Winter ulcers of the skin in Atlantic salmon, Salmo salar L.: pathogenesis and possible aetiology

Winter ulcers are sores which occur most commonly along the scaly sides of farmed Atlantic salmon, Salmo salar L., in sea water. This rather imprecise name is derived from the fact that the lesions tend to be more prevalent during the winter months. The sores appear initially as small, circular, epidermal lesions that may gradually increase in size and depth, and confluent sores may cover considerable areas of the body surface. The economic significance of the condition lies in the fact that affected fish are downgraded from `superior quality', often to `production fish', and that a high percentage of a given year class may be affected at the time of slaughtering. The aetiology is not known, and the pathogenesis is poorly understood. Recently, Lunder (1992) isolated two Vibrio spp. from manifest winter ulcers, and concluded from cohabitation experiments that the condition was transmissible. However, sores could, with few exceptions, be transmitted only to fin-clipped fish, with the area around the severed, adipose fin being most prone to the development of lesions. Thus, the microorganisms could play a role in the extensive tissue damage seen in fully developed sores, but were not likely to be primary pathogens in their own right. In the present paper, the authors present evidence which indicates that winter ulcers originate from dermal vascular occlusions which give rise to vesicles that are mechanically disrupted by breaking the epidermis (e.g. during netting and handling of the fish). A possible role of dietary iron in the thrombotic processes in discussed.