Moritella viscosa is one on the major etiological agents of winter-ulcers in Atlantic salmon (Salmo salar) in Norway. Outbreaks of ulcerative disease in farmed fish occurs across the North Atlantic region and is an impeding factor for sustainable growth within the industry. Commercially available multivalent core vaccines containing inactivated bacterin of M. viscosa reduce mortality and clinical signs related to winter ulcer disease. Two major genetic clades within M. viscosa have previously been described based on gyrB sequencing, namely typical (hereafter referred to as classic) and variant. Vaccination-challenge trials using vaccines including either variant and or classic isolates of M. viscosa show that classic clade isolates included in current commercial multivalent core vaccines provide poor cross-protection against emerging variant strains, while variant strains confer high level of protection against variant M. viscosa but to a lesser extent to classic clade isolates. This demonstrates that future vaccine regimens should include a combination of strains from both clades.

Moritella viscosa (M. viscosa) is one of the major etiological agents of winter-ulcers in Atlantic salmon (Salmo salar) in Norway. Outbreaks of ulcerative disease in farmed fish occur across the North Atlantic region, causing reduced animal welfare and economical challenges, and are of hindrance for sustainable growth within the industry. Commercially available multivalent core vaccines containing inactivated bacterin of M. viscosa reduce mortality and clinical signs related to winter ulcer disease. It has previously been described two major genetic clades within M. viscosa, typical (hereafter referred to as classic) and variant, based on gyrB sequencing. In addition, there are phenotypical traits such as viscosity that may differ between different types of isolates. Western blot using salmon plasma showed that classic non-viscous strains are antigenically different from the classic viscous type included in core vaccines. Further, Western blot also showed that there are similarities in binding patterns between Norwegian variant and classic non-viscous isolates, indicating they may be antigenically related. Vaccination-challenge trials using Norwegian gyrB-classic non-viscous isolates of M. viscosa, demonstrate that the isolates from the classic clade that are included in current commercial multivalent core vaccines, provide limited cross protection against the emerging non-viscous strains. However, a vaccine recently approved for marketing authorization in Norway, containing inactivated antigen of a variant M. viscosa strain, demonstrates reduced mortality as well as clinical signs caused by infections with the classic non-viscous M. viscosa isolated from outbreaks in Norwegian salmon farms. The study shows that there are antigenic similarities between variant and classic non-viscous types of M. viscosa, and these similarities are mirrored in the observed cross-protection in vaccination-challenge trials.

Background/Objectives: This article reports results from a multisite field trial conducted as part of the regulatory evaluation of a novel monovalent vaccine for Atlantic salmon targeting an emerging variant of Moritella viscosa, with the objective of assessing safety and performance under commercial production conditions. Methods: The trial included approximately 8 million Atlantic salmon reared at seven freshwater and eleven seawater sites in Norway. At each site, fish in the test and control groups received identical vaccination regimens, with the test group additionally receiving the test vaccine. Fish were monitored from vaccination until harvest. Safety endpoints included post-vaccination mortality and local reactions; effectiveness endpoints included outbreak-related mortality and antibody responses. Harvest quality was evaluated as an exploratory endpoint. Results: Post-vaccination mortality within 21 days was low and comparable between groups. Local reaction scores were within acceptable ranges but slightly higher for the test group. Outbreaks of winter ulcer disease caused by variant M. viscosa occurred at three seawater sites, during which test groups were associated with substantially lower mortality compared with controls. At these sites, the proportion of downgraded fish at slaughter was consistently lower in test than control groups. Across sampling points, the test group showed higher antibody titers than the control group. Conclusions: Co-administration of the monovalent variant M. viscosa vaccine with commercial core vaccines was associated with reduced disease burden during outbreaks and a favourable safety profile under commercial farming conditions. These findings support its potential relevance in vaccination programmes for Atlantic salmon farming.