Epidemiology of Renibacterium salmoninarum in Scotland and the potential for compartmentalised management of salmon and trout farming areas

Bacterial kidney disease (BKD) (caused by Renibacterium salmoninarum) can result in significant mortality in Scottish salmon farms, but is considered to be a minor issue on trout farms. Controlling R. salmoninarum infection in trout to protect farmed salmon would be effective only if the risk posed from trout is significant both in absolute terms and relative to other potential sources of R. salmoninarum. To assess this, three complementary reviews are undertaken: review of data quality on BKD in Scotland and the national level prevalence and dynamics these data imply; case studies of recent BKD outbreaks in Scotland; and an assessment of the epidemiological and management factors that maintain and spread R. salmoninarum within and between the trout and salmon industries. These are then synthesised into a conclusion on the factors required for control of BKD in salmon. Most observed spread of R. salmoninarum occurred within single species or even companies, so the majority of cases in farmed salmon are linked to other salmon (and not to trout) farms. There is substantive geographical separation of areas of production for trout and salmon and transmission between salmon and trout networks is limited. The bacterium does not survive long in water so hydrodynamic transmission is likely to be localised. Currently R. salmoninarum is extremely rare in Scottish wild fish; this has not always been the case. Wild fish therefore probably play a limited role, but might act as reservoirs or vectors. The general conclusion is that to a large extent the transmission of R. salmoninarum in salmon and trout production can be separated and so there is potential to compartmentalise BKD controls, either by host species or geographical area.

► In Scotland BKD is a more serious problem on salmon than on trout farms.
► We review: data on BKD, case histories, and contact networks.
► A substantial separation of BKD in salmon and trout farms is identified.
► Compartmentalisation of management is practical.