A bacterial disease of salmonids that is usually characterized by boils or furuncles on the skin of affected fish although this is not always the case. The causative agent is the bacterium Aeromonas salmonicida. Furunculosis is systemic in nature as it travels through the bloodstream and affects all parts of the body, especially the vital organs. The rapid multiplication of bacteria in the bloodstream causes smaller blood vessels to rupture allowing the bacteria to spread to surrounding tissue. When allowed to advance to this stage, the disease is always fatal.

Furunculosis is endemic in a number of Michigan waters but is not generally a problem in nature. In crowded hatchery settings, the disease can cause high mortalities if not recognized and treated at its onset.

Medicated feed containing the antibiotic Terramycin or Romet are often effective in treating furunculosis.