The source of F. branchiophilum is not known. The bacterium may be a normal inhabitant of the water or gills. Although C. columnaris can live in water and mud, fish probably serve as the main reservoir of infection.
The spread of the disease is very variable as it depends to a great extent on the stress factors prevailing. Bullock (1972) induced BGD in fingerling Rainbow trout in 10-14 days when the fish were subjected to crowding, DO levels of 4-5 ppm, and NH4 conc. of 1ppm.
Large losses are to expected unless treatment is started very early at onset of disease.
Dr. Bullock, Graham L.
US Fish & Wildlife Service
Nat. Fisheries Res. C.
'Pete' Bullock, one of the founding luminaries of Fish Pathology