Pathologic and immunohistochemical findings in sea bass (Dicentrarchus labrax) naturally infected with Aeromonas veronii biovar veronii.

Tissue samples from European sea bass (Dicentrarchus labrax) naturally infected with Aeromonas veronii biovar veronii were subjected to bacteriological, molecular, pathological, and immunohistochemical analyses. The fish were sourced from aquaculture farms in the Aegean region of Turkiye. Clinical and postmortem examinations revealed granuloma formation as the most prominent lesion, primarily affecting the skin, spleen, kidneys, and liver. Sequencing of the 16S rRNA gene from all isolates showed 96-98% identity with A. veronii biovar veronii in the BLASTN 2.13.0+ database. Histopathological analysis demonstrated that granulomas originated from clusters of epithelioid macrophages and progressed through five distinct stages, culminating in caseous necrosis. Immunohistochemical labeling localized A. veronii antigens within necrotic regions and within the cytoplasm of macrophages in granulomatous lesions. These findings suggest that macrophages serve as the primary host cells for the bacterium and play a key role in its intra- and inter-tissue dissemination.