Challenge models for RTFS in rainbow trout fry (Oncorhynchus mykiss)

The fish pathogen Flavobacterium psychrophilum is one of the main causes of mortality in fry of farmed rainbow trout (Oncorhynchus mykiss) and other salmonid fish. The disease following infection is often called bacterial coldwater disease (BCWD) in USA and rainbow trout fry syndrome (RTFS) in Europe. Presently no commercial vaccine exists, although several are under development. Various models for experimental infection have been carried out with varying success, including challenge through injection, bath and cohabitation. Intraperitoneal challenge and bath challenges combined with various forms of stress have shown to be reproducible. Bath challenge is more appropriate for vaccine testing, since natural transmission of infection is imitated and is also more suitable due to the small size of the fry. A bath-model using H2O2 as a stressor is currently being tested on 1.4g rainbow trout fry in four experimental groups: 1) no H2O2/no bath infection, 2) H2O2/no bath infection, 3) no H2O2/ bath infection and 4) H2O2/ bath infection. Mortality will be evaluated over approximately 25 days. The project is currently in its preliminary phase and presently focused on development of a model of infection. The overall goal is also to examine gene expression and location of transcription products in rainbow trout fry, in order to optimize vaccination or immune-stimulation. The presentation will focus on previous experimental models and the experimental design of the current model as well as the future plans for the project.