ABSTRACT: Renibacterium salmoninarum (Rs), the causative agent of bacterial kidney disease (BKD), can be transmitted vertically (i.e. from parent to progeny) and horizontally (i.e. from fish to fish). The purpose of this study was to investigate the role of the fecal-oral route of horizontal transmission among farmed salmon held in seawater. Horizontal transmission probably explained the significant increase in prevalence of Rs observed within a regularly sampled population of chinook salmon Oncorhynchus tshawytscha held in a seawater netpen. Viable Rs was detected in the seawater sampled from within a netpen of BKD-affected chinook, and once shed into seawater survives long enough to be ingested by neighbouring fish. The feces from these fish appeared to be the source of Rs in the seawater. Survival experiments revealed that Rs remained viable in seawater for up to 1 wk. A fecal-oral route of horizontal transmission was demonstrated by orally intubating Rs-laden feces into young coho salmon O. kisutch. The Rs-intubated group experienced significantly higher BKD-related mortality than the control group: 98 and 70%, respectively. This study indicates that the fecal-oral route of horizontal transmission may contribute significantly to the increasing prevalence of BKD in farmed salmon.

KEY WORDS: Renibacterium salmoninarum . Bacterial kidney disease . Horizontal transmission . Aquaculture

Published in DAO Vol. 25, No. 1-2 (1996) on May 9
ISSN: 0177-5103. Copyright © Inter-Research, Oldendorf/Luhe, 1996