Emergence and Spread of Piscine orthoreovirus Genotype 3.

Juliane Sørensen,Argelia Cuenca,Niels Jørgen Olesen,Dhamotharan Kannimuthu,Camilla Priess,Niels Henrik Henriksen,Tine Moesgaard Iburg,Niccolò Vendramin

doi:10.3390/pathogens9100823

Abstract

Piscine orthoreovirus (PRV) is a relevant pathogen for salmonid aquaculture worldwide. In 2015, a new genotype of PRV (genotype 3, PRV-3) was discovered in Norway, and in 2017 PRV-3 was detected for first time in Denmark in association with complex disease cases in rainbow trout in recirculating aquaculture systems (RAS). To explore the epidemiology of PRV-3 in Denmark, a surveillance study was conducted in 2017 to 2019. Fifty-three farms, including both flow through and RAS, were screened for PRV-3. Of the farms examined, PRV-3 was detected in thirty-eight (71.7%), with the highest prevalence in grow-out farms. Notably, in Denmark disease outbreaks were only observed in RAS. Additionally, wild Atlantic salmon and brown trout populations were included in the screening, and PRV-3 was not detected in the three years where samples were obtained (2016, 2018, and 2019). Historical samples in the form of archived material at the Danish National Reference Laboratory for Fish Diseases were also tested for the presence of PRV-3, allowing us to establish that the virus has been present in Denmark at least since 1995. Sequence analyses of segment S1 and M2, as well as full genome analyses of selected isolates, did not reveal clear association between genetic makeup in these two segments and virulence in the form of disease outbreaks in the field.

Highlights

Piscine orthoreovirus (PRV) is a segmented double stranded RNA virus belonging to the family Reoviridae, genus Orthoreovirus
The surveillance program conducted in Denmark in 2017 to 2019 revealed that there is a high prevalence of PRV-3 in Danish aquaculture
The virus is widespread within the country, and does not follow a natural pattern, as is expected by virus movement related to trade of live fish and eggs

Summary

Introduction

Piscine orthoreovirus (PRV) is a segmented double stranded RNA virus (dsRNA) belonging to the family Reoviridae, genus Orthoreovirus. PRV virions are non-enveloped particles with a double protein capsid with icosahedral symmetry [1]. The viral genome consists of ten RNA segments denoted S1-4. (S for small, ranging from 1.052 to 1.348 Kb), M1-3 (M for medium, ranging from 2.190 to 2.450 Kb), and L1-3 (L for large, ranging from 3.970 to 4.014 Kb) [1,2,3]. PRV cannot be cultivated in vitro, and only ex vivo infection of red blood cells allows certain replication [4]. To describe the different levels of variation within PRV throughout this paper, the terminology genotype is used for PRV-1, -2, and -3, and subtype to describe the groups within genotypes (e.g., PRV-3a and -3b) [5]. Three genotypes of PRV have been characterized: PRV-1, which has been shown to cause heart and skeletal muscle inflammation (HSMI) in farmed Atlantic salmon (Salmo salar) in Pathogens 2020, 9, 823; doi:10.3390/pathogens9100823 www.mdpi.com/journal/pathogens

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