Pathology associated with Edwardsiella ictaluri in catfish, Ictalurus punctatus Rafinesque, and Danio devario (Hamilton‐Buchanan, 1822)

The pathology associated with infections of Edwardsiella ictaluri in a new host, the danio, is described and compared to that observed in the channel catfish. In catfish the acute stage was characterized by petechial haemorrhage of the jaw, ventral body surface, and at the base of the fins. In chronic cases a characteristic finding was an erosion of skin and muscle overlying the skull, exposing bone and occasionally the brain. Histologically there was a diffuse granulomatous inflammation of the olfactory bulb and the telencephalon. In the catfish the infection was often systemic, involving intestine, liver, spleen, and occasionally kidney. In the danio no gross lesions were observed. Infected fish exhibited an erratic, spinning‐type swimming behaviour. Histologically, lesions usually were confined to the brain. They consisted of an acute, primarily granulocytic inflammation of the medulla oblongata (rhomboencephalon). Only one fish showed evidence of a systemic infection.