Furunculosis

Furunculosis is a serious bacterial disease of salmonids caused by Aeromonas salmonicida subsp. salmonicida, a non-motile, Gram-negative rod that produces a brown diffusible pigment and is among the most feared pathogens of trout and salmon culture. It is expressed in acute form as darkened fish with bloody discharge from the vent, exophthalmia, and furuncles (raised boils) filled with blood and bacteria under the skin, plus hemorrhaging in the pyloric caeca and internal organs; chronic carriers show no signs but shed bacteria from kidney lesions. Outbreaks occur across a wide temperature band from about 8 to 24°C, with mortality sharpest in warm, fast-growing stock. For farmers, furunculosis is historically controlled by vaccination, which is now routine in Atlantic salmon smolt production and gives strong protection through the seawater phase. Unvaccinated rainbow trout farms face recurrent risk, and infected farms may face movement restrictions. Diagnosis is by bacteriology or validated PCR; antibiotic treatment is only effective very early and must respect withdrawal periods. Biosecurity, disinfection of eggs, and fallowing between generations remain the backbone of eradication programs.