Introduction

Bacterial Kidney Disease (BKD) is a significant disease of cultured and wild salmonids with Atlantic salmon (Salmo salar), trout (Salmo trutta) and Arctic char (Salvelinus alpinus) being the natural hosts. The disease is caused by the Gram positive diplobacillus Renibacterium salmoninarum. The disease was first reported in 1933 in Atlantic salmon and was identified in 1954 following cultivation on a cysteine-blood enriched medium. BKD has been recorded in cultured salmonid stocks from North and South America, Europe, Japan and Scandinavia. Although salmonids are considered to be the primary hosts, it has been possible to induce experimental infection in non-salmonid species (for example, sablefish, Anaplopoma fimbria).

Gross Signs

External gross pathology is characterised by darkening of the body, a distended abdomen, exophthalmia and small petechial haemorrhages at the base of the pectoral fins. Ulceration and abscesses may be observed within the musculature.

Internal gross pathology is characterised by a prominent massively swollen kidney, greyish in appearance with white nodular patches apparent on the surface. Splenomegaly is apparent and a pale liver and enlargement of the gall bladder evident. A clear or cloudy ascites can be recorded and the pyloric caeca are pale with a fatty-like appearance. An opaque white pseudomembrane may form around the swimbladder, liver, spleen, heart and intestinal tract. A yellow viscous fluid occurs in the intestine and may contain blood.

Histopathology

A chronic proliferating granulomatous response affects the haematopoietic tissues. Widespread necrosis with pyknosis develops within the splenic ellipsoids. The bacterium is recorded intra- and extracellularly with focal necrosis and glomerular oedema. Multiple and large granulomas with a central caseous zone are bounded by epitheloid and other infiltrating lymphoid cells. The pancreatic fat becomes heavily infiltrated by bacteria and some colonise the muscularis externa of the pyloric caeca with an accompanying localised infiltration of leucocytes. Deposits of fibrin and collagen accumulate around the swim bladder and intestine with some hypertrophy and numerous phagocytic cells containing bacteria. Many macrophages containing bacteria are associated with a diphtheritic epicarditis around the heart. Small foci containing phagocytosed bacteria are located within the liver, which coalesce and become the centre of an inflammatory reaction.

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Early skin lesion associated with BKD in Atlantic salmon

BKD infected salmon, internal haemorrhaging and enlarged spleen in Atlantic salmon

BKD infected kidney from rainbow trout, x20, H&E stain

BKD infected spleen from rainbow trout, x20, H&E stain

BKD infected liver from rainbow trout, x20, H&E stain