Pathogenesis of Experimental <i>Edwardsiella ictaluri</i> Infections in Channel Catfish (<i>Icta lurus punctatus</i>)

The lesions observed in channel catfish (Ictalurus punctatus) as a consequence of exposure to Edwardsiella ictaluri via gut and water were studied. Channel catfish fingerlings were exposed to 10 10 bacterial cells via the gut and placed in tanks with unexposed fish. Both groups of fish were sampled periodically over a 35-day period. The gut-exposed fish exhibited gross signs of a systemic infection within 2 wk. Enteritis, hepatitis, interstitial nephritis, and myositis, all of which began as acute reactions and developed into chronic lesions, were observed histologically. Some of the contact fish developed systemic infections beginning in the intestine, thus demonstrating horizontal transmission. However, only contact fish developed the characteristic "hole in the head" lesion, which began in the olfactory sac and progressed to a granulomatous meningioencephalitis. These results indicate that both gut and nares are sites of primary infection in natural outbreaks of E. ictaluri.