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The objective of the present study was to evaluate the in vitro toxicity of bithionol and bithionol sulphoxide to Neoparamoeba spp., the causative agent of amoebic gill disease (AGD). The current treatment for AGD-affected Atlantic salmon involves bathing sea-caged fish in freshwater for a minimum of 3 h, a labour-intensive and costly exercise. Previous attempts to identify alternative treatments have suggested bithionol as an alternate therapeutic, but extensive in vitro efficacy testing has not yet been done. In vitro toxicity to Neoparamoeba spp. was examined using amoebae isolated from the gill of AGD-affected Atlantic salmon and exposing the parasites to freshwater, alumina (10 mg l -1 ), seawater, bithionol or bithionol sulphoxide at nominal concentrations of 0.1, 0.5, 1, 5 and 10 mg l -1 in seawater. The numbers of viable amoebae were counted using the trypan blue exclusion method at 0, 24, 48 and 72 h. Both bithionol and bithionol sulphoxide demonstrated in vitro toxicity to Neoparamoeba spp. at all concentrations examined (0.1 to 10 mg l -1 over 72 h), with a comparable toxicity to freshwater observed for both chemicals at concentrations > 5 mg l -1 following a 72 h treatment. Freshwater remained the most effective treatment, with only 6% viable amoebae seen after 24 h and no viable amoebae observed after 48 h. KEY WORDS: Amoebic gill disease · AGD · Bithionol · In vitro · Toxicity · Neoparamoeba spp. · Atlantic salmon · Chemotherapy · Protozoan parasite Resale or republication not permitted without written consent of the publisherDis Aquat Org 91: [257][258][259][260][261][262] 2010 ural rumen fluke infection in cattle and tapeworm infections in cats, dogs, sheep and chickens (Prasittirat et al. 1997).Both bithionol and bithionol sulphoxide have been examined as treatments for numerous fish parasites and showed mixed results. Santamarina et al. (1991) observed limited toxicity and complete in vitro efficacy against Gyrodactylus sp. in rainbow trout Oncorhynchus mykiss at 12.5 mg l -1 , with a minimum 20 mg l -1 reported as efficacious in vivo. Tojo et al. (1994b) stated that bithionol was efficacious in vivo against Ichthyobodo necator in rainbow trout at 25 mg l -1 for a 3 h freshwater bath on 2 consecutive days; however, higher concentrations exhibited some host toxicity. Moreover, Madsen et al. (2000) determined that bithionol at 0.1 mg l -1 was an effective treatment against trichodiniasis in European eels Anguilla anguilla, but found bithionol to have a relatively narrow therapeutic index. Bithionol was identified as being a potential candidate for the control of neoparmoebiasis with a demonstrated toxicity in vitro, although only 2 concentrations were tested . More recently, bithionol has displayed efficacy as a bath and oral treatment for Atlantic salmon experimentally challenged with Neoparamoeba spp., resulting in amoebic gill disease (AGD), at arbitary concentrations and dosages of 1 to 10 mg l -1 for bath treatments and 25 mg kg -1 feed (Florent et al. 2007a(Florent et al. ,...

This study examined the efficacy of bithionol as a prophylactic or therapeutic oral treatment for Atlantic salmon (AS), Salmo salar, affected by amoebic gill disease (AGD). Furthermore, it explored the interaction of bithionol oral therapy with the current standard treatment (a freshwater bath for at least 3 h). The efficacy of three medicated feeds was determined in the trial by feeding AGD-affected AS at 1% body weight (BW) day(-1) either oil coated commercial feed (control) or prophylactic and therapeutic bithionol at 25 mg kg(-1) feed. Feeding commenced 2 weeks prior to exposure to Neoparamoeba spp. at 300 cells L(-1) and continued for 49 days post-exposure (PE). Bithionol when fed as a 2-week prophylactic or therapeutic treatment at 25 mg kg(-1) feed delayed the onset of AGD pathology and reduced the percentage of gill filaments with lesions. Administration of a 3-h freshwater bath at 28 days PE significantly reduced amoeba numbers to a similar level across all treatments; in contrast, gross gill score and percent lesioned filaments were reduced to different extents, the control having a significantly higher score than both bithionol treatments. Following the freshwater bath, clinical signs of AGD increased at a similar level across all treatments, albeit controls were significantly higher than the bithionol treatments immediately following freshwater treatment. This study demonstrated that bithionol at 25 mg kg(-1) feed, when fed as a 2-week prophylactic or a therapeutic treatment, delayed and reduced the intensity of AGD pathology and warrants further investigation as a treatment for AGD-affected AS.