Furunculosis, a major cause of mortality in salmonid fishes, is caused by the Gram-negative bacterium Aeromonas safmonicida. The disease is systemic and may be characterized in several fonns: an asymptomatic form; an acute form resulting in severe internal organ involvement, with high numbers of mortalities; and a chronic form that may involve internal pathology, but with lesser numbers of mortalities over longer periods of time. Furunculosis is treated with antimicrobial therapy; three such agents are approved by the Food and Drug Administration (FDA) for use in salmonids. Because the disease is systemic, the agent is administered as a food additive.