Histopathological and ultrastructural studies on a novel pathological condition in Solea senegalensis

A new parasitic disease affecting cultured sole Solea senegalensis (Kaup, 1858) is characterised by the presence of external protuberances in the skin of the affected fish. These lesions correspond to nodules in the muscular tissue showing an abscess-like aspect. Similar lesions were found in the kidney, heart, liver and digestive tract. Histological sections of these nodules revealed the presence of a large core formed mainly of necrotic tissue surrounded with fibroblasts and macrophages. Round-shaped plasmodial organisms were found in the external layer of the nodules and usually inside macrophages or fibroblasts. These organisms were also observed in the intestinal mucosa inside phagocytic cells or parasitophorous vacuoles within the enterocytes. The morphological and ultrastructural characteristics of these organisms are similar to the morphology of some groups of parasites described as fish pathogens. The main features suggest that these organisms could be amoebae or parasites with an amoeboid or plasmodial form in their developmental cycle.