Histopathology of the thymus in Saprolegnia-infected wild brown trout, Salmo trutta L.

A comparative analysis was made of the thymic cytoarchitecture of healthy and Saprolegnia-infected wild brown trout. In Saprolegnia-infected fish, even when the thymus was not invaded by fungal hyphae, the thymic architecture was lost without signs of regionalization. Intercellular oedema, some thymocytes, hypertrophic and degenerated epithelial cells and increased phagocytic activity were also observed. In thymi invaded 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 macrophages contained engulfed dead cells. However, no inflammatory response to the fungal invasion was observed. These results confirm the immunodepressed condition of Saprolegnia-infected wild brown trout previously observed in peripheral lymphoid organs. We discuss the secretion of fungal products and/or the high levels of corticosteroids observed in these fish as possible origins of the condition.