title: Atlantic salmon (Salmo salar L.) post-smolts challenged two or nine weeks after seawater-transfer show differences in their susceptibility to salmonid alphavirus subtype 3 (SAV3).
abstract: BACKGROUND
  Pancreas disease (PD), caused by salmonid alphavirus (SAV), is an important disease affecting salmonid aquaculture. It has been speculated that Atlantic salmon post-smolts are more prone to infections in the first few weeks following seawater- transfer. After this period of seawater acclimatization, the post-smolts are more robust and better able to resist infection by pathogens. Here we describe how we established a bath immersion (BI) model for SAV subtype 3 (SAV3) in seawater. We also report how this challenge model was used to study the susceptibility of post-smolts to SAV3 infection in two groups of post-smolts two weeks or nine weeks after seawater - transfer.
  
  
  METHODS
  Post-smolts, two weeks (Phase-A) or nine weeks (Phase-B) after seawater- transfer, were infected with SAV3 by BI or intramuscular injection (IM) to evaluate their susceptibility to infection. A RT-qPCR assay targeting the non-structural protein (nsP1) gene was performed to detect SAV3-RNA in blood, heart tissue and electropositive-filtered tank-water. Histopathological changes were examined by light microscope, and the presence of SAV3 antigen in pancreas tissue was confirmed using immuno-histochemistry.
  
  
  RESULTS
  Virus shedding from the Phase-B fish injected with SAV3 (IM Phase-B) was markedly lower than that from IM Phase-A fish. A lower percentage of viraemia in Phase-B fish compared with Phase-A fish was also observed. Viral RNA in hearts from IM Phase-A fish was higher than in IM Phase-B fish at all sampling points (p < 0.05) and a similar trend was also seen in the BI groups. Necrosis of exocrine pancreatic cells was observed in all infected groups. Extensive histopathological changes were found in Phase-A fish whereas milder PD-related histopathological lesions were seen in Phase-B fish. The presence of SAV3 in pancreas tissue from all infected groups was also confirmed by immuno-histochemical staining.
  
  
  CONCLUSIONS
  Our results suggest that post-smolts are more susceptible to SAV3 infection two weeks after seawater-transfer than nine weeks after transfer. In addition, the BI challenge model described here offers an alternative SAV3 infection model when better control of the time-of-infection is essential for studying basic immunological mechanisms and disease progression.
authors.affiliation: Institute of Marine Research, Nordnesgaten 50, 5005, Bergen, Norway.|Department of Biology, University of Bergen, P.O. Box 7803, N-5020, Bergen, Norway.
authors.forename: J
authors.initials: J
authors.lastname: Jarungsriapisit
authors.affiliation: Institute of Marine Research, Nordnesgaten 50, 5005, Bergen, Norway.
authors.forename: L J
authors.initials: LJ
authors.lastname: Moore
authors.affiliation: Institute of Marine Research, Nordnesgaten 50, 5005, Bergen, Norway.
authors.forename: G L
authors.initials: GL
authors.lastname: Taranger
authors.affiliation: Uni Research Environment, Uni Research, Thormøhlensgt., 49 B, 5006, Bergen, Norway.
authors.forename: T O
authors.initials: TO
authors.lastname: Nilsen
authors.affiliation: Institute of Marine Research, Nordnesgaten 50, 5005, Bergen, Norway.
authors.forename: H C
authors.initials: HC
authors.lastname: Morton
authors.affiliation: Institute of Marine Research, Nordnesgaten 50, 5005, Bergen, Norway.
authors.forename: I U
authors.initials: IU
authors.lastname: Fiksdal
authors.affiliation: Department of Biology, University of Bergen, P.O. Box 7803, N-5020, Bergen, Norway.
authors.forename: S
authors.initials: S
authors.lastname: Stefansson
authors.affiliation: Institute of Marine Research, Matre Research Station, Matredal, Norway.
authors.forename: P G
authors.initials: PG
authors.lastname: Fjelldal
authors.affiliation: Norwegian University of Life Sciences, Faculty of Veterinary Medicine and Biosciences, P.O. Box 8146 Dep, N-0033, Oslo, Norway.
authors.forename: Ø
authors.initials: Ø
authors.lastname: Evensen
authors.affiliation: Institute of Marine Research, Nordnesgaten 50, 5005, Bergen, Norway. sonal.patel@imr.no.
authors.forename: S
authors.initials: S
authors.lastname: Patel
mesh_terms: D018354:Alphavirus Infections / Q000276:immunology / Q000662:veterinary* / Q000821:virology; D000818:Animals; D017756:Aquaculture; D001769:Blood / Q000821:virology; D004198:Disease Susceptibility; D005393:Fish Diseases / Q000276:immunology* / Q000821:virology; D006321:Heart / Q000821:virology; D006651:Histocytochemistry; D007273:Injections, Intramuscular; D008853:Microscopy; D010179:Pancreas / Q000473:pathology / Q000821:virology; D012367:RNA, Viral / Q000032:analysis; D060888:Real-Time Polymerase Chain Reaction; D020317:Salmo salar / Q000821:virology*; D012623:Seawater / Q000821:virology
chemical_list: D012367:RNA, Viral
keywords: ATPase activity; Bath challenge; Bath immersion; Condition factor; Pancreas disease; Plasma cortisol; SPDV; Salmon pancreas disease virus; Viral shedding