Abstract
Infection with SAV2 and SAV3 results in differences in viral loads, pancreas disease progression, and growth impact – what are the mechanisms behind the differences?
Fabian Kropp1 , Fadi Alnaji2 , Simen Nørstebø1 , Marco Vignuzzi2 , Øystein Evensen1, Turhan Markussen1 , and Aase B. Mikalsen1
1 Faculty of Veterinary Medicine, Norwegian University of Life Sciences, Ås, Norway.
2 A*STAR Infectious Diseases Labs, Immunos, Singapore
Introduction
Pancreas disease (PD), caused by salmonid alphavirus (SAV), is a major challenge for the aquaculture industry in Norway and Europe. Infected fish develop severe lesions in the pancreas as well as inflammation in the heart and skeletal muscle, leading to reduced fish welfare and increased economic losses for the producers, due to impaired growth and mortality. Disease outbreaks in Norway are caused by either the SAV3 or SAV2 genotypes of the virus. SAV3 infections are considered to have a more significant impact on fish health, welfare, and industry outcomes compared to SAV2, but the mechanisms behind this difference are not well understood.
Materials and methods
In this study, we generated SAV3 and SAV2 viruses from plasmid-based clones and infected groups of parr separately with equal viral doses of each genotype. Infected fish were monitored in a long-term study of 16 weeks. Quantitative analyses of viral RNA and concurrent histopathological assessments of the heart, muscle, and pancreas were performed weekly the first four weeks, and then every fourth week. In an attempt to decipher the underlying mechanisms producing the observed genotype-specific differences, additional analyses were included to study presence and characteristics of defective viral genomes (DVGs) over the time course and variations in host responses to SAV3 and SAV2 infection. Also, blood from infected fish sampled at different time points were also analyzed, targeting protein biomarkers associated with heart and pancreas damage and regenerative mechanisms.
Results
The in vivo experimental challenge trial demonstrates that SAV3, as expected, replicates to high levels in the fish during the first four weeks. This causes severe damage to the pancreas, seen as early as two weeks post challenge, and growth is completely halted. SAV2 replication is lower during this period and the infection causes less pancreas damage with regeneration starting as early as week four. Also, the lesions in the heart from SAV2 infected fish are less severe and almost insignificant. Targeted biomarkers in the blood show time-dependent increase with some differences observed between the genotypes. Preliminary results from analyses for the presence of DVGs, and their potential associations with viral replication and disease development, will also be presented.
Conclusions
The two SAV genotypes replicate at different rates and to different levels in the fish and induce distinct differences in PD severity and growth performance.