Atlantic salmon winter-ulcer disease: Combining mortality and skin ulcer development as clinical efficacy criteria against Moritella viscosa infection

The bacterium Moritella viscosa causes classical winter-ulcer disease in seawater-farmed salmonids. Despite widespread vaccination with multi-component vaccines containing the M. viscosa-antigen, disease outbreaks continue to be reported in Norway. In trials reported here, commercially available vaccines containing M. viscosa-components were used to vaccinate groups of Atlantic salmon (Salmo salar L.). In two well-controlled experimental studies, bath challenge with M. viscosa was carried out after 600, 1000, or 1500 degree days (dd) post immunization. A field study was also carried out comparing four commercial vaccines to evaluate protection against M. viscosa under commercial farming conditions. In order to increase the resolution of the clinical outcome of these M. viscosa infections studies, we used mortality and expanded the phenotypic parameters to also include skin ulceration status of survivors. The experimental laboratory challenge studies showed that immunization is effective at 600 dd and significantly protects against the clinical consequences (both mortality and skin ulceration) of M. viscosa infection, with relative protection reaching 91% compared to saline controls and 65% compared to a vaccine formulation lacking M. viscosa antigen. M. viscosa challenge was confirmed but induced only minor clinical consequences to the vaccinated groups in the field trial. The results demonstrate that the experimental bath challenge model discriminates between non-specific and specific vaccine protection against M. viscosa. The model was further successful in documenting protection utilizing cutaneous ulcer development, the most prevalent clinical manifestation of M. viscosa infection. Although the degree of protection is significantly different between the vaccines on test, immunization constitutes an important management tool providing protection against M. viscosa infections in marine farmed Atlantic salmon.