WINTER ULCER IN THE ATLANTIC SALMON SALMO-SALAR - PATHOLOGICAL AND BACTERIOLOGICAL INVESTIGATIONS AND TRANSMISSION EXPERIMENTS

The present study describes pathological changes and bacteriological f indings in 'winter ulcer' in Atlantic salmon Salmo salar L. The transm issibility of the disease was also evaluated under experimental condit ions. Skin changes were characterized by ulcers of varying size,and we re categorized as acute to subacute, chronic, and regenerative/reparat ive. In the acute stages, lesions were superficial with scale loss and mild inflammation, while in the subacute stages ulcers were present t hat extended down to the underlying muscle. Histologically, the chroni c stages were characterized by a severe inflammation of the dermis and of the interstitial muscle tissue. In the regeneration/reparation sta ges, a hyperplastic epidermis covered granulation tissue. Bacteriologi cal investigations carried out in salmon sampled from 8 different farm s with winter ulcer identified 2 groups of bacteria that were common i n affected fish. The examinations performed in the present study indic ate that both these groups belonged to the genus Vibrio, termed sp. 1 and sp. 2, respectively. Immunohistochemically, Vibrio sp. 1 and 2 wer e identified in situ associated with muscle tissue degeneration. Exper imental infection with Vibrio sp. 1 induced a disease similar to winte r ulcer in Atlantic salmon, while inoculation with Vibrio sp. 2 had no effect. Cohabitation experiments showed that winter ulcer can be tran smitted from diseased to healthy individuals, and that injection was n ot required to induce the process. However, mechanical skin lesions we re a predisposing factor for ulcer formation. The present study provid es evidence that winter ulcer is caused by an infection with a Vibrio- like bacterium, and that the disease can be transmitted through cohabi tation and injection. Pathological changes were not pathognomonic, and the severity of changes varied.