Protection against Atypical Aeromonas salmonicida Infection in Carp (Cyprinus carpio L.) by Oral Administration of Probiotic Bacterial Culture

Introduction Atypical Aeromonas salmonicida is a fish pathogen that causes several diseases in freshwater fish such as goldfish (Carassius auratus L.) (6) and eel (Anguilla japonica) (2), and also seawater fish (5). Since 1996, A. salmonicida infection characterized by the formation of ulcers on the body surface and fins (ulcer disease) has become prevalent in colored carp (Cyprinus carpio koi L.) cultivation in Japan (4). Antibiotic treatment for the ulcer disease resulted in no effectiveness because of the development of drug-resistance bacteria. Thus, the use of chemotherapeutic agents as an infection control measure has become questionable because of the acquisition of antimicrobial resistance among pathogenic bacteria. Vaccination is believed to be effective in principle against the disease, but no effective vaccine has yet been developed for ulcer disease. Another alternatives to chemotherapy and vaccination in disease control could be the use of probiotics as biological control agents. Probiotics could be applicable to fish farming to reduce the incidence of various infectious diseases (1, 7). We therefore investigated the effect of oral administration of probiotic bacterial culture on protection of carp from A. salmonicida infection. The results clearly showed that treatment of fish with small amounts of killed probiotic bacterial culture was effective in preventing A. salmonicida disease.