Immune-regulatory genes in amoebic gill disease : potential for immunomodulation

Amoebic Gill Disease (AGD) is the most significant health problem affecting Atlantic salmon, Salmo salar, culture in Tasmania, Australia. Caused by the ectoparasite Neoparamoeba sp., AGD is an emerging disease in several other countries (USA, Ireland, France, Spain, Chile, New Zealand) and affects several other cultured fish species (rainbow trout, Oncorhynchus mykiss, coho salmon, Oncorhynchus kisutch, turbot, Scophthalmus maximus). AGD and the cost of treatment, freshwater bathing, have negatively impacted on the production of Atlantic salmon in Tasmania. Consequently, alternative AGD management strategies are required. Thus an understanding of the disease processes associated with AOD pathology and the immune response to AGD is essential to the development of alternative AGD management strategies. Therefore, the immune response to AGD was partially characterised using molecular techniques, and the potential use of immunostimulants to reduce the impact of AGD on the Tasmanian salmonid industry was assessed. The transcriptional profile of selected immune-regulatory genes in the gill, liver and anterior kidney of experimentally infected Atlantic salmon and rainbow trout was assessed using semi-quantitative RT-PCR and quantitative real-time RT-PCR. Atlantic salmon interleukin-10 (IL-1)3) was shown to be up-regulated in the gills at 14 d post-inoculation (p.i.). In rainbow trout the expression of IL-1(3 and inducible nitric oxide synthase (iNOS) mRNA was significantly up-regulated in the gills at both 7 and 14 d p.i., while interleukin-8 was significantly up-regulated in the liverof AGD-affected trout at 7 d p.i. mRNA transcripts were localised by in situ hybridisation to pavement epithelial cells lining the primary and secondary lamellae of AGD-affected and control Atlantic salmon gills. These data demonstrated the involvement of the immune response to AGD at the molecular level and indicated the significance of the response at the site of infection. Intraperitoneal administration of immunostimulatory CpG oligodeoxynucleotides increased the resistance of Atlantic salmon to AGD. However, oral administration of 0-glucans, a well-known and potentially more cost-effective immunostimulant, was unable to enhance resistance to AGD. Nonetheless, a group of Atlantic salmon were able to survive an experimental AGD challenge. These findings allowed a preliminary AGD infection model to be proposed, and further our knowledge of the fish immune response to parasitic infection. The implications of this research show the potential for an effective immunological-based AGD management strategy such as use of a vaccine, immunostimulants or a selective breeding program.