Abstract
In this study, Japanese eels were injected with bacterial sonicate and then challenged with viable cells of Vibrio anguillicida. Bacterial sonicate was injected intramuscularly at a rate of 2.0 mg per 100 g body weight. After 24 hours, the treated fishes were challenged with an intramuscular injection of the viable bacteria at rates of 0.5 and 1.0 mg per 100 g body weight.
Sixty percent of challenged fishes survived for three to seven days. The fishes produced protrudent lesions at the injected areas within three days and ulcerative lesions on the fifth day. In the protrudent lesions, many neutrophils infiltrated and phagocytized bacteria extensively in the affected tissues which exhibited necrosis, edema and hemorrhage. In the viscera, reactions of neutrophils and cells belonging to the reticulo-endothelial system were extensive, which prevented bacterial dissemination.
Forty percent of challenged fishes became moribund within two days. The moribund fishes had protrudent lesions at the injected areas and exhibited the congested liver, splenomegaly and enteritis. They underwent systemic infection and the infected tissues exhibited extensive necrosis, circulatory disturbances and slight reactions of neutrophils.
Based on the results, it was clarified that the fishes which were previously treated with the bacterial sonicate, manifested extensive reactions of neutrophils as defensive responses against challenge with the viable bacteria.
