Viral Haemorrhagic Septicemia (VHS) in wild and farmed fish in Norwegian waters

Abstract

Viral Haemorrhagic septicaemia viruses (VHSV) are isolated from 82 different fish species and are globally spread in freshwater as well as marine environments. The virus can cause disease outbreaks both in wild and farmed fish. Different genotypes of the virus have been isolated in different parts of the world. Furthermore, most isolates seem to show some degree of species specificity. For instance, the genotype 1a, that is a major killer in farmed rainbow trout, shows low mortality to cod, and vice versa the genotype III from cod show low mortality to rainbow trout. However, the established view was challenged in 2007, when a VHSV of genotype III resulted in a VHS outbreak on several sites with farmed rainbow in one fjord in Norway. The farmed populations were slaughtered, according to legislative regulation. Experimental challenge studies confirmed that this genotype III isolate showed higher morality rates compared to other genotype III isolates. The source of infection is still unknown, but wild reservoirs are suspected. A major aim is to investigate wild fish along the Norwegian cost for detection of VHSV. New isolates will be characterized and further investigated along with the genotype III isolated from 2007. So far we have tested almost 3000 fish from 36 different fish species. Results from the screening and tentative challenge trials will be presented.