Characterization of salmonid alphavirus subtype 3 : recombination and adaptation

Pancreas disease (PD) affecting Atlantic salmon (Salmo salar L.) and rainbow trout (Oncorhynchus mykiss) is a major burden in European salmonid aquaculture and causes major economical losses every year. The disease is caused by Salmon Pancreas disease virus (SPDV), also referred to as salmonid alphavirus (SAV), which belongs to the genus alphavirus within the family Togaviridae. Six subtypes of SAV have so far been reported where SAV subtype 3 and a marine variant of subtype 2 is found in Norway. Currently one commercial vaccine is available, but the effect under field conditions have been debated. Documentation of virulence characteristics and field oriented genome data have been scarce and this work was initiated to enlighten these subjects. Using a SAV3 isolate cultured in both CHSE and AGK cell line, the thesis shows that adaptation to AGK cells results in an isolate with a higher replication efficiency and higher virulence in vitro, compared to CHSEadapted earlier passages. However, when tested for in vivo virulence in Atlantic salmon the results was reversed. Full-length genome sequencing revealed distinct differences between the different adapted passages.