DAO 99:23-35 (2012)  -  doi:10.3354/dao02444

European freshwater VHSV genotype Ia isolates divide into two distinct subpopulations

S. Kahns1,5,*, H. F. Skall1, R. S. Kaas2, H. Korsholm3, B. Bang Jensen1,6, S. P. Jonstrup1, M. J. Dodge4, K. Einer-Jensen1, D. Stone4, N. J. Olesen1

1National Veterinary Institute, Technical University of Denmark, 8200 Aarhus N, Denmark
2National Food Institute, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark
3Section for Aquaculture, Danish Veterinary and Food Administration, 7100 Vejle, Denmark
4CEFAS Weymouth Laboratory, Weymouth, Dorset DT4 8UB, UK
5Present address: Danish Technological Institute, 8000 Aarhus C, Denmark 6Present address: Norwegian Veterinary Institute, 0106 Oslo, Norway

ABSTRACT: Viral haemorrhagic septicaemia (VHS), caused by the novirhabdovirus VHSV, often leads to significant economic losses to European rainbow trout production. The virus isolates are divided into 4 distinct genotypes with additional subgroups including sublineage Ia, isolates of which are the main source of outbreaks in European rainbow trout farming. A significant portion of Danish rainbow trout farms have been considered endemically infected with VHSV since the first disease outbreak was observed in the 1950s. However, following a series of sanitary programs starting in 1965, VHSV has not been detected in Denmark since January 2009. Full-length G-genes of all Danish VHSV isolates that were submitted for diagnostic analyses in the period 2004−2009 were sequenced and analysed. All 58 Danish isolates from rainbow trout grouped with sublineage Ia isolates. Furthermore, VHSV isolates from infected Danish freshwater catchments appear to have evolved into a distinct clade within sublineage Ia, herein designated clade Ia-1, whereas trout isolates originating from other continental European countries cluster in another distinct clade, designated clade Ia-2. In addition, phylogenetic analyses indicate that VHSV Ia-1 strains have caused a few outbreaks in Germany and the UK. It is likely that viruses have been transmitted from infected site(s) out of the Danish environment, although a direct transmission pathway has not been identified. Furthermore, VHSV Ia-2 isolates seem to have been transmitted to Denmark at least once. Interestingly, one viral isolate possibly persisted in a Danish watershed for nearly 4 yr without detection whereas other subclades of VHSV isolates appear to have been eliminated, probably because of implemented eradication procedures.

KEY WORDS: Viral haemorrhagic septicaemia · Geographic subgroups · Molecular tracing · Genotype Ia-1 · Genotype Ia-2 · Epidemiology

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Cite this article as: Kahns S, Skall HF, Kaas RS, Korsholm H and others (2012) European freshwater VHSV genotype Ia isolates divide into two distinct subpopulations. Dis Aquat Org 99:23-35

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