Ulcerative dermal necrosis (UDN) is a chronic dermatological disease of cold water

salmonid Salmonidae is a family of ray-finned fish that constitutes the only currently extant family in the order Salmoniformes . It includes salmon (both Atlantic and Pacific species), trout (both ocean-going and landlocked), chars, freshwater whitefis ...

fish that had a severe impact on north Atlantic

Salmon Salmon () is the common name for several list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the family (biology), family Salmonidae, which are native to tributary, tributaries of the ...

and

sea trout Sea trout is the common name usually applied to anadromous (sea-run) forms of brown trout (''Salmo trutta''), and is often referred to as ''Salmo trutta'' morpha ''trutta''. Other names for anadromous brown trout are sewin (Wales), peel or peal ...

stocks in the late 1960s and 1970s–1980. Despite much investigation, the cause of UDN has not been determined. The onset of symptoms occurs after migration into freshwater. Affected fish develop severe skin lesions which begin on the head and back, and near the tail. Lesions become infected with overgrowths of oomycetes, such as ''

Saprolegnia ''Saprolegnia'' is a genus of water moulds often called cotton moulds because of the characteristic white or grey fibrous patches they form. Current taxonomy puts ''Saprolegnia'' as a genus of the heterokonts in the order Saprolegniales. Habits ...

'', giving the affected areas a slimy blue-grey appearance. The most severely affected fish frequently die before

spawn Spawn or spawning may refer to: * Spawn (biology), the eggs and sperm of aquatic animals Arts, entertainment, and media * Spawn (character), a fictional character in the comic series of the same name and in the associated franchise ** '' Spawn: Ar ...

ing. 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.

References

Fish diseases {{Veterinary-med-stub