Bacterial hemolytic jaundice (BHJ) was first described in populations of cultured yellowtail, Seriola quinqueradiata in 1980 in Japan. BHJ is now one of the most important diseases of farmed yellowtail. The taxonomic position of the causative agent, Ichthyobacterium seriolicida, was recently determined as a single clade within the order Flavobacteriales and was distinct from the known clades of the family Flavobacteriaceae, Blattabacteriaceae and Cryomorphaceae. Whole genome analysis of the type strain JBKA-6T revealed that this bacterium harbors a single circular chromosome. Polymorphic analyses of I. seriolicida identified four major clusters among I. seriolicida isolates, and all belonged to clonal complexes; thus, this bacterium is considered to be recently introduced in Japan. In passive immunization tests, significantly lower mortality was observed in fish that received the serum from a convalescent fish, indicating I. seriolicida is neutralized by the humoral component of the immune system. However, since this bacterium does not grow vigorously in vitro, control measures such as the production of an inactivated vaccine is under development.
