Geographic characterization and environmental detection of Neoparamoeba perurans the causative agent of amoebic gill disease

Amoebic gill disease (AGD) is a global disease that has major effects on Atlantic salmon aquaculture. AGD is caused by the facultative marine amoeba, Neoparamoeba perurans. The disease first appeared in farmed salmon in Washington and in Tasmania in the mid-1980s. Since then, AGD has become a cosmopolitan problem with an increasing mortality and economic impact. Outbreaks of AGD have been reported in thirteen countries across six continents. Even where gross pathology is not evident, there is PCR evidence that N. perurans is present on salmon gills in other geographic regions, thus creating the potential for further outbreaks. This research aimed at understanding the relationship between geographically diverse amoeba populations isolated from the gill of infected salmon, the parasite and the environment. This improved understanding of these relationships could help inform industry decisions associated with management of AGD associated risks. With respect to understanding the geographical relationships, PCR-based typing methods were used to compare samples obtained from Australia, Canada, Ireland, Scotland, Norway, and the United States of America (Chapter 2 and 3). Sequences of highly conserved genes were compared using Multilocus sequence typing (MLST) (Chapter 2), to create a dendrogram showing the relationships between samples. The analysis resolved low-level differences between samples. Building upon these analysis (Chapter 2), the Random Amplified Polymorphic DNA (RAPD) technique was used (Chapter 3) to help elucidate the extent of the genetic differences that were observed (Chapter 2). RAPD allowed for a better understanding of the total genetic differences contained within the isolate genomes. The analysis showed high polymorphisms between samples. Though N. perurans is a ubiquitous organism, the apparent population differences (Chapter 2 and 3) may indicate the existence of localized populations not associated with fish. Two methods were utilized to investigate the presence of N. perurans within benthic sediments (Chapter 4). These methods were further applied at two locations (Canada and Tasmania) containing commercial farms. N. perurans was present in the sediment at both locations indicating the potential of sediment as a reservoir. Finally, Chapter 5 concludes the findings of this research and relates them to the current knowledge of the N. perurans, its role in the environment and potential for risk management.