First description of clinical presentation of piscine orthoreovirus (PRV) infections in salmonid aquaculture in Chile and identification of a second genotype (Genotype II) of PRV.

Marcos G Godoy,Rudy Suarez,Yingwei Wang,Francisco Vallejos,Molly J T Kibenge,Frederick S B Kibenge,Camila Leiva

doi:10.1186/s12985-016-0554-y

Abstract

BackgroundHeart and skeletal muscle inflammation (HSMI) is an emerging disease of marine-farmed Atlantic salmon Salmo salar, first recognized in 1999 in Norway, and recently associated with piscine orthoreovirus (PRV) infection. To date, HSMI lesions with presence of PRV have only been described in marine-farmed Atlantic salmon in Norway. A new HSMI-like disease in rainbow trout Oncorhynchus mykiss associated with a PRV-related virus has also been reported in Norway.MethodsSampling of Atlantic salmon and coho salmon was done during potential disease outbreaks, targeting lethargic/moribund fish. Fish were necropsied and tissues were taken for histopathologic analysis and testing for PRV by RT-qPCR assay for segment L1 and conventional RT-PCR for PRV segment S1. The PCR products were sequenced and their relationship to PRV strains in GenBank was determined using phylogenetic analysis and nucleotide and amino acid homology comparisons.ResultsThe Atlantic salmon manifested the classical presentation of HSMI with high PRV virus loads (low Ct values) as described in Norway. The coho salmon with low Ct values had myocarditis but only in the spongy layer, the myositis of red muscle in general was mild, and the hepatic necrosis was severe. Upon phylogenetic analysis of PRV segment S1 sequences, all the Chilean PRV strains from Atlantic salmon grouped as sub-genotype Ib, whereas the Chilean PRV strains from coho salmon were more diversified, grouping in both sub-genotypes Ia and Ib and others forming a distinct new phylogenetic cluster, designated Genotype II that included the Norwegian PRV-related virus.ConclusionsTo our knowledge the present work constitutes the first published report of HSMI lesions with presence of PRV in farmed Atlantic salmon outside of Europe, and the first report of HSMI-like lesions with presence of PRV in coho salmon in Chile. The Chilean PRV strains from coho salmon are more genetically diversified than those from Atlantic salmon, and some form a distinct new phylogenetic cluster, designated Genotype II.Electronic supplementary materialThe online version of this article (doi:10.1186/s12985-016-0554-y) contains supplementary material, which is available to authorized users.

Highlights

Heart and skeletal muscle inflammation (HSMI) is an emerging disease of marine-farmed Atlantic salmon Salmo salar, first recognized in 1999 in Norway, and recently associated with piscine orthoreovirus (PRV) infection
63 Atlantic salmon were analyzed from two farm sites, which were selected from 17 Atlantic salmon farm sites studied, and 85 coho salmon were analyzed from two farm sites, which were selected from nine coho salmon farm sites studied
The farm sites were chosen in order to increase the probability of detecting HSMI disease in Atlantic salmon or HSMI-like disease in coho salmon with the goal to establish a causal relationship between PRV and HSMI

Summary

Introduction

Heart and skeletal muscle inflammation (HSMI) is an emerging disease of marine-farmed Atlantic salmon Salmo salar, first recognized in 1999 in Norway, and recently associated with piscine orthoreovirus (PRV) infection. A new HSMI-like disease in rainbow trout Oncorhynchus mykiss associated with a PRV-related virus has been reported in Norway. The main salmon species farmed in Chile include Atlantic salmon Salmo salar, coho salmon Oncorhynchus kisutch, and rainbow trout O. mykiss. Outside of Norway, PRV has been detected by RT-qPCR in farmed Atlantic salmon in Chile [8] and Ireland [9], in wild and farmed Atlantic salmon in Denmark [10], in farmed Atlantic salmon and wild chum salmon O. keta, rainbow trout O. mykiss and cutthrout trout O. clarkii in British Columbia-Canada [8], in marine-farmed Chinook salmon O. tshawytscha [11], in wild coho salmon O. kisutch from Alaska-USA [12], and recently in hatchery Chinook salmon and coho salmon in Washington stateUSA [13]. PRV was reported as being enzootic in farmed and wild salmonids on the Canada/US Pacific Coast [14]

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