Acute Aeromonas salmonicida infection in turbot (Scophthalmus maximus L.). Histopathological and immunohistochemical studies

• Experimental model of the A. salmonicida acute infection in turbot.
• A. salmonicida provokes a septicaemic syndrome in turbot.
• A. salmonicida causes vascular and tissular damage during the acute infection.
• IHC allows studying the distribution of A. salmonicida antigen in the lesions.
• IHC is a diagnostic tool to detect A. salmonicida in paraffin-embedded samples.

Aeromonas salmonicida subsp. salmonicida (A. salmonicida) is a highly pathogenic bacterium for turbot culture, which represents a great threat in fish farming. In this study, gross and histological lesions in turbot experimentally inoculated with A. salmonicida were analysed. In addition, the distribution of the bacterial antigen in tissues was investigated by using an immunohistochemical method specifically developed for this purpose. A. salmonicida caused septicaemia in challenged fish, characterized by vascular and necrotic changes, and the presence of bacterial colonies in several organs. Immunoreactivity against the bacterial antigen was evidenced in the coelomic cavity, blood vessels, and associated to the necrotic foci. These findings indicated the rapid onset of bacteraemia and of the tissue colonization by A. salmonicida and a direct relation between the presence of the bacteria and the development of lesions. Immunoreactivity against bacterial antigen was also located in the cytoplasm in both circulating and tissue monocyte/macrophages, suggesting the phagocytosis of the bacterium during the initial phase of the infection. To our knowledge, this is the first description of the morphohistological changes induced by acute A. salmonicida infection in turbot. The immunohistochemical method represents a new tool for the diagnosis of the disease and to gain insight in some aspects of its pathogenesis.