Detection and abundance of Paramoeba species in the environment
CSIRO Oceans and Atmosphere Hobart
Nick G. Elliott
Amoebic gill disease is the single biggest disease afflicting the salmon industry in Tasmania. Paramoeba sp. have been identified as the causal organism but relatively little is known of the biology and life cycle of Paramoeba prohibiting the development of cost-effective management practices to contain the effects of the disease. In 1990 AGD was estimated to cost the industry $500,000 per annum. In past years the incidence of AGD has been increasing and a major producer in the Huon Estuary has estimated that AGD now costs their company $300,000 per annum in direct and in-direct costs. Without strategies to reduce or contain AGD, the cost to the industry is likely to continue to increase. The infecting source of Paramoeba in the environment is currently not known but could be either seawater, sediments, or surfaces of various structures including nets, cage frames and pontoons. If the infecting source could be identified, appropriate management strategies could be devised to minimize the ability of Paramoeba to infect farmed fish by reducing the numbers of Paramoeba in the environment or by minimizing contact with Paramoeba. For example, if structures such as nets harbour Paramoeba, decontamination procedures could be devised. Alternatively, if the sediments and/or the water column are identified as infection sources then avoidance practices could implemented by appropriate placement of cages. Knowledge of the location of the infecting sources of Paramoeba would therefore reduce the detrimental effects of AGD on the farming of Atlantic Salmon and significantly reduce the current high cost in controlling the disease by freshwater bathing.
1. Obtain a library of Paramoeba isolates and a range of amoebae from infected Atlantic salmon and the environment, sequence regions of the small-subunit ribosomal DNA to identify DNA sequences specific to Paramoeba species
2. Using DNA sequences specific to Paramoeba species, develop and test a diagnostic assay procedure based on PCR technology
3. Using the diagnostic assay developed, identify the major sources and reservoirs of Paramoeba in the environment, in and adjacent to fish farms.
Amoebic gill disease (AGD) remains the major disease associated with sea-cage culture of Atlantic salmon in Tasmania. AGD is associated with a Paramoeba species infecting the gills. Current treatment involves multiple freshwater bathes for all fish. This treatment has a significant financial impact on the industry, a cost estimated to be equivalent to over 10% of the industry GVP. Prevalence of the disease and costs associated with the freshwater treatment are increasing.
While Paramoeba sp. had been identified as the causative agent for AGD, relatively little is known of the biology and life cycle of the organism; including, prior to this project, the species identity of the pathogen. Likewise the source of the infecting Paramoeba in the environment is unknown. A selective and sensitive method for detecting the particular Paramoeba species in the environment was required to assist with the management of AGD.
Key Words: Neoparamoeba pemaquidensis, Atlantic salmon, Salmo salar, aquaculture, species identification, AGD, PCR amplification, DNA extraction