Tissue localization of Aeromonas salmonicida in Atlantic salmon, Salmo salar L., following experimental challenge

Three groups of Atlantic salmon, Salmo salar L., were exposed to live, colo ny-forming, radiolabelled Aeromonas salmonicida bacteria in a bath challeng e: (1) fish with artificial wounds; (2) fish with a reduced epidermal mucus layer caused by removal of the mucus layer on two occasions by a swabbing procedure; and (3) a control group of untreated fish. Fish were killed 2, 6 and 24 h after challenge, and radioactivity (cpm g(-1)) was measured in th e blood, mucus, skin, wound area, gills, anterior kidney, posterior kidney, spleen, midgut and hindgut. The highest levels of radioactivity were measu red in the wound areas and in the gills. There was a significant positive c orrelation between the levels of radioactivity in the gills and blood, and between the mucus and skin at 2 h post-challenge. Two hours after the bath challenge, live A. salmonicida bacteria were found in the blood of fish in the 'swabbed' and 'artificial wound' groups, and not in the control group. Twenty-four hours after the bath challenge, the kidney of fish from all gro ups contained viable bacteria, whereas the blood was negative.