Piscine orthoreovirus-1 (PRV-1) infection dynamics: Investigation through temporal profiling of factors affecting viral susceptibility, infectiousness, and persistence in Atlantic salmon

Abstract

In recent years, Norway has accounted for >50% of the global commercial production of Atlantic salmon. This growth in production has also resulted in an increased emergence of several viral diseases. Heart and skeletal muscle inflammation (HSMI), caused by Piscine orthoreovirus-1 (PRV-1), is one of the most frequently detected viral diseases following transfer to seawater. PRV-1 has also been detected in freshwater hatcheries, broodstock facilities, and has been shown to be persistent for at least 65 weeks during the freshwater stages of Atlantic salmon production. However, little is known regarding PRV-1 viral kinetics and HSMI symptoms during and after smoltification. In this thesis, Atlantic salmon persistently infected with PRV-1 were smoltified under laboratory conditions, and the viral load and development of HSMI disease symptoms were monitored for 28 weeks post-smoltification (WPS). Long- term infected salmon had higher viral load during smoltification (0-2 WPS) in whole blood, which was detected at similar levels in blood, heart, muscle, head kidney, and spleen until at least 6 weeks post-seawater transfer (WPST). At 28 WPS, some of the fish tested negative for PRV-1 in whole blood. Histological examination showed no recurrence of HSMI disease development during or after smoltification. Infectiousness of PRV-1-positive erythrocytes collected at various stages of the potentially persistent infection period, was then investigated using an established PRV-1 challenge model via intraperitoneal (IP) injection in naïve Atlantic salmon fry. Testing showed that blood from persistently infected fish remained infectious and resulted in infection of naïve salmon fry at 4 WPC with PRV-1 being detected in the organ package and heart tissue. In Norway, the use of triploid fish in aquaculture has been suggested as a solution to the challenges of crossbreeding between farmed escapees and native wild stocks. However, little is known about the spread of viral infection, particularly PRV-1, in triploid populations. Therefore, in another study included in this thesis, the susceptibility to PRV-1 infection and subsequent viral kinetics were monitored in groups of Atlantic salmon post-smolts with different ploidy levels. Results showed all ploidy level groups reaching a peak viral load at 4 WPC in blood. Histology showed HSMI disease development in heart tissues in both diploid and triploid fish. Overall, these findings provide information regarding the kinetics of PRV-1 infection during and after smoltification and provide valuable information on the susceptibility of triploid compared to diploid Atlantic salmon to PRV-1 infection.