Localised initially in the North Atlantic ocean (Norway, Scotland, Canada), the infectious salmon anaemia virus is now found in all countries where salmon are farmed. The most recent major epidemic affected the whole of 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. This virus, which is spread by healthy carriers, also constitutes a threat to trout farming. Indeed, using experimental infection through injection of the virus or immersion in a contaminated tank, some trout varieties 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, is specific insofar as its gene pool is carried by 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 different proteins coded 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 its pathogenic potential without diminishing the antigenic potential. To ensure their safety, these modifications are targeted to prevent any accidental return to virulence; reference is then made to attenuated viruses, with optimum vaccine efficacy.

With the aim of developing live attenuated viral strains INRA scientists focused on obtaining the sequences of the ends of viral RNA molecules. These regions are essential for the correct functioning of the viral genome as they contain the signals for gene transcription and genomic replication. Because the viral genome is spread over eight autonomous RNA molecules, it was also necessary to obtain the end of each of the fragments. To achieve this, an ISAV strain was cultured on salmon cells in vitro and the viral RNA was subsequently extracted. Using reverse transcription, cDNA fragments of each viral RNA were amplified. The researchers then fully sequenced the copies obtained. They characterised the ends of the eight fragments using RACE (rapid amplification of cDNA ends). The full sequence of the ISAV genome is a prerequisite to both a clearer understanding of the functioning of this virus, and revealing its interactions with its host, the salmon. New perspectives for the creation of live attenuated viruses, which constitute highly effective vaccine agents, can now be envisaged.

While sequencing the ISAV genome, the scientists also developed and characterised four monoclonal antibodies directed against three viral proteins (hemagglutinin esterase, or 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.

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