Aeromonas salmonicida is a gram-negative pathogenic bacterium that impacts both wild and cultivated fish. The bacterium is found worldwide with the exception of South America. A. salmonicida has a wide range of fish hosts including Trout, Pike, Perch and Halibut, and is especially critical to farmed Salmon. It is the causative agent of the disease furunculosis. This disease can have devastating results in aquaculture, the high density of fish in fish farms means the disease can spread rapidly. The main route of transmission appears to be via contact with infected fish or contaminated water. Outbreaks tend to appear in spring-autumn period and mortality can be triggered by stress factors such as crowding, poor water quality and high temperatures. The disease generally appears to develop as a septicaemia and is often fatal. Affected fish often show darkening of skin and lethargy. Haemorrhages may occur at the bases of fins and internally at the abdominal walls, heart and liver. Chronic infections can lead to internal damage such as inflammation of the lower intestine and spleen. In acute outbreaks fish may dierapidlywithfewsigns,juvenile,spawningandimmunocompromisedfishareatthehighest risk.