At the end of the 19th century in the United Kingdom one particular disease was considered a significant threat to certain wild salmon fisheries and data gathered from these fish provided the basis of a detailed description of the condition known today as ulcerative dermal necrosis (UDN).
Historically UDN has occurred in Ireland, France, Sweden and Norway but in the UK this condition had largely disappeared or was not reported since the 1970s.  Research from this period suggests that UDN can persist for 3-4 years in individual rivers and then disappear.

Recently, Marine Scotland Science diagnosed Ulcerative Dermal Necrosis (UDN) from Atlantic salmon stock that originated from the River Spey. This condition is a naturally occurring condition of wild fish and despite significant research no recognised cause including no link to an infectious agent.  Furthermore UDN has never been reported from farmed fish.

UDN is believed to start during homeward migration and is primarily a lesion of epidermal and dermal layers of the head area.  Confirmation requires histological examination of early skin lesions which is considered the only specific signs of the disease.  However, secondary infection by Saprolegnia, a normal part of the river ecological system, reduces the likelihood of a correct diagnosis.