Acquired Immunity of Yellowtail, Seriola quinqueradiata Recovered from Experimental Infection with β-hemolytic Streptococcus sp.

Abstract

Immunity of yellowtail, Seriola quinqueradiatarecovered from experimental infection with β-hemolyticStreptococcussp. was examined. Yellowtail showing vertebral deformity caused by the sublethal dose of infective bacterium were reared for 75 days. Fishes those recovered from this symptom were free of viable bacteria. The agglutinating antibody titers in the sera isolated from recovered fish against the causative bacterium were 1 : 4-1 : 16. After reinfecting the recovered fish, no clear symptom was observed, furthermore number of viable bacteria in the blood, spleen, and kidney were decreased faster than those of the control fish. Moreover, increase of viable bacterial cell number was not observed in the brain of the recovered fish. These results indicate that fish recovered from β-hemolytic streptococcal infection acquired resistance to subsequent reinfection.