By Ronald J. Roberts
Fisheries Research, Volume 17, Issues 1-2, Pathological Conditions of Wild Salmonids, June 1993, Pages 3-14, ISSN 0165-7836, DOI: 10.1016/0165-7836(93)90003-P.

Ulcerative dermal necrosis is predominantly a condition of the head skin of salmonid fish native to the waters of the British Isles. It has, however, occureed in the Baltic, noerthwest France and possibly elsewhere. It has never been reported in farmed fish. It was firsty reported in teh 19th century. It probably occured prior to 1868 but the main epizoolit was first in 1877 when it was referred to as Salmon Disease.
The disease largely disappeared until the mid 1960s when it first occurred in southwest Ireland and then gradually spread to almost all rivers of the British Isles and subbsequently to neighbouring areas. It had disappeared again by the mid 1970s. Salmon and grlse (Salmo salar0 and sea trout (Salmo trutta) are susceptible and there is evidence that some brown trout may also show clinical signs. The disease is characetristically found in adult wild fish as tey congregate at sea prior to entering fresh water adn during their upstream river migration.
The lesion is a progressive cytolytic necrosis of epidermal Malpighian cells, of the pemphigoid type which is restricted to spefic sites ion the head. On entry to fersh water, these rapidly ulcerate and may become infected with a number of opportunitist pathogensm, principal of which is the oomycete Saprolegnia diclina. Once this occurs the lesion extended by fungal activity, death being due either to secondary bacterial infection of the ulcer or, more often, circulatory failure resulting from the osmotic haemodilution induiced by the large area of ulceration. The condition is primarily found in the colder months of the year and usually persists at high for 3 or 4 years in a river system after which it gradually disappears.
Attempts to isolate a specific viral or bacterial agent from the lesions, or from other organs, have failed and the evidence that is an infectious disease is very limited. Investigation into other more complex possible aetiologies have been equally unsuccesfully. The charcetristics early lesion, the pemphigoid bulla has, however, also been induced y photosensitisation and this may represent a further possibility for its investigation. Currently the disease is in a period of remission and no outbreaks have been reported for some years. It may be no coincidence that the inccidence of epizzotics has appeared always to coincide with very high wild salmonid stock levels.

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