Virulence of Yersinia ruckeri for Four Indigenous Fish Species in Japan

Enteric redmouth disease (ERM), which is caused by Yersinia ruckeri, has not been reported in Japan. In the present study, virulence of the bacterium were assessed by experimental chal­ lenges with the rainbow trout Oncorhynchus mykiss and 4 indigenous fish species in Japan: yamame (masu salmon parr) O. masou masou, amago O. masou macrostomus, whitespotted char Salvelinus leucomaenis and ayu Plecoglossus altivelis. When intraperitoneally (ip) injected with the bacterium at a dose of 7.1 x 10 2 CFU/g fish body weight (BW), rainbow trout, char, yamame, and amago showed the cumulative mortalities of 100%, 60%, 30% and 30%, respectively. The cumulative mortalities of ip­ injected ayu with doses of 1.5 x 10 6 , 1.5 X 10 5 , 1.5 X 10 4 and 1.5 x 10 3 CFU/g BW were 87%, 47%, 13% and 0%, respectively. In addition, the cumulative mortalities of ayu immersed in water containing 1.7 x 10 8 and 1.7 x 10 7 CFU/mL of the bacterium were 60% and 6.7%, respectively. Typical clinical signs of ERM, such as reddening of the wall of the oral cavity, tongue, upper jaw, lower jaw, operculum, bases of fins, or skin around the anus, were observed in all dead salmonids but not in ayu. Instead, exophthal­ mos, hemorrhage in the eyes, ascites or hypertrophy of the kidney were often observed in dead ayu. These results indicate that Yersinia ruckeri is a potentially dangerous bacterium to freshwater aquacul­ ture in Japan.