HISTOPATHOLOGY OF THE RENAL AND SPLENIC HEMATOPOIETIC TISSUES OF COHOSALMON ONCORHYNCHUS-KISUTCH EXPERIMENTALLY INFECTED WITH RENIBACTERIUM-SALMONINARUM

We report the histopathological changes occurring in the renal and spl enic haemopoietic tissues of coho salmon Oncorhynchus kisutch experime ntally infected with renibacterium salmoninarum. In both tissues we we re able by light microscopy to distinguish 3 different grades in the p rogression of the infection. The grades of infection were characterize d according to the location of the bacteria and the tissue injuries. U ltrastructural observation revealed sinusoidal cells, macrophages, ret icular and barrier cells to be infected by the pathogen, and necrosis of the tissue to be general in advanced stages of the infection. Despi te destruction of the haemopoietic tissue, plasmacytopoietic foci were frequently observed in both organs. Other major changes were the appe arance of epithelioid cells and an increase in the number of barrier c ells. These 2 cell types might be involved in the local defence respon se to the pathogen, but they may also act as a reservoir and prolifera tion locus for R. salmoninarum.