Understanding amoebic gill disease

Amoebic gill disease (AGD) is a severely debilitating disease, which mainly affects the salmonid industry. It causes high production losses worldwide, including Tasmania, where it is the main health problem in farmed Atlantic salmon. Without the use of control procedures such as fresh water bathing and/or by maintaining fish population densities at a level commensurate with proper hygiene, this disease can often lead to the death of over 50% of infected salmon. AGD is caused by the parasitic amoeba, Neoparamoeba perurans, which binds to and inhabits the gill epithelium of growing fish. Pathology is associated with severe epithelial hyperplasia, fusion of gill lamellae, lowering of oxygen tension and pH of the blood and eventual death of the fish. In order to understand this disease process, research has been carried out, to study the immune response of fish to primary and secondary infections, the genetic basis of resistance to infection as well as how the parasite binds to the host gill tissue and induces a severe host response. Results have shown that N. perurans contains within its surface glycocalyx, highmolecular- weight mucin -like glycoproteins. These surface mucins are immunodominant, according to studies showing that they are the major antigens recognized by antibody in serum from fish exposed to the parasite, as well as when immunized with either whole parasites or subcellular fractions. Thus far, attempts to vaccinate salmon using the purified, high-molecular-weight surface antigen (HMWA) have failed to induce significant protective immunity. However, these studies have greatly aided our understanding of the molecular basis of the amoeba binding and infection process.