Clinical assessment of chloramine-T and freshwater as treatments for the control of gill amoebae in Atlantic salmon, Salmo salar L.

Abstract

Infections of gill amoebae that manifest as amoebic gill disease (AGD) occur in Atlantic salmon in Tasmania. The treatment of choice is freshwater bathing; however, the effectiveness of this treatment has declined over time. In this experiment, cage trials of chloramine-T (Cl-T) to treat AGD in Atlantic salmon were conducted over 3 months, and involved an initial bath in either freshwater or seawater with Cl-T, followed by a second bath 6 weeks later. Amoeba densities were reduced to 50-80% of original values for both treatments. Neoparamoeba sp. density was not affected by bathing, and was not significantly different over the course of the experiment. Lesion prevalence was higher for Cl-T-treated fish than for freshwater-treated fish, with overall prevalence levels of 14.30 ± 1.00% and 8.03 ± 0.57% respectively. This was also seen for gross gill scores. In the fortnight after each of the two baths, Cl-T-treated fish had significantly higher lesion levels, although this difference was then resolved by 4 weeks post bathing. The use of Cl-T in seawater is at least as effective as freshwater at reducing amoebae density, and may be a more practical alternative when freshwater is in short supply. © 2005 Blackwell Publishing Ltd.