The Australian salmon industry is free from the major infectious diseases affecting salmon industries in other countries. Amoebic Gill Disease is the main infectious disease affecting the salmon industry in Tasmania. While this disease occurs in other countries, it does not appear to be as severe as in Australia. The disease-related fish mortalities are greatly reduced by freshwater bathing, however the disease treatment results in greater production costs and reduces the Australian salmon industry’s ability to compete in international markets. While development of a vaccine to protect against the causative agent is a desirable solution for the industry, it must be appreciated that this is a challenging task and requires a long-term view. In the meantime, the industry needs more knowledge of AGD epidemiology and more tools to improve management of fish with AGD on the farms, which would improve AGD treatment and control and decrease the risk of large outbreaks of AGD. As a direct result this should lead to reduced production costs. Improved understanding of the epidemiology of the disease will also provide a valuable basis on which to develop strategies for vaccine application in the future.
The ultimate goal is to have an AGD surveillance system in place, to provide information about the host, agent and environment which is relevant to prevention and management of the disease. The results will improve our understanding of factors contributing to AGD outbreaks and will develop a best industry practice protocol aimed at the reduction of AGD effects on the salmon industry in Tasmania. Information from the surveillance system will facilitate better decisions on the timing of treatments as well as provide long term data for analysis to identify additional management strategies aimed at minimisation of disease risks and economic impacts. Specifically, farm-level surveillance data will eventually provide a basis for measuring spatial and temporal trends in AGD occurrence in both the host and potential reservoirs as well as environmental and production factors associated with changes in AGD occurrence. Analysis of retrospective data will permit the identification of improved management strategies as well as providing farm managers with a more reliable basis on which to make decisions on prevention and control. This project will provide ways of value adding to the data collected by the salmon industry in general. It will protect individual company interests with respect to privacy of commercial information.
Final report
We have identified reservoirs of the amoeba causing AGD in Atlantic salmon in Tasmania. The amoebae were widespread and easily isolated from marine and estuarine sediments and cage netting. There was no apparent relationship between the presence of the amoebae in the environment and AGD prevalence in the cages. Development of methods to isolate amoebae from water samples and to accurately quantify the amoebae is needed if the relationship between the amoebae in the environment and AGD outbreaks is to be fully understood. However, computer simulations suggested that once fish in a cage are infected with AGD, they become the main source of amoebae and other reservoirs become insignificant.
This project provided specialised training in introductory and advanced epidemiology, sampling design, design of field trials and data analysis to all stakeholders, including researchers, government veterinarians and industry. Potential for AGD data sharing and analysis for the whole industry was discussed. A trial AGD database was set up, however industry showed no interest in using it. A computer model was developed based on data from one farm collected during 2003. While the simulations provided interesting insights and identified knowledge gaps, the model could not be validated using data from the same farm from 2002, proving that it has serious limitations. Increased understanding of the disease outbreaks dynamics is needed before a predictive model of AGD can be developed.
