The virus disease of the rainbow trout, known as trout disease, infectious swelling of the kidneys and liver degeneration and - in Denmark - as Egtved disease, extremely depends on the season. During the warm season it generally disappears completely, but it still exists latently. Sometimes the disease does not appear in the next winter.
The trout disease also occurs, besides the normal forms, as a secondary form. The normal symptoms of the disease are then completely missing with the fishes attached by that illness. The animals merely behave abnormally, which indicates a damage of the nervous system.
Two species of cyprinid (Squalius cephalus and Leuciscus rutilus) were immune to the trout disease during the experiment. An artificial infection did not succeed. It was also impossible to infect brook trout (Salmo fario). The American species, S. fontinalis, however, was extremely susceptible.
Various vitamin preparations, Streptomycin and also a Sulphonamide (Eleudron) had, in most cases, negative results after one oral use. A proved positive effect reducing the mortality up to 30 per cent, however, could be attained by using a Vitamin-A-preparation (Vogan), as well as a combination of Vitamin T and Terramycin.
Malachite green had no effect on the development of the infection and of the illness, not even when the trouts were kept in a strong dyed solution for nearly a month.