Early Changes in Pigmented Macrophages in Head Kidney of Channel Catfish Infected withAeromonas hydrophila

Abstract

Channel catfish Ictalurus punctatus with scarified skin were exposed to the bacterial pathogen Aeromonas hydrophila. Systemic infections developed in 80% of the exposed fish, and the remaining exposed fish had infections limited to the cutaneous lesion. Skin of control fish was injured in the same manner as for the exposed fish, but control fish were not exposed to A. hydrophila nor was A. hydrophila recovered from them. None of the fish died during the 3-d experiment, and gross lesions in head kidneys, including changes in the size or relative weight, were not found in infected fish. Macrophage aggregates in head kidney of systemically infected fish increased 68% in mean area, 28% in number, and 111% in total relative volume. Volume of lipofuscin, the predominate pigment found in channel catfish head kidney, in systemically infected fish was about twice that of control fish. Volume of hemosiderin was about four times as great and macrophages containing hemosiderin were about three times as numerous in systemically infected fish as in control fish. The amount of melanin in macrophages did not change in infected fish. Significant differences in macrophage aggregates and pigments were not found between controls and fish with only superficial infections. Microscopic lesions, other than changes in macrophage aggregates and pigments, were not found in the head kidney. Increases in macrophage aggregates, lipofuscin, and hemosiderin in head kidney of channel catfish were useful for quantification of injury caused by a systemic bacterial infection.