Inter-Research > DAO > v101 > n3 > p197-206  
Diseases of Aquatic Organisms

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DAO 101:197-206 (2012)  -  DOI:

Low virulent infectious salmon anaemia virus (ISAV-HPR0) is prevalent and geographically structured in Norwegian salmon farming

Trude M. Lyngstad1,*, Anja B. Kristoffersen1, 2, Monika J. Hjortaas1, Magnus Devold3, Vidar Aspehaug3, Rolf B. Larssen4, Peder A. Jansen

1Norwegian Veterinary Institute, PO Box 750 Sentrum, 0106 Oslo, Norway
2University of Oslo, Department of Informatics, PO Box 1080, Blindern, 0316 Oslo, Norway
3PatoGen Analyse AS, PO Box 1527, 6025 Ålesund, Norway
4Norwegian School of Veterinary Science, PO Box 8146 Dep, 0033 Oslo, Norway

ABSTRACT: Infectious salmon anaemia (ISA) is a severe disease in farmed Atlantic salmon Salmo salar that has caused epidemic outbreaks in most salmon-producing countries worldwide. The disease is caused by virulent ISA virus (ISAV). Low virulent variants of the virus, characterised by a full-length sequence in the highly polymorphic region of segment 6 in the virus genome, have been reported with increasing frequencies. These variants of the virus, termed HPR0, have been proposed to be ancestors of virulent ISAV. We examined this idea through studies of the phylogeographic and environmental distribution of ISAV-HPR0, as well as phylogeographic associations between virulent ISAV and ISAV-HPR0. Samples from 232 fish groups were screened for ISAV. Real-time RT-PCR was used for detection of ISAV, and the ISAV haemagglutinin esterase (HE) gene was characterised for positive samples. A Mantel test was used to test phylogeographic associations between pairs of ISAV-HPR0 HE gene sequences. A rank test was used to test associations between HE gene sequences from virulent ISAV and ISAV-HPR0. ISAV-HPR0 was detected in fish groups both in freshwater and marine environments, and in juveniles, on-grown marine salmon and broodstock salmon. Genetic and geographic distances between pairs of ISAV-HPR0 HE gene sequences were positively correlated, suggesting that the population of ISAV-HPR0 is geographically structured. Finally, we found a spatial association between fish groups with virulent ISAV (n = 21) and fish groups with ISAV-HPR0 (n = 27), supporting the hypothesis that ISAV-HPR0 may undergo a transition to virulent ISAV.

KEY WORDS: Infectious salmon anaemia virus · ISAV · HPR0 · Atlantic salmon · Norway

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Cite this article as: Lyngstad TM, Kristoffersen AB, Hjortaas MJ, Devold M, Aspehaug V, Larssen RB, Jansen PA (2012) Low virulent infectious salmon anaemia virus (ISAV-HPR0) is prevalent and geographically structured in Norwegian salmon farming. Dis Aquat Org 101:197-206.

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