Transmission experiment in Atlantic salmon (Salmo salar) with an Atlantic Canadian isolate of Moritella viscosa

In Atlantic Canada, Moritella viscosa is often cultured from Atlantic salmon skin ulcers in the summer and autumn months when water temperature is > 10 °C. The objectives of our study were to: 1) describe the development of skin ulcers associated with an Atlantic Canadian M. viscosa isolate; and 2) determine if this isolate could be transmitted horizontally between fish via water under laboratory conditions. Atlantic salmon in salt water were bath-immersed with M. viscosa (5.6 × 10⁶ cfu ml-1) at 10.9 ± 0.2 °C. After observing skin lesions, water from each bath-challenge tank was diverted to a “delayed-challenge” tank with previously unexposed fish. The initial bath-challenge fish expressed clinical signs of skin ulcers within four days of exposure. Fish that were not sacrificed during our trial fully recovered by 26 days post-challenge. No clinical signs of disease were observed in fish from the delayed-challenge tanks. Our results suggest skin lesions induced by an M. viscosa isolate from Atlantic Canada have similar pathology to winter ulcer disease described in Europe. Most bath-challenged fish were not systemically infected until the development of visible skin ulcerations. None of the delayed-challenge fish were infected with M. viscosa, which suggests horizontal transmission via the water was not sufficient to cause disease in our study.