IMPACT OF STRESSORS ON TRANSMISSION POTENTIAL OF RENIBACTERIUM SALMONINARUM IN CHINOOK SALMON

Renibacterium salmoninarum (Rs) is the causative agent of bacterial kidney disease (BKD), a significant disease of Chinook salmon in the Great Lakes. Rs can cause a long-term persistent infection and infected fish may not necessarily show disease. The standard susceptible-infected-recovered (SIR) population framework for disease does not model Rs infection dynamics well. Fenichel et al. 2009 (Ecol. Apps V19:747) proposed a conceptual framework for Rs, which we have modified here as the ‘susceptible-latent-diseased-latent’ (SLDL) model. Under this model, fish are 1) susceptible /naïve, 2) exposed to Rs with latent infections, and 3) diseased and presumably infectious. The model hypothesizes that salmon move back and forth between the latent and diseased states and this process is driven by stressors. The goal of any model is to identify uncertainty and generate testable hypotheses. Here we will empirically evaluate how potential stressors impact both host and pathogen. Next, we will evaluate how transmission risk changes during the infection cycle and in response to stressors. This will be accomplished by conducting laboratory challenges of Chinook salmon fed different diet regimes or held at different temperatures. In Aim 1, specific pathogen free (SPF) Chinook salmon will be fed one of four diets formulated to contain elevated omega-3 fatty acids using fish oil or elevated omega-6 fatty acids using soybean/corn oil and the thiamine content will be either replete or deficient. Fish will be injected with a Great Lakes strain of Rs using an established challenge model. Fish will be held at an optimal temperature (12°C) and sampled periodically until 16 weeks post-challenge (pc). Transmission potential, bacterial load and disease state will be assessed on sampled fish. At 2 weeks pc, a subset of challenged fish will subjected to a cold or heat stress. The temperature stress experiment will be conducted using only fish fed the thiamine-replete diet with elevated omega-3 fatty acids. Fish will be sampled periodically out to 16 weeks pc. Transmission potential will be assayed as bacterial shedding. Disease status of individual fish will be assessed for external and internal clinical signs, blood measurements and histopathology of the post kidney. Bacterial load will be assessed using culture and quantitative PCR.