Epidemiology of amoebic gill disease

Amoebic gill disease (AGD) is the main disease affecting the salmon industry in Australia, however inadequate information is available on the epidemiology of amoebic gill disease (AGD) and the biology of the pathogen, Neoparamoeba pemaquidensis (Page, 1987). Thus far no convenient mass screening test was available. In this project a pathogen specific and non-lethal dot blot test was developed and validated against indirect fluorescence antibody testing (IFAT), the 'gold standard'. The agreement between the 300 paired gill mucus samples that were analysed using both tests was high, with a corrected kappa value of 0.88. The overall aim of this project was to investigate distributions and seasonal patterns of the pathogen, identify risk factors for the disease and reservoirs of N. pemaquidensis, and develop and review husbandry methods in order to reduce AGD prevalence. Results of an infection trial implied that transmission of AGD infections in the field do not only occur from fish to fish, but also from water to fish. Therefore distribution of paramoebae in the water column and seasonal patterns were investigated. The spatial and temporal distribution of paramoebae was determined using the dot blot test and most probable number techniques for the identification and quantification respectively. Associations between paramoebae densities and environmental conditions were also explored. Potential reservoirs were investigated in both field and laboratory trials. In a laboratory study it was determined that dead AGD infected fish may be a reservoir of N. pemaquidensis when left in sea cages. In the laboratory trial, N. pemaquidensis remained on infected gills for at least 30 hours after death of the host, and these protozoa from dead infected fish could colonise gills of previously uninfected dead fish. This would potentially increase the bio-burden of N. pemaquidensis on infected farms. AGD was not detected in wild fish and wild fish did not seem to be a reservoir of the pathogen. Five different husbandry options were evaluated in extensive field trials with the aim to minimise the impact of AGD. Three of the husbandry options seemed beneficial in reducing either cost due to the disease and/or AGD prevalence on Tasmanian salmon farms. All three options could easily be incorporated into existing management plans.