The pathology of "saddleback" disease of underyearling Atlantic salmon (Salmo salar)

Abstract

This study describes the pathology of "saddleback" disease of underyearling Atlantic salmon (Salmo salar) cultured at the Mactaquac Fish Hatchery near Fredericton, New Brunswick. A gram-negative, flexible, bacterium had a constant association with the characteristic lesion. It is thought that the bacteria enter the dorsal fin and then spread into the surrounding dermis by moving through the connective tissue and collagen layers. This causes sloughing of the overlying epidermis and some scales, and results in a symmetrical band of discoloration on the flanks of the body on either side of the dorsal fin. Invasion of the underlying epimysium and perimysium leads the bacteria deep into the dorsal fin levators and the epaxial muscles on either side of the dorsal fin. Evidence suggests the bacteria can enter muscle cells and destroy the cell from within. The bacterium was not found in any other tissue.