Understanding Salmon gill poxvirus disease; an emerging threat for Atlantic salmon farming

Researchers at The Norwegian Veterinary Institute (NVI) and National Institute of Health (NIH) USA, published a ground breaking paper on the first poxvirus genome from a fish. The virus is in the same family as the smallpox agent and is associated with an emerging gill disease of Atlantic salmon. We established tools to detect the virus, salmon gill poxvirus (SGPV), and monitoring is therefore possible. This project, SALPOX, will develop models needed to achieve fundamental characterization of the virus and the disease it is associated with. Further attempt were initiated to cultivate SGPV in cell cultures. Since experiences from the field and challenge experiments, suggest that stress may be a decisive factor for the induction of SGPVD, we attempted to cultivate pox in ASG-10 cell line in the presence of cortisol. No cytopathic effect (CPE) was observed in the tested cultures and qPCR results indicated lack of SGPV replication in the tested cell culture. Furthermore, since some report suggest that sonication and freeze-thaw of poxviruses (especially avipox and mammalian pox) prior to inoculation onto a cell line, has effect on their ability to attach and replicate in a cell culture, ASG-10 cells were exposed to sonicated and freeze-thawed samples of SGPV-positive Atlantic salmon gill homogenates and inoculum from concentrated water samples. No cytopathic effect (CPE) was observed in the tested cultures and qPCR results indicated lack of SGPV replication. We also tested the susceptibility/permissibility of the insect cells SF9 cells to SGPV. Although SF9 cells incubated with virus-containing materials showed higher mortalities as compared to that incubated with control (non-virus containing) materials, but neither of the qPCR nor the RNAscope offered reliable evidences that SGPV could replicate in SF9 cells.

SALPOX has clearly demonstrated that cortisol injection of salmon infected with SGPV lead to clinical disease. This confirms experiences from the field, where stress is suspected as a risk factor for development of SGPVD in SGPV infected animals and handling and other types of stressful interventions should be avoided. Further, SALPOX has shown that the host gill transcriptome during an outbreak reflects the pathological observations and indicates immunosuppression. This can help explain the high levels of coinfections in gills after an SGPVD outbreak, and suggest that transcriptional biomarkers can be used alongside pathological evaluations, and in the short term, help to understand SGPVD and ultimately prevent the disease and complex gill diseases. The new NRC financed projects IMMUNOPOX and TRACEPOX benefit highly from the knowledge generated in SALPOX and the research on SGPV and the disease in may cause will continue in the next years in these projects

Researchers at The Norwegian Veterinary Institute (NVI) and National Institute of Health (NIH) USA, have just published a ground breaking paper on the first poxvirus genome from a fish, associated with an emerging disease of Atlantic salmon. Previous lack of diagnostic tools has hampered the control of this disease. Our new tools open new possibilities, but funding is needed. This project, SALPOX, will develop models needed to achieve fundamental characterization of SGPVD. Western Fisheries research center, USA, will contribute in establishment of a culture system for SGPV and National Institutes of Health, USA will provide pox expertise advice. Nofima will contribute on infection mechanisms and transcriptome analysis and Norwegian University of Life Sciences and VESO Vikan will provide facilities for infection trials. Overall this project will provide basic tools and models that ultimately is the fundament for making good decisions regarding health management, especially during the smolitification period, to reduce losses related to the emerging SGPVD.