Outbreak of viral haemorrhagic septicaemia (VHS) in seawater-farmed rainbow trout in Norway caused by VHS virus Genotype III

Abstract

We describe the finding of a novel viral haemorrhagic septicaemia virus (VHSV) Genotype III strain that caused disease of both a neurological and septicaemic nature in seawater-farmed rainbow trout Oncorhynchus mykiss in Storfjorden, Norway. In November 2007, an outbreak of VHS associated with slightly elevated mortality was confirmed at a seawater site rearing rainbow trout (90 to 440 g). Within 3 to 4 mo, the disease was recognised in 3 neighbouring sea sites with ongrowing rainbow trout. The clinical, gross pathological and histopathological findings were in accordance with VHS, and the diagnosis was confirmed by the detection of VHSV in brain and internal tissues by immunohistochemistry, cell culture and reverse transcriptase PCR (RT-PCR). Sequence analysis of the G-gene revealed that the isolated virus clustered with VHSV Genotype III and that the Norwegian isolate represents a unique strain of VHSV. The pathogenicity of the virus strain to rainbow trout and Atlantic salmon Salmo salar was examined using infection experiments. In immersion trials, the Norwegian isolate produced a cumulative mortality of 70% in rainbow trout, while nearly 100% mortality was obtained after intraperitoneal injection of the virus. For Atlantic salmon, no mortality was observed in immersion trials, whereas 52% mortality was observed after intraperitoneal injection. The Norwegian isolate thus represents the first VHSV of Genotype III pathogenic to rainbow trout.

A low-pathogenic variant of infectious salmon anemia virus (ISAV-HPR0) is highly prevalent and causes a non-clinical transient infection in farmed Atlantic salmon (Salmo salar L.) in the Faroe Islands

“…Such factors are known to drive the faster evolution of other fish RNA viruses within aquaculture, such as viral hemorrhagic septicemia virus (VHSV) (Einer-Jensen et al, 2004). In this case, adaptation of the virus to an alternate 'pathogenic' lifestyle has occurred several times within aquaculture (Einer-Jensen et al, 2004;Dale et al 2009). In the wild, owing to a lack of available hosts, such a strategy would probably represent an evolutionary dead end and lead to viral extinction.…”

Immunohistochemistry and pathology of multiple Great Lakes fish from mortality events associated with viral hemorrhagic septicemia virus type IVb

“…Since VHSV has become very topical in the Great Lakes basin and the molecular tests used are very sensitive (Hope et al 2010), there is a possibility that every mortality event from which VHSV nucleic acid or virus is obtained will be ascribed to the virus. Immunohistochemistry (IHC) is routinely used to localize a pathogen to a lesion pattern and has been used previously for other genotypes of VHSV (Evensen & Olesen 1997, Brudeseth et al 2005, Dale et al 2009). …”

Sublethal concentrations of ichthyotoxic alga Prymnesium parvum affect rainbow trout susceptibility to viral haemorrhagic septicaemia virus

“…Results comparing gene sequences of the Finnish VHSV strains support the hypothesis that wild fish populations were the source of the primary infection (Raja-Halli et al 2006). Similarly, the infection route was also assumed to be from wild fish during outbreaks of VHS in marine rainbow trout farms in Sweden during 1998 and 2000 (Skall et al 2005) and in Norway during 2007 (Dale et al 2009). …”

“…Marine rainbow trout farms in Denmark have experienced several VHSV outbreaks since 1982 (Jørgensen 1992, Dale et al 2009). Some of the earlier outbreaks were due to transfer of virus-infected fish from freshwater facilities (Hørlyck et al 1984), but intensive efforts to eradicate VHSV (Olesen 1998) from freshwater farms in Denmark have successfully eliminated this infection route (Bang Jensen et al 2014).…”