Very often one or both gill-covers become partially glued down to the shoulder, and the gill-openings obstructed, but I have never yet been able to meet with any sign of disease on the gills themselves. This is contrary to common belief, but I can only say that such is my experience. On the other hand, the disease frequently invades the cavity of the mouth, and often more or less destroys the membranous veil which lies behind the teeth, and plays a curious part in respiration. Thus, although, so far as I have been able to observe, the respiratory organs are not directly attacked, the performance of the respiratory function may be very seriously obstructed.
If the mucous membrane lining the mouth be reckoned with the integument, of which it really is a part, it may be said that the affection under discussion is strictly a cutaneous disease, comparable to ringworm among men. However badly a fish may have been diseased, there is no trace of the affection in the abdominal cavity or iii any of the viscera, and the muscles and deep-seated bones appear healthy. Some say that the liver is enlarged and soft, but I have seen the liver quite healthy in very severe cases.
The fish appear to suffer considerable irritation from the disease; but how far this is a primary symptom, and how far it results from the entanglement of multitudes of minute grains of sand in the fluffy coat of the diseased skin, is uncertain. Badly diseased fish in aquaria, the water of which contains no suspended particles of sand, do not show signs of any particular irritation.
The mortality among salmon, sea trout, and freshwater trout caused by this disease is very considerable.
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