Histopathology and Pathogenesis of Experimental Infection with Edwardsiella tarda in Channel Catfish

Abstract

The histopathology and pathogenesis of Edwardsiella tarda-induced edwardsiellosis in juvenile channel catfish Ictalurus punctatus were characterized. The lateral body surfaces of fish were scraped before infection by immersion in a bath of E. tarda for 30 min. Scattered cutaneous petechiae and ulcerations were seen on the mouth, operculum, isthmus, abdomen, and lateral body after infection. Injured skin developed focal ulcerative necrotizing dermatitis, increased mucus, and irregular areas of skin depigmentation scattered over the entire body. Internal organs were congested, and livers showed patchy discoloration and petechiae. Histologically, the liver, head kidney, trunk kidney, and spleen had severe multifocal necrotizing inflammation. Scraped controls did not show gross or microscopic lesions except for mild dermatitis of injured skin at the scraping site. Bacteria in hepatic lesions were specifically identified by immunohistochemistry that used rabbit anti-E. tarda serum. Colony-forming units of E. tarda per gram of tissue or milliliter of blood peaked 3 d postinfection (PI) in the liver, trunk kidney, and blood. No bacteria were isolated from infected fish after 6 d PI. Clinical signs and internal gross lesions declined by 8 d PI and were absent thereafter. No histological lesions were observed in the intestine, stomach, heart, gills, or brain. Infected fish were sluggish and had rapid opercular movements and pale gills before death. Histological lesions were similar to those caused by E. ictaluri in channel catfish.