Viral Haemorrhagic septicaemia virus (VHSV), the causative agent of viral haemorrhagic septicaemia (VHS), is widespread among several fresh water and marine fish species, both farmed and wild. The virulence of different VHSV strains to different fish species varies, and rainbow trout Oncorhynchus mykiss is regarded as one of the most susceptible species. The presence of VHSV in wild fish has been considered as a threat to the salmonid farming industry.
The recent outbreak of VHS in sea farmed rainbow trout in Western Norway (Storfjorden) was caused by VHSV of a genotype that has hitherto only been detected in wild marine species and has never earlier been associated with outbreak of VHS in farmed salmonids.
The proposal suggests an approach to this objective by screening saltwater populations of wild fish species, including salmonids, for VHSV with subsequent genetic characterization of detected virus. Experimental pathogenicity studies performed on susceptible species and development of a risk-based surveillance system will contribute to the understanding of possible transmission of VHSV from wild to farmed fish and vice versa.
Viral Haemorrhagic septicaemia virus (VHSV) has been isolated from 82 different fish species and are globally spread. The virus causes disease outbreaks both in wild and farmed fish. Different genotypes of the virus are isolated in different parts of the world, and most isolates show some species specificity. For instance the genotype 1a, which is highly pathogenic to farmed rainbow trout, shows low mortality to cod in infection trials. Furthermore, VHSV genotype III isolated from cod shows low mortality to rainbow trout. In 2007 VHSV of genotype III resulted in a VHS outbreak in rainbow trout in several fish farms in one fjord in Norway. The mortality rates were moderate and the fish were slaughtered. Experimental studies confirmed that this genotype III isolate showed higher mortality rates compared to other genotype III isolates from marine fish. The source of infection is still unknown.
- The Institute of Marine Research, Bergen
- FRS Marine laboratory, UK
- National Veterinary Institute, DK