Pathology of experimental enteric septicaemia in channel catfish, Ictalurus punctatus (Rafinesque), following immersion-exposure to Edwardsiella ictaluri

Abstract. Fingerling channel catfish, Ictalurus punctatus (Rafinesque), were exposed experimentally to Edwardsiella ictaluri by immersion for 1 h in water containing 5 × 10⁸ colony-forming units (CFU) of the bacterium per ml. Ninety per cent of the fish developed lesions typical of enteric septicaemia of catfish (ESC), 93% of affected fish developed the acute form of ESC and 7% developed chronic ESC. Acute disease was characterized grossly by cutaneous haemorrhage and ulceration, and microscopically by enteritis, olfactory sacculitis, hepatitis and dermatitis. The earliest lesions of acute ESC, i.e. enteritis and olfactory sacculitis, were observed microscopically at 2 days post-exposure (PE); gross lesions, primarily mild subcutaneous haemorrhage at the base of fins, were first apparent at 4 days PE. Chronic ESC, seen most commonly 3–4 weeks PE, was characterized by dorsocranial swelling and ulceration, granulomatous olfactory neuritis/perineuritis, and meningoencephalitis involving the olfactory bulbs, olfactory tracts and olfactory lobes of the brain. Gross and microscopic lesions in the acute and chronic forms of experimental ESC were similar to the lesions reported in naturally occurring ESC. Definitive pathogenesis of acute and chronic ESC remains to be determined.