Genomic Epidemiology of Salmonid Alphavirus in Norwegian Aquaculture Reveals Recent Subtype-2 Transmission Dynamics and Novel Subtype-3 Lineages.

Viral disease poses a major barrier to sustainable aquaculture, with outbreaks causing large economic losses and growing concerns for fish welfare. Genomic epidemiology can support disease control by providing rapid inferences on viral evolution and disease transmission. In this study, genomic epidemiology was used to investigate salmonid alphavirus (SAV), the causative agent of pancreas disease (PD) in Atlantic salmon. Our aim was to reconstruct SAV subtype-2 (SAV2) diversity and transmission dynamics in recent Norwegian aquaculture, including the origin of SAV2 in regions where this subtype is not tolerated under current legislation. Using nanopore sequencing, we captured ~90% of the SAV2 genome for n = 68 field isolates from 10 aquaculture production regions sampled between 2018 and 2020. Using time-calibrated phylogenetics, we infer that, following its introduction to Norway around 2010, SAV2 split into two clades (SAV2a and 2b) around 2013. While co-present at the same sites near the boundary of Møre og Romsdal and Trøndelag, SAV2a and 2b were generally detected in non-overlapping locations at more Southern and Northern latitudes, respectively. We provide evidence for recent SAV2 transmission over large distances, revealing a strong connection between Møre og Romsdal and SAV2 detected in 2019/20 in Rogaland. We also demonstrate separate introductions of SAV2a and 2b outside the SAV2 zone in Sognefjorden (Vestland), connected to samples from Møre og Romsdal and Trøndelag, respectively, and a likely 100 km Northward transmission of SAV2b within Trøndelag. Finally, we recovered genomes of SAV2a and SAV3 co-infecting single fish in Rogaland, involving novel SAV3 lineages that diverged from previously characterized strains >25 years ago. Overall, this study demonstrates useful applications of genomic epidemiology for tracking viral disease spread in aquaculture.

Time-course study of injection site inflammatory reactions following intraperitoneal injection of Atlantic cod (Gadus morhua L.) with oil-adjuvanted vaccines.

The inflammatory response of Atlantic cod (Gadus morhua L.) following vaccination with oil-based vaccines has not been previously characterized in any detail. In this study, groups of Atlantic cod were intraperitoneally injected with commercial oil-adjuvanted vaccines ALPHA JECT 3000 (AJ 3000) and AJ 6-2. A water-based vaccine ALPHA MARINE Vibrio (AVM), an experimental liposome vaccine and physiological saline (placebo) were also included for comparison. Histopathological changes at the injection sites were evaluated semi-quantitatively at 1, 2, 4, 8, 12, 16, 20 and 25 weeks post-vaccination (p.v.), parallel with the examination of vaccine antigen retention. Gross intra-abdominal lesions were only examined at 12 and 25 weeks. The results show that the onset of inflammation in all vaccinated groups was rapid to develop, with intense cellular infiltrations predominated by mononuclear cells especially in groups injected by oil-based vaccines. Inflammation induced by AVM and liposome vaccines resolved within 12 weeks. In contrast, oil-adjuvanted vaccines produced mild, persistent but ultimately decreasing reactions. Persistent antigens were observed in oil-based and liposome vaccines. The results show that the cod inflammatory response is similar to other bony fish species. The findings also suggest that cod has an efficient innate immune system that is able to rapidly remove or sequester antigens from the injection site leading to the down-regulation of inflammation. Oil-adjuvanted vaccines appear to be well-tolerated by this species and show promise as a possible approach for disease control.

Sequential study of antigen persistence and concomitant inflammatory reactions relative to side-effects and growth of Atlantic salmon (Salmo salar L.) following intraperitoneal injection with oil-adjuvanted vaccines.

The persistence of antigens at the injection site (area around pyloric caeca and spleen), concomitant inflammatory reaction and granuloma development were monitored at 3, 6 and 12 months following intraperitoneal injection with multivalent, oil-adjuvanted vaccines in Atlantic salmon. Parallel assessment of side-effect profiles and growth rate were also performed. Antigen persistence was examined by use of a monoclonal antibody that recognises Aeromonas salmonicida lipopolysaccharide in an immunohistochemical method for in situ identification of bacteria or bacterial fragments. The inflammatory reaction was monitored using standard histological techniques. The amount of persistent antigens and size of inflammation/adhesions were estimated semi-quantitatively. A steady decrease in the quantity of antigens at the injection site was observed from 3 to 12 months. Antigens were consistently found in inflamed tissues located in the pancreatic region. The size of inflammation increased during the first 6 months but declined thereafter. These findings suggest that persistent antigens at the injection site may act as inflammatory stimulants that induce and perpetuate the inflammatory reaction, eventually leading to adverse side-effects.