Microbiology and histopathology of 'saddleback' disease of underyearling Atlantic salmon, Salmo salar L.

Abstract. Heavy mortalities of underyearling Atlantic salmon parr have occurred at the Mactaquac Fish Culture Station in New Brunswick as the result of 'saddleback' disease. The organism causing this lesion was isolated and identified as Flexibacter columnaris. This bacterium was transmitted to other salmon parr, where it produced the typical saddle lesion from which it was re-isolated. The condition was more readily transmitted at 20°C than at 15°C. Transmission required entrance to the dermis through a break in the epidermal surface and was unsuccessful in intact fish. The morphology of the saddleback isolate and its histopathological effect on the fish tissues is described. F. columnaris appeared to be restricted to the dermis and underlying muscle of the saddle area when histological techniques were used, although it was also isolated from the kidneys of some affected fish. The gills, kidney and other organs of the fish did not seem to be affected by the presence of F. columnaris.