Abstract
Viral haemorrhagic septicaemia ( VHS ), caused by the novirhabdovirus viral haemorrhagic septicaemia virus ( VHSV ), causes significant economic problems to European rainbow trout, O ncorhynchus mykiss ( W albaum), production. The virus isolates can be divided into four distinct genotypes with additional subgroups. The main source of outbreaks in European rainbow trout farming is sublineage Ia isolates. Recently, this group of isolates has been further subdivided in to two subclades of which the I a‐2 consists of isolates occurring mainly in C ontinental E urope outside of D enmark. In this study, we sequenced the full‐length G ‐gene sequences of 24 VHSV isolates that caused VHS outbreaks in P olish trout farms between 2005 and 2009. All these isolates were identified as genotype I a‐2; they divided however into two genetically distinct subgroups, that we name P ol I and P ol II . The P ol I isolates mainly caused outbreaks in the southern part of P oland, while P ol II isolates predominantly were sampled in the north of P oland, although it seems that they have been transmitted to other parts of the country. Molecular epidemiology was used for characterization of transmission pathways. This study shows that a main cause of virus transmission appears to be movement of fish. At least in P olish circumstances trading practices appear to have significant impact on spreading of VHSV infection.