Infectious agents and their physiological correlates in early marine Chinook salmon (Oncorhynchus tshawytscha)

: The early marine life of Pacific salmon is believed to be a critical period limiting population-level survival. Recent evidence suggests that some infectious agents are associated with survival but linkages with underlying physiological mechanisms are lacking. While challenge studies can demonstrate cause and effect relationships between infection and pathological change or mortality, in some cases pathological change may only manifest in the presence of environmental stressors; thus, it is important to gain context from field observations. Herein, we examined physiological correlates with infectious agent loads in Chinook salmon during their first ocean year. We measured physiology at the molecular (gene expression), metabolic (plasma chemistry) and cellular (histopathology) levels. Of 46 assayed infectious agents, 27 were detected, including viruses, bacteria and parasites. This exploratory study identified: : a strong molecular response to viral disease and pathological change consistent with jaundice/anemia associated with Piscine orthoreovirus, strong molecular signals of gill inflammation and immune response associated with gill agents `Candidatus Branchiomonas cysticola’ and Parvicapsula pseudobranchicola, a general downregulation of gill immune response associated with Parvicapsula minibicornis complementary to that of P. pseudobranchicola. Importantly, our study provides the first evidence that the molecular activation of viral disease response and the lesions observed during the development of the PRV-related disease jaundice/anemia in farmed Chinook salmon are also observed in wild juvenile Chinook salmon. :