HISTOPATHOLOGY OF THE THYMUS IN SAPROLEGNIA-INFECTED WILD BROWN TROUT, SALMO-TRUTTA L

A comparative analysis was made of the thymic cytoarchitecture of heal thy and Saprolegnia-infected wild brown trout. In Saprolegnia-infected fish, even when the thymus was not invaded by fungal hyphae, the thym ic architecture was lost without signs of regionalization. Intercellul ar oedema, some thymocytes, hypertrophic and degenerated epithelial ce lls and increased phagocytic activity were also observed,In thymi inva ded by the fungus, hyphae occurred isolated or in close apposition to epithelial cells, and the thymic disorganization was more noticeable. Most thymocytes were pyknotic and both epithelial cells and macrophage s contained engulfed dead cells. However, no inflammatory response to the fungal invasion was observed. These results confirm the immunodepr essed condition of Saprolegnia-infected wild brown trout previously ob served in peripheral lymphoid organs. We discuss the secretion of fung al products and/or the high levels of corticosteroids observed in thes e fish as possible origins of the condition.