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Localised initially in the North Atlantic (Norway, Scotland, Canada), the infectious salmon anaemia virus is now found in all countries where salmon are farmed. The most recent major epidemic occurred in Chile in 2007 and eloquently illustrated the considerable losses that this virus can cause: production fell dramatically within two years, from 400,000 tonnes to 250,000 tonnes in 2009. Spread by healthy carriers, this virus also constitutes a threat to trout farming. Indeed, using experimental infection - injection of the virus or immersion in a contaminated tank -, some rainbow trout strains have displayed mortality rates of up to 30%.

The pathogenic agent (ISAV, for Infectious Salmon Anaemia Virus) belongs to the family of Orthomyxoviridae. This family of viruses (which also include influenza viruses) displays a negative-stranded RNA genome consisting of eight RNA molecules instead of one DNA molecule. In living organisms, RNA molecules serve as a DNA copy for protein synthesis. During the 1980s, this reversed situation, which is observed in numerous viruses, led to the development of a "reverse genetics" technology which consisted in synthesising and cloning a DNA copy of the viral genome to produce complementary DNA (cDNA). This then enabled manipulation of the genome of these viruses using conventional molecular biology tools. It has thus become possible to identify the full set of proteins encoded by the viral genome, and to determine those implicated in virulence. By making precise modifications to the viral genome, it should then be possible to attenuate viral pathogenicity without diminishing the antigenic potential. To ensure their safety, these modifications are targeted so that they will prevent any accidental return to virulence; reference is then made to attenuated viruses or live attenuated vaccines, with optimum protective efficacy.

It was with the aim of developing live attenuated viral strains that the INRA scientists focused on resolving the sequences of the ends of each viral RNA molecule. These regions are essential for the correct functioning of the viral genome as they contain signals for gene transcription, genome replication and encapsidation. But because the viral genome consists of eight independent RNA molecules, it was necessary to obtain the end of each segment. To achieve this, an ISAV strain was cultured on salmon cells in vitro and then the viral RNA was extracted. Using reverse transcription, cDNA fragments of each viral RNA were amplified and fully sequenced. The ends of the eight fragments were finally characterised by RACE (rapid amplification of cDNA ends). The full-length sequence of the ISAV genome is an absolute prerequisite for the elaboration of a functional reverse genetics system for this important virus and will lead to a better understanding of viral protein functions and virus interactions with its host, the salmon. New perspectives for the design of live attenuated vaccines can now be envisaged.

In parallel, the scientists also developed and characterised four monoclonal antibodies directed against three viral proteins: hemagglutinin esterase glycoprotein (HE), the nucleoprotein (N) and the matrix protein (M) associated or not with actin. These antibodies represent essential tools for the diagnosis of this viral disease and have enabled the design of different tests: an indirect immunofluorescence test, a seroneutralisation test, Western blot and Immunoprecipitation assays.

For further information:

• The reverse genetics applied to fish RNA viruses. Biacchesi S., Vet Res. 2011; 42(1): 12. 2011
• Completion of the full-length genome sequence of the infectious salmon anemia virus, an aquatic orthomyxovirus-like, and characterization of mAbs. Merour Emilie; LeBerre Monique; Lamoureux Annie; et al. J Gen Vir 2011 Volume: 92, 528-533
• Fish genotype significantly influences susceptibility of juvenile rainbow trout, Oncorhynchus mykiss (Walbaum), to waterborne infection with infectious salmon anaemia virus. Biacchesi S.; Le Berre M.; Le Guillou S.; et al. J Fish Dis. 2007. Volume: 30 Issue: 10 Pages: 631-636