Cohabitation Challenge Demonstrates Minimal Transmission of Tenacibaculosis From Atlantic Salmon ( Salmo salar L.) to Coho Salmon ( Oncorhynchus kisutch W.)

ABSTRACT Tenacibaculosis, a disease caused by Tenacibaculum spp., is of concern in British Columbia (BC, Canada) due to the potential for interspecific transmission from Atlantic salmon ( Salmo salar L.) to Pacific salmon. However, the susceptibility of most Pacific salmonids remains unknown. In this study, Atlantic salmon post‐smolts were bath‐exposed to an Atlantic salmon‐derived BC isolate of Tenacibaculum maritimum (isolate 2.1C) at 10 6 or 10 8 CFU mL −1 and cohabitated with naïve Atlantic or coho salmon ( Oncorhynchus kisutch W.) for 41 days. Tenacibaculum maritimum bath‐exposed and cohabitant Atlantic salmon exhibited widespread morbidity (~95%) with multifocal ulcerative epidermal lesions, accompanied by intralesional T. maritimum , and generally over 35% mortality. Coho salmon that cohabitated with T. maritimum bath‐exposed Atlantic salmon experienced ≤ 5% ( n = 4, 1 fish per tank) morbidity and mortality, with T. maritimum reisolated and confirmed through qPCR. The four afflicted coho salmon had similar external lesions as infected Atlantic salmon. Concurrently, these coho salmon lacked visceral fat, a feature not associated with tenacibaculosis. These findings suggest that under the tested conditions, BC coho salmon are unlikely to, but can develop tenacibaculosis from infected Atlantic salmon, and compromised body condition may be required for disease development.