Research Document 2019/018
Characterization of Renibacterium salmoninarum and bacterial kidney disease to inform pathogen transfer risk assessments in British Columbia
By Rhodes, L. D. and Mimeault, C.
Bacterial kidney disease (BKD) is a persistent, debilitating condition that affects all species of salmonids in freshwater or marine phases, including Atlantic Salmon (Salmo salar). Experimental studies indicate that Pacific salmon such as Chinook (Oncorhynchus tshawytscha), Sockeye (O. nerka), and Chum Salmon (O. keta) are more susceptible to infection and disease than Atlantic Salmon or Rainbow Trout (O. mykiss). The etiologic agent, Renibacterium salmoninarum, can be transmitted horizontally and vertically, and infection or disease can be detected at all life history stages. Both symptomatic and asymptomatic fish shed bacteria, and ingestion of bacteria is likely the most common route of horizontal infection. Organisms, rather than the environment (e.g., sediments) are likely to serve as reservoirs for the bacterium, including non-salmonid marine fish such as prey (Pacific Herring (Clupea pallasii)), sympatric species (Threespined Stickleback (Gasterosteus aculeatus)), Shiner Perch (Cymatogaster aggregata)), and potential parasitic fish (Western River Lamprey (Lampetra ayresii)), Pacific Lamprey (Entosphenus tridentatus)). Infected but asymptomatic salmonids are another important reservoir. Although a minimum infectious dose in seawater has not been experimentally determined, Chinook Salmon can be infected with as few as 700 bacterial cells mL-1 when exposed for 24 hours. Although R. salmoninarum has relatively short persistence in unfiltered seawater (>50% loss within eight hours), association with organic particles such as feces can extend viability to many days. Determination of incubation period and minimum infectious dose depends upon exposure dosage, water temperature, and species of salmon. Typical incubation periods for susceptible species exposed by immersion can range from 21 to 50 days. Water temperature effects on incubation period and mortality exhibit a nonlinear pattern. In laboratory studies, pathogenicity increases as temperatures increase from 8°C, then declines at ~ 15°C and up through higher temperatures. These observations contrast with field observations of increased clinical signs at higher temperatures, but there are often confounding factors (e.g., smoltification, crowding, decreased dissolved oxygen) associated with field observations. Among the factors associated with dissemination of BKD and R. salmoninarum, anthropogenic actions are the leading cause. This has been supported both by genomic analyses and by epidemiology around management efforts to control the disease. While there are a range of hygiene and rearing practices available, sensitive and specific surveillance with sufficient temporal resolution is the initial tool for management, as it will provide information on the effectiveness of hygiene and biosecurity. When an outbreak occurs, management actions should be followed up by effectiveness monitoring. This process can provide credibility for the actions and help to minimize the risk of transmission from net pen stocks to wild salmon populations.
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