PATHOGENESIS OF EXPERIMENTAL EDWARDSIELLOSIS IN JAPANESE FLOUNDER PARALICHTHYS-OLIVACEUS

Japanese flounder Paralichthys olivaceus were artificially infected wi th Edwardsiella tarda by three different methods, viz., intraperitonea l injection (IpI), oral intubation (OI), and immersion (IMM). Moribund stages were attained earliest (4-5 days) in IpI group, followed by IM M (7-10 days) and OI (9-10 days) groups. However, almost the same tren d in the fate of E. tarda, monitored by viable count, was observed reg ardless of inoculation methods. The viable number of the pathogen cont inuously increased in all the four tissues tested (blood, intestine, l iver, 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. Histopa thological examination demonstrated abscess formations in the liver to gether with massive hepatocytic necroses. Kidney conditions were chara cterized 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 lesi ons in these three tissues were characterized by migration off. tarda- laden phagocytic cells. Phagocytosis and intraphagocytic multiplicatio n of the pathogen in these tissues were confirmed by enzyme labeled an tibody technique (ELAT) and electron microscopy.