FISH DISEASE LEAFLET 78

BACTERIAL KIDNEY DISEASE OF SALMONID FISHES CAUSED BY RENIBACTERIUM SALMONINARUM

G. L. Bullock and R. L. Herman

U.S. Fish and Wildlife Service, National Fisheries Research Center-Leetown, National Fish Health Research Laboratory, Box 700, Kearneysville, West Virginia 25430

UNITED STATES DEPARTMENT OF THE INTERIOR, Fish and Wildlife Service, Research and Development, Washington, D. C. 20240, 1988

Revision of Fish Disease Leaflet 60 (1980), same title, by G. L. Bullock.

Introduction

Bacterial kidney disease (BKD) is a systemic infection that commonly causes high mortality in populations of wild and propagated salmonids. The disease course is typically chronic, but acute outbreaks sometimes occur, especially at moderate temperatures (1318°C). The first report of BKD described an infection in Atlantic salmon (Salmo salar) in Scotland. In the United States, the disease was first reported by Belding and Merrill (1935), who described gross external and internal pathology in brook trout (Salvelinus fontinalis). The bacterium from trout was morphologically identical with the organism described from salmon. Because focal abscesses occur in the kidneys, spleen, and liver, BKD has also been called white boil disease.

Etiology and Disease

The causative agent is a small (0.4x0.8 mm), nonmotile, asporogenous, grampositive diplobacillus. Ordal and Earp (1956) made the initial isolation and, on the basis of morphology, classified it as a Corynebacterium. A taxonomic study of 25 isolates by Austin and Rodgers (1980) showed diversity among strains. One group of 6 strains was related to Corynebacterium pyogenes, a second group of 12 represented a new taxon, and 7 strains did not fall in either group.

On the basis of biochemical characteristics of strains of the causative bacterium, Sanders and Fryer (1980) proposed the name Renibacterium salmoninarum. Studies of the fatty acid profiles of R. salmoninarum strains, as well as numerical taxonomic studies, have supported the integrity of the genus Renibacterium (Goodfellow et al. 1985). Immunoelectrophoretic analyses of R. salmoninarum strains indicate seven common components (AG), among which antigen F is the major surface antigen (Getchell et al. 1985). Immunoblot techniques revealed the presence of antigenic proteins attached to the cell wall (Fiedler and Draxl 1986).

Because R. salmoninarum is a slowgrowing organism that typically requires 1 to 3 weeks to isolate, culture techniques are not usually used for diagnosis. Presumptive diagnosis is based on the presence of typical clinical signs and small, grampositive diplobacilli in infected tissues.

Several serological procedures have been developed for definitive diagnosis. Immunodiffusion procedures have been used for diagnosis of overt kidney disease (Chen et al. 1974; Kimura et al. 1978), but the more rapid direct and indirect fluorescent antibody tests (FAT) are more commonly used for diagnosis of both overt and subclinical infections (Bullock and Stuckey 1975; Bullock et al. 1980). Ochiai et al. (1985) adapted the direct FAT test for detection of R. salmoninarum in fixed and paraffin-embedded tissue sections. Kimura and Yoshimizu (1981) used staphylococci specifically sensitized with antibody against R salmoninarum to develop a coagglutination test. The most recently developed serological procedures for diagnosis and detection of BKD are an enzymelinked immunosorbent assay (ELISA) procedure that detects soluble antigen of R. salmoninarum (Pascho and Mulcahy 1987) and a filtration FAT technique (Elliott and Barila 1987) that concentrates the organism and facilitates its detection in ovarian fluids.