Antiparasitic Efficacy of Glutaraldehyde against Ichthyobodo salmonis and Trichodina spp. in Chum Salmon Oncorhynchus keta and Barfin Flounder Verasper moseri, with a Toxicological Assessment on Host Fish

Abstract

Ichthyobodo (Euglenozoa) and Trichodina (Ciliophora) are important ectoparasites in aquaculture, causing substantial disease burdens in salmonids and flounder. With formalin use increasingly restricted, effective alternatives are required. This study assessed the antiparasitic efficacy and acute toxicity of glutaraldehyde (GA) against I. salmonis and T. truttae on chum salmon Oncorhynchus keta and T. hokkaidoensis on barfin flounder Verasper moseri. Infected fish were treated with GA (28–44 mg/L) for 15, 30, and 60 min, with 93 mg/L formaldehyde (FA) serving as a positive control. Mortality was recorded at 24 h post-initiation (hpi), and parasite counts were quantified microscopically. Acute toxicity tests involved 1-h exposures to GA (chum salmon: 93–3,682 mg/L; flounder: 35–866 mg/L) and FA (chum salmon: 93–2,327 mg/L; flounder: 93–2,270 mg/L), with mortality monitored for 96 h. GA achieved >90.0% clearance of I. salmonis and T. truttae on chum salmon. In flounder, GA eradicated >99.0% of T. hokkaidoensis, with 28 mg/L for 30 min achieving 99.7% clearance without fish mortality. The 1% lethal concentration values of GA (salmon: 63 mg/L; flounder: 102 mg/L) exceeded the treatment concentrations effective for parasite clearance. These findings highlight GA as a promising alternative for managing Ichthyobodo and Trichodina infections in aquaculture.