HISTOPATHOLOGY OF THE THYMUS OF COHO SALMON ONCORHYNCHUS-KISUTCH EXPERIMENTALLY INFECTED WITH RENIBACTERIUM-SALMONINARUM

We report on the histopathological changes occurring in the thymus of coho salmon Oncorhynchus kisutch experimentally infected with Renibact erium salmoninarum. Coho salmon were intraperitoneally infected, and t he thymi were collected weekly from 1 to 7 wk post-infection, and proc essed for ultrastructural study. The thymus appeared to be infected on ly in fish collected at 6 and 7 wk post-infection. The first stage of the infection was characterized by the presence of a low number of bac teria scattered in the connective tissue of the capsule. Further progr ession of the thymic infection was characterized by the rupture of the capsular-thymic barrier, and by the bacterial colonization of the sub capsular and inner zones of the parenchyma, where the bacteria were lo cated mainly in macrophages, but also apparently in pale reticular-epi thelial cells and in thymocytes. Other reticular-epithelial cells, and the vascular trabeculae, remained free of R. salmoninarun during the initial stages of the infection. In severely infected thymi, necrosis of the parenchyma of the subcapsular and inner zones, and bacterial in vasion of the trabeculae occurred. The integrity of the pharyngeal epi thelium covering the thymus was not affected during the infection.