Abstract:Acute bacterial septicemic infection in sea cage-cultured juvenile cobia (Rachycentron cannadum) has occurred at Penghu (Pescodores) Islands, Taiwan, since October of 1999 with a 30-80% mortality rate. Diseased fish exhibited no apparent surface lesions; however, the kidney and spleen demonstrated swelling and white tubercles. The bacterium was identified as Photobacterium damselae ssp. piscicida, based on the results of the API-20E system and Bionor Mono-Pp kit. The pathogenicity of the isolate to cobia was confirmed by intraperitoneal injection and immersion. From histological examinations, multifocal necrosis and/or granulomatatous inflammation were found in tissue sections. Drug sensitivity testing indicated this pathogen were sensitive to amoxicillin, chloramphenicol, oxolinic acid, oxytetracycline, trimethoprim-sulfamethoxazole (SXT), and flumequine in 1999. However, amoxicillin-, chloramphenicol-, and oxytetracycline-resistant Photobacterium damselae ssp. piscicida were isolated in 2000. SXT-, oxolinic acid- and flumequine-resistant P. damselae ssp. piscida were isolated from sea cage-cultured cobia at Penghu in 2002, 2006, and 2007, respectively.