Demonstration and manipulation of acquired resistance to amoebic gill disease in Atlantic salmon, Salmo salar L.

Amoebic gill disease (AGD), caused by Paramoeba sp., is the most serious infectious disease infecting sea caged Atlantic salmon, Salmo salar L., in Tasmania, Australia. After AGD had been described from Tasmanian salmonids, it also emerged as a problem in other countries. Major outbreaks have occurred in France, Spain and Ireland and disease, although not to the same extent, has been reported in the US and New Zealand. In Tasmania at present, treatment consists of a series of freshwater baths given during the critical high temperature, high salinity periods of the year. These baths last between two to three hours and during a normal summer fish are usually treated two to three times. However, there have been summers when up to eight baths have been undertaken and it appears recently, that this scenario is becoming more common. Because of the time and monetary costs associated with AGD, the Tasmanian industry is constrained with regards to increasing production and thus it's competitiveness in international and domestic markets. Any strategy that would eliminate or even reduce the number of baths required would be of substantial commercial value. All attempts to find a practical chemotherapeutic agent have been fruitless, thus immunisation and/or immunostimulation appeared to offer the best hope of providing an alternative treatment strategy. This project reports for the first time, the presence of varying degrees of resistance to AGD in fish that have been previously infected and then treated using freshwater bathing and demonstrates that this resistance can be modulated via the use of the immunostimulant, levamisole. It is also the first report of the use and efficacy of levamisole as an immunostimulant of the nonspecific defence system of Atlantic salmon. In order to achieve the results obtained a number of novel and improved experimental techniques were developed and are reported here.