Furunculosis in rainbow trout (Salmo gairdneri) raised in sea water

Ulcerative skin lesions were encountered in rainbow trout raised in sea water by a commercial concern in the Western Cape, South Africa. Grossly, the lesions resembled furunculosis but, histopathologically, they differed from typical furunculosis in that bacterial colonies were rarely found in the organs, and also the kidneys and spleens were minimally involved. The causative organism was identified as an achromogenic Aeromonas salmonicida that shared characteristics with all 3 subspecies, salmonicida, masoucida and achromogenes. This is the first report of an outbreak of this disease in South Africa.