Carrying situation in red sea bream artificially infected with Edwardsiella tarda

¹Owase Fisheries Research Laboratory, Mie Prefecture Fisheries Research Institute, Owase, Mie 519-3602, ²Graduate School of Biosphere Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8528, Japan

We examined bacterial concentration and granuloma formation at the early and late phases of infection in red sea bream Pagrus major artificially infected with an atypical Edwardiella tarda. Fish at the early and late phases were defined as fish at 1 day and 30 days after infection, respectively. Concentrations of E. tarda in blood were higher at the early phase than at the late phase, while those in kidney were higher at the late phase than at the early phase. The concentrations in blood and kidney at the late phase were less than 10^1.0 CFU/mL and 10^7.1-7.8 CFU/g, respectively. Granuloma formation in kidney was found only at the late phase. The IFAT-positive areas were scattered in the kidney tissue at the early phase, but only in granulomas at the late phase. These results suggest that E. tarda can survive in granulomas, however, detection rates of E. tarda from kidney by a platinum loop were 29-35% at the late phase but 100% at the early phase. The former low rate is seemingly due to the difficulty of destroying the granulomas by the platinum loop.