Effect of dietary inclusion of n-acetyl cysteine on mucus viscosity and susceptibility of rainbow trout, oncorhynchus mykiss, and atlantic salmon, salmo salar, to amoebic gill disease

The treatment of amoebic gill disease (AGD) in cultured Atlantic salmon, Salmo salar L., using mucolytic agents has been previously reported. The agent L-cysteine ethyl ester reduces salmonid mucus viscosity and potentially increases the flushing of the gill. In the present study, the effects of the mucolytic agent N-acetyl cysteine (NAC) were assessed. Cutaneous mucus from rainbow trout, Oncorhynchus mykiss Walbaum, and Atlantic salmon was shown to have reduced viscosity when mixed in vitro with 100 or 200 mu g/mL NAC. Saltwater-acclimated rainbow trout and Atlantic salmon were fed an oil-incorporated, NAC-medicated diet (8 g NAC/kg diet) for up to 24 d and challenged with inoculation of 300 cells/L Neoparamoeba spp., the etiological agent of AGD. Control fish were fed normal oil-coated pellets and received no NAC. NAC medication failed to reduce the severity of gill lesions associated with AGD even though the mucus viscosity from medicated fish was less than that of controls. Oral NAC medication does not appear to be an effective method for controlling AGD in salmonids despite reducing cutaneous mucus viscosity.