The infectious disease of hatchery rainbow trout, known in Europe under various names, is now identified as "Viral hemorrhagic septicemia," in agreement with a decision reached during the first European symposium on fish disease at Turin, Italy, in 1962.
The specific symptoms and pathologic-anatomical lesions exhibited by the disease during the first five years are described. VHS may develop an acute, a chronic, and a nervous form. The acute form is characterized by scattered hemorrhages which are especially evident in the skeletal muscles, in the air bladder, and in perivisceral adipose tissue; also by necrobiotic lesions of hepatocytes and kidney cells, a high mortality, and a rapid course of the disease. The chronic form exhiits low mortality, strong anemia, general hydroemia, and hemorrhages are generally absent. The nervous form shows swimming ataxia and no pathologic-anatomical lesions.
The etiology is ascribed to a virus, in accordance with most authors, while a nutritional deficiency is now only considered to be a secondary factor. The high infectiosity of the disease is emphasized, and the results of new transmission tests are reported. According to these, VHS can be transmitted by bacteria-free pathological material, intraperitoneally inoculated, or spread on the gills. The incubation time (water temperature15 to 16 degrees C) is seven to fifteen days.
Prophylactic measures are discussed. The failure of any specific treatment (antibiotics, sulfonamides, etc) is confirmed. The poor results obtained by vitamin-supplement should be considered as the effects of a symptomatic treatment, and confirm once again the viral etiology of the disease.