Efficacy of dietary live or heat‐killed Bacillus subtilis in goldfish (Carassius auratus) infected with Ichthyophthirius multifiliis

BACKGROUND: The beneficial effects of Bacillus subtilis on growth, immune response, and disease resistance against various diseases in different fish species have been proved. However, there are no data concerning this probiotic effect on skin mucosal immunity in fish infected with Ichthyophthirius multifiliis (Ich). Ich has a high mortality rate in both edible and ornamental fish and consequently is concerned with heavy economic losses. OBJECTIVES: Thus, we assessed the efficacy of live and heat‐killed B. subtilis on skin immunity and histopathology in goldfish (Carassius auratus) infected with Ich. METHODS: Goldfish (144 fish, 2.38 g average weight) were stocked in nine glass tanks each in three replicates. Fish were fed 10(9) CFU g(−1) live or heat‐killed B. subtilis for 80 days. RESULTS: Probiotic administration in both viable and non‐viable forms could enhance the growth performance in goldfish. Probiotic therapy also reduced the density of the parasite and histopathological level on skin and gill tissues of the treated fish. Real‐time polymerase chain reaction analysis showed a higher expression of lysozyme and tumour necrosis factor‐α in the treated groups compared to the control group. CONCLUSIONS: These data demonstrated the beneficial effect of B. subtilis as probiotic and paraprobiotic on growth performance and disease resistance to Ich infestation in goldfish.