Amoebic gill disease of Atlantic salmon : resistance, serum antibody response and factors that may influence disease severity

Amoebic gill disease (AGD) is a condition of some marine-cultured fish worldwide and is the result of Neoparamoeba spp. infection. If AGD-affected fish are left untreated, major mortalities can occur. In Tasmania, Australia, fresh water bathing remains the only treatment for AGD, a practice that represents approximately 10-20% of production costs. Therefore, development of a preventative measure such as vaccination is a priority for Tasmanian salmon growers. In this project, resistance of Atlantic salmon to AGD and the development of a serum antibody response to Neoparamoeba spp. were assessed. Sera from AGD-affected Atlantic salmon were screened to identify potential candidate antigens for an AGD vaccine. Atlantic salmon exposed to Neoparamoeba spp. and subsequently challenged with AGD demonstrated resistance in terms of increased survival compared to AGD-narve fish. In addition, antibodies that bound cell-surface carbohydrate epitope(s) of Neoparamoeba spp. were detected in the sera of some fish after secondary exposure to Neoparamoeba spp .. In light of this, further screening of sera from Atlantic salmon exposed to Neoparamoeba spp. in the laboratory or during sea-cage culture was conducted. Antibodies present in the sera of some AGD-affected Atlantic salmon predominately bound carbohydrate residues expressed on the cell-surface of Neoparamoeba spp. Ideally an AGD vaccine would contain peptide antigen(s) that can easily be produced by recombinant DNA technology. Therefore, an alternative approach to identify candidate vaccine antigens for an AGD vaccine was needed. Some pathogenic amoebae colonise host tissues via lectin-mediated attachment and lectins have shown promise as candidate vaccine antigens. The in vivo effect of mucus and saccharides on the ability of Neoparamoeba spp. to cause AGD was investigated. The number of AGD lesions was significantly reduced when amoebae were incubated in mucus or any of the range of saccharides assessed. These data suggest that colonisation of Neoparamoeba spp. on gill tissues of Atlantic salmon may be lectin-mediated and the ensuing infection can enhance resistance to Neoparamoeba spp. However, very few AGD-affected Atlantic salmon develop a serum antibody response to Neoparamoeba spp. suggesting that the development of antibody-mediated protection of Atlantic salmon during Neoparamoeba spp. infection is unlikely.