Date Published: July 5, 2017
Publisher: Public Library of Science
Author(s): Helena Hauge, Niccolo Vendramin, Torunn Taksdal, Anne Berit Olsen, Øystein Wessel, Susie Sommer Mikkelsen, Anna Luiza Farias Alencar, Niels Jørgen Olesen, Maria Krudtaa Dahle, Jodie L. Rummer.
A new disease in farmed rainbow trout (Onchorhyncus mykiss) was described in Norway in 2013. The disease mainly affected the heart and resembled heart and skeletal muscle inflammation (HSMI) in Atlantic salmon (Salmo salar L.). HSMI is associated with Piscine orthoreovirus (PRV), and a search for a similar virus in the diseased rainbow trout led to detection of a sequence with 85% similarity to PRV. This finding called for a targeted effort to assess the risk the new PRV-variant pose on farmed rainbow trout and Atlantic salmon by studying infection and disease pathogenesis, aiming to provide more diagnostic knowledge. Based on the genetic relationship to PRV, the novel virus is referred to as PRV-Oncorhynchus mykiss (PRV-Om) in contrast to PRV-Salmo salar (PRV-Ss). In experimental trials, intraperitoneally injected PRV-Om was shown to replicate in blood in both salmonid species, but more effectively in rainbow trout. In rainbow trout, the virus levels peaked in blood and heart of cohabitants 6 weeks post challenge, along with increased expression of antiviral genes (Mx and viperin) in the spleen, with 80–100% of the cohabitants infected. Heart inflammation was diagnosed in all cohabitants examined 8 weeks post challenge. In contrast, less than 50% of the Atlantic salmon cohabitants were infected between 8 and 16 weeks post challenge and the antiviral response in these fish was very low. From 12 weeks post challenge and onwards, mild focal myocarditis was demonstrated in a few virus-positive salmon. In conclusion, PRV-Om infects both salmonid species, but faster transmission, more notable antiviral response and more prominent heart pathology were observed in rainbow trout.
Rainbow trout (Onchorynchus mykiss) farming is common in many parts of the world. This salmonid is highly domesticated, fast-growing, and an attractive food source. In 2015, European production of rainbow trout was estimated to be 386 thousand tons, making it the second most farmed fish species in Europe after Atlantic salmon (Salmo salar L.), with 1.57 million tons of salmon produced during the same period . Rainbow trout are primarily produced as portion-sized fish in fresh water and river raceways, whereas larger rainbow trout is primarily sea-reared on the north European coast together with Atlantic salmon.
Due to the emergence of this novel PRV variant in Norway, it was of primary importance to assess the risk associated with PRV-Om infection in rainbow trout and Atlantic salmon. To do so, it was necessary to establish a challenge model under controlled conditions, to evaluate the transmissibility of the virus from infected fish to naïve cohabitants, to assess the pathogenicity of the novel variant in farmed salmonids and provide scientific based evidence for improved diagnostics.