A histological study of the response to challenge with vibriosis in ayu, Plecoglossus altivelis Temminck and Schlegel, vaccinated by immersion and injection with Vibrio anguillarum bacterin

. The histological responses of ayu, Plecoglossus altivelis , given an intramuscular injection of a formalin‐killed bacterin of Vibrio anguillarum are described. Lesions at the site of injection showed muscular necrosis and infiltration of inflammatory cells by the second day after injection, and production of granulation tissue from the fifth to the fourteenth day. Protective responses against vibriosis were studied histopathologically in ayu that were vaccinated by intramuscular injection with formalin‐killed Vibrio bacterin and by immersion in sonicated Vibrio bacterin, and challenged by a subcutaneous injection with viable Vibrio on the fourteenth day after the vaccination. Efficacy of both methods was confirmed by the survival of vaccinated fish after the challenge. There was slight bacterial multiplication in the fish, and bacterial phagocytosis by infiltrated neutrophils and tissue necrosis in the injected area on the third day after the challenge. In contrast, non‐vaccinated fish died within 24h of challenge, with extensive bacterial multiplication and tissue necrosis in the injected area and visceral organs.