Background. Ichthyobodosis (“costiosis”) is induced by the flagellate Ichthyobodo necator in both wild and cultured host populations. Fish farmers for decades have been using formaldehyde (FA) for control but this compound is problematic due to its carcinogenic and allergenic nature. Alternative auxiliary substances for bath treatment are needed and therefore we decided to compare efficacies of sodium percarbonate (SPC), peracetic acid (PAA), and hydrogen peroxide (HP) for control of ichthyobodosis in rainbow trout, Oncorhynchus mykiss. Materials and methods. An experimental infection of Ichthyobodo necator on rainbow trout (2–3 g) was established (total parasite population up to 142 000 flagellates per fish) and duplicated controlled water bath treatments were conducted with specified water quality criteria. Four compounds: SPC (40, 80, and 120 mg · L–1), PAA (0.1, 0.2, and 0.3 mg · L–1), HP (15 and 30 mg · L–1), and FA (40 and 80 mg · L–1) were tested. The compounds were applied for 2 h (as static freshwater bath treatments at 13°C) in 20-L aerated tanks (containing 10 L of the treatment solution). Results. FA at a concentration of 80 mg · L–1 and PAA at 0.3 mg · L–1 showed almost complete elimination of parasites but it was found that also SPC at a concentration of the 120 mg · L– was highly effective in eliminating I. necator. In addition, SPC and PAA (80 mg · L–1 and 0.2 mg · L–1, respectively) also significantly reduced the parasite load compared to untreated control fish. HP significantly reduced parasite populations at a concentration of 30 mg · L–1, but the compound elicited adverse reactions in fish. Conclusion. It is suggested that SPC and PAA may serve as alternatives to formaldehyde for control of ichthyobodosis in rainbow trout farms.

Ichthyobodo necator, rainbow trout, Oncorhynchus mykiss, bath treatment