DETECTION OF AEROMONAS-SALMONICIDA IN ATLANTIC SALMON WITH ASYMPTOMATIC FURUNCULOSIS INFECTIONS

Standard bacteriology and enzyme-linked immunosorbent assay (ELISA) were employed to detect Aeromonas salmonicida in samples of kidney, intestine and mucus from pre-smolt Atlantic salmon Salmo salar with stress-inducible furunculosis infections. No A. salmonicida was isolated by bacteriological culture of samples from 30 unstressed salmon. In the same samples, ELISA detected antigen in 15 intestinal samples, but none in any of the kidney or mucus samples. The frequency of stress-inducible infections in a paired group of Atlantic salmon was 7/29 when the kidneys of the stressed fish were examined bacteriologically and 13/29 when ELISA results from kidney, intestinal content and mucus were combined. If ELISA results from kidneys alone were considered the frequency of detection was 8/20. In 2 separate experiments brown trout enrichment was used to detect A. salmonicida in pre-smolt Atlantic salmon with stress-inducible infections and compared to the standard stress test. The pathogen was detected in the combined mucus, fins and gills of 7/20 and 14/20 fish and in the blood of 3/20 and 2/20 fish respectively. The frequency of stress-inducible infections in paired groups of fish from the same populations was 10/20 and 16/20 respectively. These data suggest that the intestine may be the primary location of A. salmonicida in salmon with stress-inducible infections and that colonisation of the mucus, fins and gills may also occur. The implications of these findings for the design of methods to eliminate such infections are discussed.