Immune responses and survival in Salmon Gill Pox virus disease: IMMUNOPOX

A serious gill disease in Atlantic salmon was discovered in the early 1990 ies in a Norwegian smolt producing farm. It was suspected to be caused by a virus and this was confirmed many years later, in 2015, when the virus genome was sequenced and diagnostic methods established. Salmon gill poxvirus (SGPV) infects epithelial cells, primarily gills and can lead to severe disease, (SGPVD). In some farms SGPVD and mortalities can be extreme and up to 70% mortalities is reported. in some salmon, extensive pathology is not only seen in the gills, but laso in spleen and kidney in the form of signs of massive blood cell break down. This project, IMMUNOPOX will investigate how the fish combat or fail to combat pox. The project will study host immunity, viral infection and signs of disease and relate this to the disparate outcomes of mortality versus good health. Overall IMMUNOPOX will contribute with basic knowledge of how salmon can fight poxvirus and may also shed some light on the immunesystem of salmon that will be an important steppingstone to make efficient vaccines. In the long term, IMMUNOPOX will ultimately contribute to a fundament for making good decisions regarding health management to reduce losses related to the emerging SGPVD. In WP1, we have performed challenges with yolk sac larvea and shown that the storage temperature of the SGPV infectious material affects the virus infectivity and the success of the experimental infection procedure. Further, we have demonstrated that salmon alevin are susceptible to SGPV and can infect not only epithelial cells in gills but also in oral cavity and skin. These results are expected to be published early in 2024. Next, in WP 2 we want to investigate immune responses that salmon generate when infected with SGPV. To do this, we have produced, three recombinant surface proteins from salmon gill poxvirus in vitro. Now, we will immunize salmon with the three recombinant proteins and collect serum samples serving as positive control samples. We are working until the end of 2023 to scale up the recombinant proteins to produce enough amount for the vaccination trial. In WP4 we investigate the processes leading to the blood cell break down in spleen and kidney in some salmon suffering from SGPVD. Previously, transcriptome and histopathology of gills in salmon suffering from a natural outbreak of SGPVD was published. From these same animals, we are now in the process of investigating the spleen and kidney transcriptome and histopathology. The preliminary results indicate an acute and transient response in spleen and kidney of salmon suffering from SGPVD. Further investigation of the transcriptome to understand processes in spleen and kidney of salmon with SGPVD is ongoing and results are expected to be published in 2024.

In the 2015 The Norwegian Veterinary Institute (NVI) and National Institute of Health (NIH), USA published the Salmon Gill Pox Virus (SGPV) genome and the associated gill disease in Atlantic salmon. Recent studies show that SPGV disease has a wide distribution, is a significant welfare problem and has a huge negative economical impact. The ongoing NRC fincanced project SALPOX ongoing at NVI has resulted in a successful experimental model recreating the enigmatic paradox of healthy recovery versus the acute lethal pox syndrome affecting both gill and the general circulation. This opens for studies as it was previously impossible to generate knowledge of the determinants of survival versus acute death. IMMUNOPOX will over the time course of repeated experimental challenges characterize host immunity, viral infection and pathology with regard to their nature, levels and localization and relate to the disparate outcomes of mortality versus good health. Norwegian University of Life Sciences will contribute with studies of the role of the newly discovered gill mucosal associated immune tissues. This will extend the existing knowledge of SGPVD pathology and test the hypothesis that survivors of SGPVD become immune to SGPVD upon rechallenge. Finally, integration of the data into suggested immuno- and patho-genesis respectively will be made as a foundation for future research to shift the balance towards a healthy outcome. Overall IMMUNOPOX will contribute with basic knowledge of host immune responses during SGPVD, that may also shed new light on the salmon immune system, that will be an important steppingstone to make efficient vaccines. In the long term,IMMUNOPOX will ultimately contribute to a fundament for making good decisions regarding health management to reduce losses related to the emerging SGPVD.