was present already in unfertilised eggs and that there was a decrease in C3 content towards hatch, whereas the results using reverse transcription-polymerase chain-reaction showed a continuous increase of C3 mRNA content towards hatch. These findings indicate a transfer of maternal complement component C3 to the offspring where the content of C3 decreases and the level of C3 mRNA increases towards hatch. Immunohistochemical experiments revealed that only the liver of spotted wolffish larvae contained complement component C3, suggesting that the liver is the main site of production.