Efficacy of green tea extract on removal of the ectoparasitic flagellate Ichthyobodo necator from chum salmon, Oncorhynchus keta, and masu salmon, O. masou

The efficacy of green tea extract (GTE) on removal of the ectoparasitic flagellate Ichthyobodo necator (Henneguy, 1883) from the fins and skin surface of infected chum salmon, Oncorhynchus keta, and masu salmon, O. masou, fry is reported. Immersion of infected fry (body weight 0.2-0.9 g) in 0.03% GTE for 30-60 min or 0.3-0.9% GTE for 1-5 min resulted in a decrease of 87.6-100% in chum salmon and 86.5-99.6% in masu salmon in the number of parasites on the dorsal and anal fins in comparison to untreated controls (statistically significant, p < 0.01). Scanning electron microscopy showed no parasite present on the skin surface of infected chum salmon fry after a 1 min bath in 0.9% GTE. To clarify the contribution of (-)-epigallocatechin gallate (EGCg), which comprises 14% of GTE, to the efficacy, a 60-min bath with 0.0042% EGCg, equivalent to 0.03% GTE, and a 5-min bath with 0.126% EGCg, equivalent to 0.9% GTE, was evaluated; the number of parasites on the fins decreased 78.3% and 83.5%, respectively. Neither the long (0.03% GTE for 60 min) or short bath treatments (0.3% GTE for 15 min, 0.6-0.9% GTE for 5 min, or 2.7% GTE for 3 min) resulted in any mortality in normal (parasite-free) alevin and fry (0.4-0.7 g) of either species, indicating the safety of the treatments. Moreover, seawater challenge tests of chum salmon fry indicated that the treatments had no effect on the seawater adaptability of normal fish and restored seawater adaptability to infected fish by removing the parasite. Large-scale application of the treatments to approximately 900,000 chum salmon fry (0.4-0.6 g) infected with the parasite in two hatcheries showed that baths in 0.03% GTE for 30 min or 0.6% GTE for 2.5 min resulted in statistically significant decreases of 76.2-90.7% or 98.8-99.4%, respectively (p < 0.01), in the number of the parasites on the skin surface and fins in comparison to untreated controls. Cumulative mortalities for 48 h after the treatments were 0.08-0.14%. From our results, we conclude that treatment of I. necator-infected salmonids with GTE is safe, efficacious, and applicable to treatment of large numbers of fish in hatcheries, and that one of the efficacious constituents is EGCg.