Differences in patterns of meningoencephalitis due to bacterial kidney disease in farmed Atlantic and chinook salmon

A range of neural pathology, in particular meningitis with sporadic encephalitic extension, can develop in salmonids infected with Renibacterium salmoninarum, the agent of bacterial kidney disease (BKD). Tissues from 134 Atlantic salmon and 164 chinook salmon were selected on the basis of their having a positive diagnosis of BKD, with evidence of multi-tissue infection including the brain (214 fish), or brain involvement in the absence of systemic lesions attributable to BKD (58 fish). Although meningitis was a feature of BKD in both species, encephalitis was more common in the Atlantic salmon. Specifically, a higher portion of the Atlantic salmon had encephalitis accompanying meningitis (P = 0.0159), encephalitis in the absence of meningitis (P = 0.0756), and brain lesions (meningitis or encephalitis) in the apparent absence of systemic lesions (P = 0.0067). These findings suggest either that some aspects of the pathobiology of R salmoninarum are dictated by the host species, or that the farm management methods used to deal with BKD are sufficiently different for the two species of salmon that they affect the pathology of the disease.