Studies on the pathology of bacterial kidney disease (Renibacterium salmoninarum) in coho (Oncorhynchus kisutch) and Atlantic salmon (Salmo salar)

Abstract

With the intensification of the aquaculture industry along the west coast of Canada, there has been a concomitant increase in the prevalance of bacterial kidney disease (BKD) (Renibacterium salmoninarum)(Rs) in production and government hatchery facilities. Due to the paucity of information on the pathogenesis and epizootiology of Rs, treatment and control measures have been confounded and emergance of BKD exacerbated. Initially, to enhance specificity over conventional histochemistry for demonstration of Rs in histological sections an avidin biotin conjugated immunoperoxidase technique was developed for us to monitor the histopathological manifestation of BKD. To ascertain the temporal and spatial hlstogenesis of BKD. coho (Oncorhvnchus kisutch) and Atlantic (Salmo salar) salmon ware challenged experimentally by intraperitoneal injection and naturally by cohabitation with Rs-innoculated fish than serially sampled. Histopathology revealed lesions consistent with past case reports and investigations, as well as previously undescribed manifestations including, inflammatory call kinetics in the renal perivascular compartment, cresentic glomerulonephritis, extrarenal dissemination (via septic emboli or direct extension), and ovarian follicular cell accumulation of Rs. Pseudocyst formation, renal interstitial hyperplasia and meningoencephalitis were also characterized. The granulomatous response in coho salmon was predominantly histiocytic, whereas, in Atlantic salmon a tuberculoid response was more apparent. In either species a profound call mediated immunity was adduced. To further resolve the nature of the inflammatory response, coho and Atlantic salmon were immunosuppressed by administration of suprapharmacologic doses of glucocorticosteroids. On challange with Rs both species incurred an earlier onset and greater rate of mortality than immunocompetent cohorts. The granulomatous response appeared irregular and expansive with exuberent intra- and extracellular Rs growth. These observations may be attributed to inhibition of prostaglandin and leukotrienne synthesis, as well as an inability to immunologically prima phagocytes. To evaluate the inflammogenic potential o£ somatic, cell- wall associated, and soluble fractions of Rs, coho and Atlantic salmon were injected and serially sampled for histopathology. Past studies on Rs virulance determinants and pathogenic mechanisms have focused almost exclusively on a soluble protein, designated p57. In this investigation no histological alterations were appreciated in fish challenged with cell-wall associated or soluble (p57) fractions, however, mild, multifocal pyogranulomata were noted in the renal interstitium of coho and Atlantic salmon challenged with the somatic or peptidoglycan fraction of Rs. Peptidoglycans of a number of mammalian pathogens are strongly inflammogenic, poorly biodegradable, and persist in host tissue for protracted periods. Chemical resolution and in vivo evaluatlon of subcellular components of Rs is warranted to further resolve the pathogenesis of BKD in salmonid species.