Comparative genomic analysis of a new Vibrio strain (J383), and the evaluation of vaccines against Moritella viscosa in farmed Atlantic salmon (Salmo salar)

Winter Ulcer Disease (WUD) is a prominent health concern for marine fish, particularly affecting farmed salmonids. It has hemorrhagic effects and leads to dermal lesions and ulcers. This disease often reduces the fish’s market value and increases mortality rates. Moritella viscosa, a gram-negative bacterium, is mainly, but not exclusively, linked to the appearance of WUD, especially during the colder months. Notably, however, there is significant variation in the prevalence and characteristics of WUD in the Eastern North Atlantic (Europe) as compared to in Eastern Canada. The disease in Europe often presents in the winter, and the severity and frequency of outbreaks is affected by water temperature and salinity. In contrast, in Eastern Canada, ulcerative disease outbreaks generally occur in the summer and mid-autumn, and mortality persists until September. Although various polyvalent vaccines are used against M. viscosa, several outbreaks of this disease have been reported in Eastern Canada in recent years. It is unknown whether undescribed bacterial pathogens are causing ulcerative disease in vaccinated farmed Atlantic salmon in this region, or whether current vaccines against M. viscosa are ineffective. This study analyzed the phenotypical and genomic characteristics of a new Vibrio spp. (Vibrio sp. J383) isolated from internal organs of vaccinated farmed Atlantic salmon displaying clinical signs of ulcer disease (Chapter 2). Additionally, the susceptibility of vaccinated farmed Atlantic salmon to M. viscosa challenge was determined, and the haemato-immune response of immunized farmed Atlantic salmon to M. viscosa vaccine preparations was investigated (Chapter 3).