Ulcerative dermal necrosis

Sea trout affected by UDN with typical secondary Saprolegnia infections

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 late 1960s, 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.

Although the worst effects of the disease were seen in the 1970s and 1980, even now large numbers of salmon will succumb to the disease after spawning. This is thought be due in part to their weak post-spawning condition, and lack of food for several months whilst in the river.

Those fish that do make it back to the sea are thought to make a good recovery.