This project seeks to investigate the use of a probiotic dip treatment called Stembiont for the prevention, or the reduction, in the severity of Amoebic Gill Disease (AGD) in salmon. Stembiont consists of 3 natural strains of live bacteria, isolated from salmon and its marine saltwater environment. Stembiont is a water-based product that is safe for both humans and animals. AGD is a gill disease of Atlantic salmon in salt water and is caused by the single-celled amoeba, Paramoeba perurans. First described in Tasmania in 1985, AGD is now present in most Atlantic salmon-producing countries. AGD is now considered to be one of the major health challenges on the slamon farming industry, having the most impact economically. Poor gill health in salmon can lead to direct mortalities, increased susceptibility to other diseases, and reduced performance and welfare. There are no vaccines or other preventative veterinary medicines currently available for AGD. Existing treatment options include bathing the fish in freshwater for 4 hours, however re-infection can occur within 10 days. Multiple freshwater bathing treatments are normally required during the saltwater stage of the salmon production cycle, which can last up to 2 years. Freshwater bathing, although effective at removing the amoeba from the gills, can result in increased levels of fish mortality due to the stress associated with exposure to freshwater when the salmon have reached a saltwater lifestage. Also, in salmon polyculture systems, the co-habiting cleaner fish species (which have a much lower tolerance to freshwater than salmon) are also subjected to the treatment, and are at an even higher risk of stress and increased mortality. There is evidence that the use of Stembiont can improve the bacterial community on and in farmed fish, thereby preventing disease-causing bacteria or microorganisms from causing infection in the fish. The objective of this study is to determine if Stembiont is effective in treating or preventing AGD infection.

Fish will undergo a single procedure comprising two interventions. Fish will be exposed to the AGD parasite so that they develop AGD infection. Half of these fish will be treated with probiotic and half will not (control group). Fish will then be randomly selected to be anaesthetised at several timepoints in order to facilitate assessment of the health of their gills, to determine if the probiotic treatment has been effective at reducing infection levels. The maximum duration of the procedure for any fish is 5 weeks, however due to the random sampling process, it is likely that the majority of fish will be on study for a shorter period of time.

Currently no alternatives to the use of live fish are available to investigate the effect of the probiotic treatment of salmon in preventing or treating AGD infection. In order to prove that this probiotic treatment can prevent, treat, or reduce the severity of AGD infection, there is no alternative but to demonstrate this in infected fish.