Ulcerative dermal necrosis

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Ulcerative dermal necrosis (UDN) is a chronic dermatological disease of cold water salmonid fish that had a severe impact on north Atlantic Salmon and sea trout stocks in the 1970s and 1980.

Affected fish developed severe skin lesions over large parts of their body which penetrated into skeletal muscle. The onset of symptoms only occurred after migration into freshwater. Lesions became quickly infected with overgrowths of saprolegnia fungus giving the affected fish an appearance of being covered in slimy white pustules. The most severely affected fish frequently die before spawning.