Juvenile chinook salmon (Oncorhynchus tshawytscha) were experimentally infected with Renibacterium salmoninarum (Rs), the causative agent of bacterial kidney disease (BKD), to investigate the effects of BKD on three aspects of juvenile salmonid performance: (1) predator avoidance ability; (2) smoltification; and (3) physiological responses to stress. For these experiments, fish with different Rs-infection profiles (created by using an immersion challenge method) were sampled to assess physiological change and subjected to various performance tests during disease progression. When equal numbers of Rs-challenged and unchallenged fish were subjected to predation by northern pikeminnow (Ptychocheilus oregonensis) or smallmouth bass (Micropterus dolomieui), Rs-challenged fish were eaten in significantly greater numbers than controls by nearly two to one. A progressively worsening infection with Rs did not alter the normal changes in gill ATPase and condition factor associated with smoltification in juvenile chinook salmon. A dramatic proliferation of BKD was associated with maximal responses of indicators of smoltification, suggesting that the process of smoltification itself can trigger outbreaks of disease. When Rs-infected fish were subjected to three 60-s bouts of severe handling that were separated by 48-72 h, this experience did not lead to higher infection levels or increased mortality when compared to diseased fish that did not receive the stressors. Furthermore, the kinetics of plasma cortisol, glucose, and lactate over 24-h following each stressor were similar between fish with moderate to high BKD and those that had low or no detectable infection. Fish with moderate to high Rs infections had higher titers of cortisol and lactate prior to each application of the stressor and were also unable to consistently elicit a significant hyperglycemia in response to the stressors when compared to fish with low infection levels. During all experiments, fish consistently developed decreased hematocrits and blood glucose levels and increased levels of cortisol and lactate as the disease worsened, indicating that BKD is stressful, particularly during the later stages. Collectively, these results illustrate the impact of BKD on juvenile salmonids and have also ascribed some ecological significance to this disease beyond that of direct pathogen-related mortality. / Graduation date: 1999

A series of experiments with Aeromonas salmonicida and infectious hematopoietic necrosis virus (IHNV) were carried out to determine dynamics of the spread of infection in chinook salmon (1.2-1.98g) and rainbow trout (1.2-3.1g). It was found in experiments with A. salmonicida that fish infected by bath immersion became infectious at 4 days postexposure (dpe), one day prior to dying from furunculosis. In cohabitation experiments with a single infected fish donor, an average of 75% disease specific mortality was obtained. There was suggestive evidence that there is a positive relationship between holding volumes and furunculosis prevalence in cohabitation experiments with single donor fish. Median day to infection was inversely correlated with density. The threshold density at density of 1.97 fish/L was approximately 30 times less than the density of 0.08 fish/L, 13.33 and 320 fish respectively. Reproductive ratio (R₀) and transmission coefficient (β) in the furunculosis epizootic were 3.23 and 0.021 (individuals*day)⁻¹ respectively. The mortality rate (α) of infected animals was 28.7% per day. The models constructed successfully mirrored the results of laboratory experiments. Data produced by simulation of the models were significantly associated with the data obtained from laboratory experiments for susceptible (S) class and also for infected class. In similar experiments carried out with IHNV, it was found that donor fish became infectious 3 dpe. The virus levels in donor fish and prevalence levels were also highly associated. Smaller volumes of that led to higher prevalence levels than observed in bigger volumes with 60 or 30 fish in each. A single donor fish was able to transfer virus to recipient fish. However, unlike the A. salmonicida experiment, transmission was insufficient to initiate a full-scale infectious hematopoietic (IHN) epizootic. Estimated parameters for dynamics of infection were approximately half of the values for A. salmonicida (R₀=2.57,β=0.008 (individuals*day)⁻¹ and α=0.15). The models constructed for IHNV spread were used to simulate the results of density experiment. However, it was not possible to test the association between susceptible and infected classes due to inadequate number of infected fish. / Graduation date: 2001