Evaluation of bithionol as a bath treatment for amoebic gill disease caused by Neoparamoeba spp.

This study examined the toxicity of bithionol to Atlantic salmon Salmo salar and rainbow trout Oncorhynchus mykiss in fresh- and seawater and the efficacy of bithionol as a 1 h seawater bath treatment for amoebic gill disease (AGD). To examine toxicity, fish were bathed for 1, 3 and 6 h in bithionol, an anti-protozoal at 0, 1, 5, 10, 25 and 35 mg L−1 with toxicity determined by time to morbidity. Efficacy was examined by bathing AGD-affected Atlantic salmon and rainbow trout for 1 h at bithionol concentrations of 1–25 mg L−1. Efficacy was determined by examining gill amoeba counts and identifying percent lesioned gill filaments at 1 and 24 h after bath exposure to bithionol. For both species, bithionol was determined to be toxic at 25 and 35 mg L−1 exhibiting median lethal times (LT50s) ranging from 21 to 84 min. Morbidity occurred in the 5 and 10 mg L−1 treatments, however, due to sampling regime there were not enough fish available to calculate LT50s. Only bithionol at 1 mg L−1 was considered non-toxic with no signs of morbidity. Bithionol was more toxic in seawater than freshwater and had no acute effects on gill Na+/K+ ATPase and succinic dehydrogenase, or plasma osmolality and chloride concentration. Bithionol at 1 mg L−1 reduced percent lesioned gill filaments in Atlantic salmon and rainbow trout by 33 and 27%, respectively, compared to the seawater control. Similarly, numbers of amoeba were reduced by 33 and 43% for Atlantic salmon and rainbow trout, respectively, when compared to the seawater control. Furthermore, bithionol reduced percent lesioned gill filaments as much as did the current industry standard of freshwater. This study demonstrated that a 1 h seawater bath containing 1 mg L−1 bithionol could be an improvement to the current method of treatment for AGD-affected Atlantic salmon and rainbow trout.