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Using genomics to expand the knowledge of a non-toxic strain (avirulent) of Infectious Salmon Anemia Virus (ISAV)

Description

Miramichi River Atlantic Salmon (pictured) was one of four wild populations examined in the project. These fish were provided by the Miramichi Salmon Conservation Centre at the parr stage and grown to smolt stage at SABS prior to commencement of the challenge.

Since the initial identification of Infectious Salmon Anemia Virus (ISAV), a variety of strains with variable virulence (toxicity) have been discovered. They range from the essentially avirulent strains, such as the HPR0 type, to the highly virulent HPR4 strain and many types in between. The foundation for this research lies in the idea that ISAV-HPR0 can be transmitted among a population and create a state similar to what is referred as “herd immunity”, where fish exposed to ISAV-HPR0 would be naturally immune to more virulent ISAV strains, thus reducing the risk of disease outbreaks. This project attempted to infect salmon with ISAV-HPR0 and study the immune response at the site of entry (gills), transmission among a population, viral persistence, and fish resistance to further exposure with virulent forms of ISAV. Additionally, this work was intended to promote different techniques for revealing the presence of the virus, including non-lethal detection possibilities via the gills as current methods depend solely on destroying fish and measuring virus amounts in tissues.

Work: Given that the HRP0 virus cannot be grown by traditional cell culture, it became necessary to develop a model for ISAV-HPR0 infection in salmon. Measuring the persistence of the HPR0 virus and its distribution within tissues would follow. From there, the optimal tissue for detection (gills, kidney, and/or other tissue) was to be determined. The project then attempted to verify the genetic response, at the cellular level, of salmon gill tissues to HPR0. As ISAV-HPR0 does not destroy cells in culture, contrary to other ISAV strains, such study will provide insight into the process of cell death used by toxic ISAV strains. The study would then continue with the investigation of the infectious potential of HPR0, by exposing non-infected salmon with those fish known to be infected with HRP0. Finally, the research focused on fish previously infected with HPR0 to determine if they were able to resist an exposure to virulent ISAV-HPR4 strain (immunity challenge).

Results: The initial HRP0 infection trial (using infected gill tissue from a local source), was unsuccessful as the presence of the virus in the experimentally infected fish could not be confirmed. A larger sample of live fish was obtained from a hatchery that was experiencing an ISA-HRP0 outbreak for another trial. These fish that were HPR0 exposed reached a 50% death rate when challenged with HRP4 infected fish. It is therefore thought that although most of these fish had likely been exposed to HPR0, those that survived were the only ones that had a widespread infection, meaning only those that survived displayed immunity. A set of fish were tested using a genomic signature and while data analysis is still ongoing, a statistically significant difference between animals (those who displayed a response typical of an active immunity against ISAV versus those with no response) would suggest a possible link for HRP0 immunity and further exploration would be warranted.

To make the best use of time and funding that resulted because of the problems with the initial acquisition of HPR0 virus, an alternative trial was chosen by the research team in which three new strains (NS1, NS2, NL1) that were implicated in viral outbreaks in 2012 were assessed for their virulence as compared to HRP4. Next generation genomic techniques were used to measure the response of salmon to the ISA infection and to obtain the full genetic code of each ISA strain. The amount of virus in tissues of the infected salmon gave a picture of virus progression, with gills first being infected, then blood, and kidney. From the results, it would appear that in the long term, blood remains a good source for non-lethal carrier detection. Fish who recovered from the challenge were then tested for their infectious or carrier status by exposing them to uninfected fish. Although the virus was barely detectable in a small proportion of fish after recovery, new fish became infected rapidly. Gill samples were selected for further testing to measure the response in this tissue at the initial stage of infection and to compare the more resistant fish groups. Data analysis is ongoing.

Scientific title for this project:

HPR0 non-pathogenic Infectious Salmon Anemia Virus (ISAV) in vivo: a comparative genomic study

Program Name

Genomics Research and Development Initiative (GRDI)

Year(s)

2011 - 2014

Principal Investigator(s)

Nellie Gagné
lead

Francis Leblanc

Mark Laflamme

Brian Glebe

Steve Leadbeater

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