Fish Rhabdoviruses Viral Haemorrhagic Septicaemia Virus (vhsv) and Perch Rhabdovirus (prv): Study of Viral Strains and the Disease Epidemiology in Finland

Viral haemorrhagic septicaemia virus (VHSV) was diagnosed after clinical symptoms for the first time in 2000 from four rainbow trout Oncorhynchus mykiss (Walbaum) farms in Åland and Pyhtää in Finland. Phylogenetic analysis based on the full-length VHSV glycoprotein (G) and nonvirion (NV) genes of the Finnish VHSV isolates in 2000–2004 revealed that all isolates are closely related and grouped in the genotype Id, which suggests the same origin of infection. Finnish isolates were shown to be 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 located close to the presumed ancestral source. Infection with the VHSV genotype Id has spread since then, and the same genotype had been isolated from rainbow trout farms in three separate locations: Åland in the Baltic Sea, and Uusikaupunki in the Gulf of Bothnia, and Pyhtää in the Gulf of Finland. The majority of isolations have been from Åland, and since 2009 have only been from there. The VHSV genotype Id was isolated from Pyhtää only in 2000 and 2001 and from Uusikaupunki once in 2004 and 2008. The pathogenicity of rainbow trout genotype Id isolates was analysed in infection experiments with rainbow trout fry. The cumulative mortalities induced by waterborne and intraperitoneal challenge were approximately from 13% to 40% and 66 % to 90%, respectively, depending on the size of the rainbow trout fingerlings. The Finnish brackish water VHSV genotype Id isolates induce lower mortality than freshwater VHSV isolates in infection experiments but they could represent an intermediate stage of marine isolates evolving towards pathogenicity in rainbow trout. The occurrence of viral haemorrhagic septicaemia virus (VHSV) was examined in the main spawning stocks of wild European river lamprey, Lampetra fluviatilis, in the rivers of Finland from 1999 to 2008. In total, 2621 lampreys as 262 pooled samples were examined virologically during 1999–2008. VHSV was isolated from five lamprey samples from the mouth of the rivers Lestijoki and Kalajoki, which flow into the Bothnian Bay of the Baltic Sea from Finland. The full-length VHSV G gene sequence revealed that the isolates were closely related to the VHSV strains isolated earlier from herring and sprat, Sprattus sprattus (L.), in Gotland and were therefore assigned to VHSV genotype II. The virulence of the lamprey VHSV genotype II isolate was evaluated by an experimental infection trial in rainbow trout fry. No mortality was induced post-infection by either waterborne or intraperitoneal challenge. To clarify the role of wild fish, especially Baltic herring, Clupea harengus membras (L.), in the epidemiology of VHSV in brackish waters, Baltic herring with no visible signs of disease were collected from the Archipelago Sea, the Gulf of Bothnia and from the Eastern Gulf of Finland. In total 7580 herring as 758 pooled