ABSTRACT: We report the histopathological changes occurring in the renal and splenic haemopoietic tissues of coho salmon Oncorhynchus kisutch experimentally infected with Renibacterium salmoninarum. In both tissues we were able by light microscopy to distinguish 3 different grades in the progression of the infection. The grades of infection were characterized according to the location of the bacteria and the tissue injuries. Ultrastructural observation revealed sinusoidal cells, macrophages, reticular and barrier cells to be infected by the pathogen, and necrosis of the tissue to be general in advanced stages of the infection. Despite destruction of the haemopoietic tissue, plasmacytopoietic foci were frequently observed in both organs. Other major changes were the appearance of epithelioid cells and an increase in the number of barrier cells. These 2 cell types might be involved in the local defence response to the pathogen, but they may also act as a reservoir and proliferation locus for R. salmoninarum.

KEY WORDS: BKD . Renibacterium salmoninarum . Histopathology . Haemopoietic tissue . Kidney . Spleen . Oncorhynchus kisutch . Fish

Published in DAO Vol. 24, No. 2 (1996) on February 1
ISSN: 0177-5103. Copyright © Inter-Research, Oldendorf/Luhe, 1996