Relationship between resistance of salmonids to furunculosis and recovery of Aeromonas salmonicida from external mucus

Fish were sampled at the Ed Weed State Fish Hatchery (South Hero, Vermont, USA) in September 1992. Aeromonas salmonicida was common, with concentrations as high as 10⁵ to 10⁷ colony-forming units per gram of mucus, and readily recovered from most mucus samples obtained from furunculosis-sensitive populations of brook trout (Salvelinus fontinalis), lake trout (Salvelinus namaycush), and Atlantic salmon (Salmo salar). The pathogen was the predominant microorganism and accounted for greater than 85% of the total number of bacteria isolated from the mucus of these fish. By comparison, A. salmonicida was recovered only from two rainbow trout (Oncorhynchus mykiss), and bacterial frequencies did not exceed 10³ colony-forming units per gram of mucus. The pathogen was not recovered from the mucus of steelhead (O. mykiss) or Rome brown trout (Salmo trutta) selectively bred for resistance to furunculosis, even though there was widespread contagion throughout the hatchery and fish were cultured on a common, unprotected water supply.