Changes in the spleen of the channel catfish Ictalurus punetatus Rafinesque indueed by infection with Aeromonas hydrophila

Abstract. A virulent, 2,3-butanediol positive culture of Aeromonas hydrophila , subspecies hydrophila , was fed or injected intraperitoneally into channel catfish. The positive infection and tissue involvement were monitored by the 2,3-butanediol test with tissue homogenate enriched with a buffered nutrient broth containing dextrose. The results revealed that the fish infected orally showed very little or no spleen involvement. Fish infected intraperitoneally, however, showed spleen involvement in a very short time, accompanied by the rapid onset of an acute systemic infection.
Light and electron microscope studies of the spleen revealed that the pathogenic bacteria were mainly located within the reticular sheath of the ellipsoids and only sparsely in the surrounding regions, and that intense phagocytic activity of macro-phages had taken place in the ellipsoids. It was also observed that the phagocytized bacteria divide extracellularly, and probably intracellularly, resulting in the destruction of the endothelial and reticular cells of the ellipsoids. This finding substantiates the cytolytic and fibrolytic capacities of A. hydrophila for destroying host defence systems and may contribute to its invasive powers. Other cytological changes in cells in spleen after infection are described, and the resistance of A. hydrophila to intracellular enzymatic digestion by the macrophages is discussed.