The bacterial fish pathogen Flavobacterium psychrophilum, the cause of Rainbow Trout Fry Syndrome (RTFS), results in significant mortality in farmed rainbow trout; unless it is treated with antibiotics. Presently no commercial vaccine exists. More knowledge is required to elucidate the immune response in rainbow trout against F. psychrophilum in order to create preventive measures against RTFS. A limited number of studies have been carried out so far and have relied on samples from either naturally infected or injection-challenged fish. The use of naturally infected fish introduces many possible sources of error. Injection is a suboptimal approach for investigations regarding the immune response, since mucosal surfaces are bypassed. F. psychrophilum has a limited ability to cause disease in experimental bath challenges without applying a stressor. Recently, a bath model utilized H2O2 before pathogen exposure to elevate mortality. The model was used to examine the immune response to infection in rainbow trout fry (≈1 g); both with and without preceding H2O2 treatment. Samples from the head kidney were taken before pathogen exposure and 4 hours, 48 hours, 125 hours and 192 hours after exposure. The regulation of several immune relevant genes was examined and the relative bacterial load was assessed. Although it is not determined how H2O2 increases mortality, it is assumed to be due to stress. Exposure to H2O2 prior to infection altered the regulation of several genes, and several correlations between pathogen load and gene expression were observed.
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16th International Conference on Diseases of Fish and Shellfish, 2013