Moritella viscosa bypasses Atlantic salmon epidermal keratocyte clearing activity and might use skin surfaces as a port of infection

Moritella viscosa is considered the main causative agent of winter ulcer disease in salmonid fish. In order to obtain more details on route of infection, we challenged Atlantic salmon (Salmo salar) epidermal keratocytes with M. viscosa and performed an Atlantic salmon immersion challenge. Although keratocytes were able to remove M. viscosa from surfaces, their engulfment capability appeared inefficient with reduced ability to reepithelialise superficial wounds (scale less skin surfaces) challenged with the bacterium. The immersion challenge revealed a significant connection between exposure area and mortality. Enhanced invasion ability and mortality was observed by M. viscosa exposure of the head and gill region compared to exposure of: the right side of the body; the left side of the body; or the body from pectoral to caudal fin (p = 0.04). Ulcer development corresponded to area exposed (p = 0.002), suggesting skin ulcer formation to result primarily from direct skin surface colonization. Ulceration of surfaces exposed to M. viscosa in parallel with occurrence of septicaemia suggests that both skin and gills may act as possible initiation sites for M. viscosa infections.