Outbreaks of Edwardsiella ictaluri Infection in Ayu Plecoglossus altivelis in Japanese Rivers

From middle August to early October in 2007, mortalities in wild ayu Plecoglossus altivelis occurred in the rivers of Tokyo Metropolis, Yamaguchi Prefecture and Hiroshima Prefecture, Japan. Hemorrhagic ascites was observed in almost all the examined fish. Some fish showed reddening of body surface, anus or bases of fins and exophthalmos. A single species of bacteria was isolated from the kidney, spleen and ascites of all the examined fish and identified as Edwardsiella ictaluri. Pathogenicity against ayu of the bacterial isolates was demonstrated by experimental infections, with the LD50 being approximately 104 CFU/fish. The dead fish exhibited similar disease signs to those of naturally affected fish. From these results, it was concluded that the present mortality of wild ayu was caused by E. ictaluri infection. This is the first report on E. ictaluri infection of fish in Japan.