Abstract
In 2017, a PCR‐based survey for Piscine orthoreovirus‐3 (PRV‐3) was conducted in wild anadromous and non‐anadromous salmonids in Norway. In seatrout (anadromous Salmo trutta L.), the virus was present in 16.6% of the fish and in 15 of 21 investigated rivers. Four of 221 (1.8%) Atlantic salmon (Salmo salar L.) from three of 15 rivers were also PCR‐positive, with Ct‐values indicating low amounts of viral RNA. All anadromous Arctic char (Salvelinus alpinus L.) were PCR‐negative. Neither non‐anadromous trout (brown trout) nor landlocked salmon were PRV‐3 positive. Altogether, these findings suggest that in Norway PRV‐3 is more prevalent in the marine environment. In contrast, PRV‐3 is present in areas with intensive inland farming in continental Europe. PRV‐3 genome sequences from Norwegian seatrout grouped together with sequences from rainbow trout (Oncorhynchus mykiss Walbaum) in Norway and Coho salmon (Oncorhynchus kisutch Walbaum) in Chile. At present, the origin of the virus remains unknown. Nevertheless, the study highlights the value of safeguarding native fish by upholding natural and artificial barriers that hinder introduction and spread, on a local or national scale, of alien fish species and their pathogens. Accordingly, further investigations of freshwater reservoirs and interactions with farmed salmonids are warranted.
Highlights
A cross‐sectional study designed to investigate the occurrence of Piscine orthoreovirus‐3 (PRV‐3) in wild adult anadromous and non‐anadromous salmonids in Norwegian watercourses was conducted (Table 1 and Figure 1)
Seatrout that are exposed to effluents from hatcheries or infected coastal waters are at risk. Both coun‐ ties represented in this study have a steady production of rainbow trout in the sea (Directorate of fisheries; Statistics for aquaculture 2018), and sequences obtained from wild seatrout had 99%–100% identity with piscine orthoreovirus virus (PRV)‐3 in rainbow trout
Reports from continental Europe provide evidence that PRV‐3 is present in inland farming of rainbow trout and brown trout (Adamek et al, 2018; Bigarre, 2016; Dhamotharan et al, 2018) elaborate on the sequence identity of PRV‐3 found in seatrout
Summary
A cross‐sectional study designed to investigate the occurrence of PRV‐3 in wild adult anadromous and non‐anadromous salmonids in Norwegian watercourses was conducted (Table 1 and Figure 1). From each of the two regions, the two seatrout samples with lowest Ct‐values were selected for verification of posi‐ tive real‐time RT‐PCR findings by conventional RT‐PCR with forward primer 5’‐GAC‐CAA‐CAT‐AAC‐GTT‐TCA‐GGC‐3’ and reverse primer 5’‐ATC‐CAA‐CCA‐CTA‐AAA‐CCG‐AGA‐3’ ampli‐ fying a 423 bp fragment of the sigma 3 protein gene followed by Sanger sequencing and conditions as previously described (Olsen et al, 2015). In wild Atlantic salmon, PRV‐3 was detected in four of 221 sam‐ ples with Ct‐values close to the limit of detection (34.6–40). These results could not be confirmed by sequencing due to low amount of viral RNA.
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