Induction of innate immunity in fish by humus extract and its protective effect

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Humic substances are formed during the decomposition of organic matter in humus, and are found in many natural environments in which organic materials and microorganisms have been present. Humus extract exhibited anti-bacterial activity against 9 species 15 strains of fish pathogens. Oral administration of humus extract to carp induced effective protection against experimental atypical Aeromonas salmonicida infection. Mortality of fish and development of skin lesions such as hemorrhages and ulcers were significantly suppressed in carp treated with 10 %, 5 % or 1 % humus extract adsorbed on dry pellets. The median surviving days was also greater in fish treated with 10% or 5% humus extract than in untreated fish. Atypical A. salmonicida was isolated from ulcerative lesions of part of dead fish, but Aeromonas hydrophila and Flavobacterium sp. were also isolated from these fish, verifying bacterial population changes during the progression of skin lesions. Oral administration of humus extract to ayu induced effective protection against experimental atypical Flavobacterium psychrophilum infection. Mortality of fish (65%) and development of skin lesions such as ulcers and hemorrhages were significantly suppressed in ayu treated with humus extract. F. psychrophilum was isolated from ulcerative lesions of part of dead fish, and bacterial DNA could be amplified using PCR in dead control fish. Surviving rate of ayu treated with humus extract was 96%, and no gross skin lesion was observed. No bacteria was isolated and no bacterial DNA was amplified in survived fish. These results clearly show that treatment of fish with humus extract is effective in preventing A. salmonicida disease and cold water disease.