Pathogenesis of experimental Edwardsiellosis in Japanese flounder Paralichthys olivaceus

Japanese flounder Paralichthys olivaceus were artificially infected with Edwardsiella tarda by three different methods, viz., intraperitoneal injection (IpI), oral intubation (OI), and immersion (IMM). Moribund stages were attained earliest (4-5 days) in IpI group, followed by IMM (7-10 days) and OI (9-10 days) groups. However, almost the same trend in the fate of E. tarda, monitored by viable count, was observed regardless of inoculation methods. The viable number of the pathogen continuously increased in all the four tissues tested (blood, intestine, liver, and kidney) for 7 days after inoculation and reached the highest levels at moribund stages. The viable counts in the intestine, liver and kidney tissues were always higher than those in the blood. Histopathological examination demonstrated abscess formations in the liver together with massive hepatocytic necroses. Kidney conditions were characterized by necrotic lesions involving nephrons and abscess formations. Intestine showed only sporadic necrosis in the villar epithelia and abscess in the tunica propria. The abscesses and diffuse necrotic lesions in these three tissues were characterized by migration off. tarda-laden phagocytic cells. Phagocytosis and intraphagocytic multiplication of the pathogen in these tissues were confirmed by enzyme labeled antibody technique (ELAT) and electron microscopy.