In Finland, viral haemorrhagic septicaemia virus (VHSV) was diagnosed for the first time in 2000 from 4 rainbow trout farms in brackish water. Since then the infection has spread and, by the end of 2004, VHSV had been isolated from 24 farms in 3 separate locations: 2 in the Baltic Sea and 1 in the Gulf of Finland. The pathogenicity of 3 of these isolates from 2 separate locations was analysed in infection experiments with rainbow trout fry. The cumulative mortalities induced by waterborne and intraperitoneal challenge were approximately 40 and 90%, respectively. Pair-wise comparisons of the G and NV gene regions of Finnish VHSV isolates collected between 2000 and 2004 revealed that all isolates were closely related, with 99.3 to 100% nucleotide identity, which suggests the same origin of infection. Phylogenetic analysis revealed that they were closely related to the old freshwater isolates from rainbow trout in Denmark and to one old marine isolate from cod in the Baltic Sea, and that they were located close to the presumed ancestral source. As the Finnish isolates induce lower mortality than freshwater VHSV isolates in infection experiments, they could represent an intermediate stage of marine isolates evolving towards pathogenicity in rainbow trout. KEY WORDS: Viral haemorrhagic septicaemia virus · VHSV · Rainbow trout · Epidemiology Resale or republication not permitted without written consent of the publisherDis Aquat Org 72: [201][202][203][204][205][206][207][208][209][210][211] 2006 also induce mortality in experimental infections (Kocan et al. 1997). In contrast, the European marine VHSV isolates originating from wild fish have exhibited no or very low pathogenicity in rainbow trout in immersion experiments .The role of these virus strains in maintaining infection in the marine environment remains unclear. Several VHSV strains isolated from fish farms have a high level of genetic similarity with some wild marine fish VHSV isolates (Einer-Jensen et al. 2004, Snow et al. 2004. The close genetic similarity of the pathogenic and non-pathogenic VHSV strains indicates that only small differences in the virus genome may be involved in the determination of VHSV virulence for rainbow trout (Betts & Stone 2000). Since RNA viruses are known to be highly adaptable and to have high mutation rates, VHSV in wild marine fish could pose a permanent threat to rainbow trout farming, especially in the marine environment.VHSV is an enveloped negative-strand RNA virus belonging to the genus Novirhabdovirus of the family Rhabdoviridae (Walker et al. 2000). The VHSV genome is a non-segmented, single-stranded RNA molecule with a length of approximately 11 200 nucleotides. The genome consists of 6 genes in the order 3'-N-P-M-G-NV-L-5', encoding 5 structural proteins: nucleocapsid-(N), phospho-(P), matrix-(M), glyco-(G) and RNA polymerase (L) protein and 1 non-structural (NV) protein (Schütze et al. 1999).VHSV isolates have been shown to cluster into 4 different genotypes, which seem to correlate with the geographical...