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The present study describes pathological changes and bacteriological findings in 'winter ulcer' in Atlantic salmon Salmo salar L. The transmissibility of the disease was also evaluated under experimental conditions. Skin changes were characterized by ulcers of varying size, and were categorized as acute to subacute, chronic, and regenerative/reparative. In the acute stages, lesions were superficial with scale loss and mild inflammation, while in the subacute stages ulcers were present that extended down to the underlying n~uscle. Histologically, the chronic stages were characterized by a severe inflammation of the dermis and of the interstitial muscle tissue. In the regeneration/reparation stages, a hyperplastic epidermis covered granulation tissue. Bacteriological investigations carried out in salmon sampled from 8 different farms with winter ulcer identified 2 groups of bacteria that were common in affected fish. The examinations performed in the present study indicate that both these groups belonged to the genus Vibrio, termed sp. 1 and sp. 2, respectively. Immunohistochemically, Vibrio sp. 1 and 2 were identified in situ associated with muscle tissue degeneration. Experimental infection with Vibno sp. 1 induced a disease similar to winter ulcer in Atlantic salmon, while inoculation with Vibrio sp. 2 had no effect. Cohabitation experiments showed that winter ulcer can be transmitted from diseased to healthy individuals, and that injection was not required to induce the process. However, mechanical skin lesions were a predisposing factor for ulcer formation. The present study provides evidence that winter ulcer is caused by an infection with a Vibrio-like bacterium, and that the disease can be transmitted through cohabitation and injection Pathological changes were not pathognomonic, and the severity of changes varied.

'Winter ulcer' in the Atlantic salmon Salmo salar. Pathological and bacteriological investigations and transmission experiments