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Diseased Salmon’ – Follow Up Report


Further to the Diseased Salmon in autumn article published on this site in mid November, The Tweed Foundation sent some samples for analysis. Here, the Foundation’s senior biologist, Dr Ronald Campbell, reports on the results, and what some anglers on Tweed this autumn were erroneously claiming was UDN.

  1. Tissue samples from the skin, gills, heart, liver, kidney, “pancreas” and gut of four moribund or recently dead salmon were taken at the end of November and sent to the Veterinary Diagnostic Services at the Institute of Aquaculture, Stirling University for analysis.
  2. While no signs of “major infectious disease processes” were identified, it was found that: “The principal changes present in these fish samples are a thickening of epithelium and decrease of mucous cells at the skin surface and resultant osmotic damage to epithelial surfaces resulting in epithelial sloughing. In advanced cases the epithelium is completely lost resulting in open ulceration and water-logging of underlying tissues. It is thought likely that these changes may be under the control of sex hormones which increase the speed of cell turnover so that differentiation into specialised cell types such as mucous cells takes place less often. This, in turn, leads to a loss of the normal thick mucous layer and surface cells become affected by osmotic damage and slough.”  In other words, the outer layer of the skin of these fish was disintegrating meaning that the fish were losing control over the flow of water in to their bodies (as the body fluids of fish are more concentrated solutions than fresh water, water will flow in to their bodies to equalise concentrations with the water outside unless prevented by the skin and other defence mechanisms).
  3.  Healing, however, is possible, as the report adds; “Under the right water conditions, epithelial cells can migrate to cover the ulcerated areas and healing may take place. If, on the other hand, the ulceration is extensive, the osmotic damage produced will lead to death from loss of salts and proteins at the skin surface and the fish eventually die of heart failure.” In other words, new skin can grow to cover the damaged areas and allow the fish to control its body fluids again.
  4. However, damage to the outer skin will allow colonisation of the exposed body tissues by fungal spores and the resulting fungal growth will further extend the damage and reduce the chances of healing, increasing the likelihood of the fish dying.
  5. What causes the skin to disintegrate in the way seen this Autumn is unknown, but it is a condition and not a disease. Similarly, in humans, Eczema is a skin condition and not an infectious disease.
  6. The skin condition seen this Autumn was not the UDN of the 1880s and 1960s although this was also a condition and not a disease. UDN occurs in salt water, the fish come in to rivers with it and it is characteristic of cold water temperatures. It’s classic symptom is a lesion / hole in the top of the head that can penetrate right down to the brain case, but fish can heal as water temperatures increase towards Summer.
  7. The conditions that could have given rise to the skin problems seen this Autumn were the warmth of the water (this was the warmest November on record) combined with the early entry of large numbers of fish in August and September which meant they were crowded together in warm water for a longer period than usual.