Efficacy of bithionol as an oral treatment for amoebic gill disease in Atlantic salmon Salmo salar (L.) Renee L. Florent ⁎ , Joy A. Becker, Mark D. Powell Aquafin CRC, School of Aquaculture, Tasmanian Aquaculture and Fisheries Institute, University of Tasmania, Locked Bag 1370 Launceston, Tasmania 7250, Australia Received 30 October 2006; received in revised form 18 April 2007; accepted 28 April 2007 Abstract This study examined the efficacy of bithionol as an oral treatment for Atlantic salmon Salmo salar affected by amoebic gill disease (AGD). The current commercial management strategy of AGD is a costly 3 h freshwater bath. It is labour intensive and the number of baths needed appears to be increasing; hence, there is an effort to identify alternative treatments. Efficacy was examined by feeding AGD-affected Atlantic salmon twice daily to satiation with bithionol, an antiprotozoal, at 25 mg kg - 1 feed. Three seawater (35‰, 17 °C) re-circulation systems were used each consisting of three tanks containing 32 Atlantic salmon smolts with an average (±SEM) mass of 90.4 g (±5.2). Three feeds were examined in the trial including bithionol, plain commercial control and oil coated commercial control. Feeding commenced 2 weeks prior to exposure to Neoparamoeba spp. at 300 cells L - 1 and continued for 28 days post-exposure. Efficacy was determined by examining gross gill score and identifying percent lesioned gill filaments twice weekly for 4 weeks post-exposure. Bithionol when fed as a two-week prophylactic treatment at 25 mg kg - 1 feed delayed the onset of AGD pathology and reduced the percent lesioned gill filaments by 53% and halved the gill score from 2 to 1 when compared with both the plain and oil controls during an experimental challenge. There were no palatability problems observed with mean feed intake of bithionol over the trial duration with fish consuming higher levels of the bithionol diet compared to both the oil and plain controls. This study demonstrated that bithionol at 25 mg kg - 1 feed, when fed as a two-week prophylactic treatment for Neoparamoeba spp. exposure, delayed and reduced the intensity of AGD pathology and warrants further investigation as an alternative to the current freshwater bath treatment for AGD-affected Atlantic salmon. © 2007 Elsevier B.V. All rights reserved. Keywords: Salmo salar; Amoebic gill disease (AGD); Bithionol; Neoparamoeba spp; Oral treatment; Protozoan parasite; Aquaculture 1. Introduction Bithionol, a phenolic compound related structurally to hexachlorophene, has been used worldwide as a parasiticide for cattle, sheep, chickens, dogs and cats (Enzie and Colglazier, 1960). It is reported to have surfactant antimicrobial properties and thus is also effective against bacteria, moulds and yeast (Kim and Choi, 1998). It has been used as an alternative to praziquantel against human trematode and cestode infections, in particular paragonimiasis (Enzie and Colglazier, 1960; Takeuchi et al., 1984), and has been widely used in veterinary medicine, as it is active against flukes and cestodes (Mourot et al., 1987). Bithionol has Aquaculture 270 (2007) 15 – 22 www.elsevier.com/locate/aqua-online ⁎ Corresponding author. Tel.: +61 3 6324 3862; fax: +61 3 6324 3804. E-mail address: rflorent@utas.edu.au (R.L. Florent). 0044-8486/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2007.04.082