Understanding the Pathogenesis of Red Mark Syndrome in Rainbow Trout (Oncorhynchus mykiss) through an Integrated Morphological and Molecular Approach

Red mark syndrome is a non-lethal widespread skin disease mainly reported in rainbow trout and caused by a Midichloria-like organism. Despite extensive research, its etiology and pathogenesis are still uncertain. In the present study, the authors used an integrated morphological and molecular approach, including gene expression, to elucidate the immune response and the complex immune interaction between the host and Midichloria-like organism. The results lead to the conclusions that the most severe skin lesions were characterized by a high level of inflammatory cytokines sustaining and modulating the severe inflammatory process. In contrast, in the moderate form, the response was driven to produce immunoglobulins and IL-10 to control the severity of the disease. Humoral immunity elicited during MLO infection appeared to have a fundamental role in controlling the severity of the skin disease, possibly through bactericidal antibody-mediated mechanisms.