The dynamics and effects of bacterial kidney disease in Snake River spring Chinook salmon (Oncorhynchus tshawytscha)

Abstract

Bacterial kidney disease (BKD) is endemic in Snake River spring chinook salmon (Oncorhynchus tshawytscha) stocks of both wild and hatchery origin, and may be partially responsible for their poor returns in recent years. BKD is particularly difficult to control due to the chronic nature of the disease. Renibacterium salmoninarum, the causative agent of BKD, can survive and reproduce intracellularly within the host, thus avoiding the humoral immune system. Numerous factors and processes are important in determining the survivorship or mortality of individual salmon infected with R. salmoninarum. Here, a number of these are explored using mathematical models and statistical techniques applied to data from the published literature. Antigen density, an important factor as the p57 antigen of R. salmoninarum has immunosuppressive properties, is related to bacterial load, and possible explanations for the observed relationship are suggested. The likelihood of vertical transmission of the disease (from spawner to offspring) or of in-ovum antigen inclusion induced immunotolerance is related to ovarian fluid infection levels. The effects of temperature on the ability of the immune system to fight off the disease and on the growth rate of the bacteria are explored. These factors are then brought to bear on the results of a broodstock segregation study, suggesting that immunotolerance and temperature effects likely account for many of the observed results.