Atlantic salmon challenged with extracellular products from Moritella viscosa

Skin ulcers in Atlantic salmon Salmo salar in the Canadian east coast salmon aquaculture industry lead to high mortality rates. This condition is clinically similar to winter ulcer disease in Norway with the exception that it occurs at temperatures above 10°C. Moritella viscosa is thought to be the causative agent for winter ulcer disease in Norway, and it is occasionally also isolated from skin ulcer cases in Atlantic Canada. This bacterium is known to produce cytotoxins. The objective of this study was to determine if extracellular products (ECP) from an Atlantic Canadian strain of M. viscosa could induce a tissue response similar to what is observed with M. viscosa infections in Atlantic salmon in eastern Canada. We injected fish subcutaneously with ECP and monitored the development of skin lesions. We sampled fish with early skin lesions and ulcers to describe the pathology associated with the condition. Samples were taken for histopathology, bacterial culture, and quantitative PCR (qPCR). All experimental fish expressed early skin lesions, with 5 fish (8.3%) developing deep skin ulcers after 12 d post-exposure. Our results suggest the ECP of M. viscosa from the east coast of Canada induces a similar tissue response to what is described in ulcer disease in Atlantic salmon. These extracelluar products may partially explain the pathology associated with M. viscosa.