Atypical Aeromonas salmonicida Infection in Cultured Marbled Sole Pleuronectes yokohamae

In spring of 2003, a mass mortality occurred in a population of marbled sole Pleuronectes yokohamae reared at a hatchery of Miyagi Prefecture, Japan. Since the first case, outbreak of the disease has been repeated every year in the hatchery. The affected fish weighing 1-5 g showed hemorrhagic lesions on the body surface. The maximum cumulative mortality reached more than 80%. The prominent histopathological feature was hemorrhage and necrosis in the gill, muscle, liver, heart and kidney associated with extensive bacterial multiplication. A single species of bacterium was isolated from the kidneys of the diseased fish using trypticase soy agar containing 1% NaCl at 20°C. The isolates were identified as atypical Aeromonas salmonicida by biochemical, serological and molecular phylogenetic analyses. Experimental infections confirmed that an isolate was highly pathogenic not only to marbled sole, but also to Japanese flounder Paralichthys olivaceus and spotted halibut Verasper variegatus, with LD50 of less than 102 CFU/fish (1.4-6.8 g body weight) by intramuscular injection.