Viral Protein Kinetics of Piscine Orthoreovirus Infection in Atlantic Salmon Blood Cells

Piscine orthoreovirus (PRV) is ubiquitous in farmed Atlantic salmon (Salmo salar) and the cause of heart and skeletal muscle inflammation. Erythrocytes are important target cells for PRV. We have investigated the kinetics of PRV infection in salmon blood cells. The findings indicate that PRV causes an acute infection of blood cells lasting 1-2 weeks, before it subsides into persistence. A high production of viral proteins occurred initially in the acute phase which significantly correlated with antiviral gene transcription. Globular viral factories organized by the non-structural protein mu NS were also observed initially, but were not evident at later stages. Interactions between mu NS and the PRV structural proteins lambda 1, mu 1, sigma 1 and sigma 3 were demonstrated. Different size variants of mu NS and the outer capsid protein mu 1 appeared at specific time points during infection. Maximal viral protein load was observed five weeks post cohabitant challenge and was undetectable from seven weeks post challenge. In contrast, viral RNA at a high level could be detected throughout the eight-week trial. A proteolytic cleavage fragment of the mu 1 protein was the only viral protein detectable after seven weeks post challenge, indicating that this mu 1 fragment may be involved in the mechanisms of persistent infection.