Furunculosis

Lake trout with external furuncle boils caused by furunculosis. (American Fisheries Society)

Furunculosis is a serious bacterial disease found principally in hatchery-raised salmonid species. It may also occur in warmwater fish, including carp and pike.

The disease was first reported in 1890 and was named furunculosis because of the formation of furuncles, or boil-like lesions, in various tissues of the body. Severe outbreaks occasionally occur, but due to advances in fish culture, sanitation, and drug therapy, most episodes can be controlled. Outbreaks of drug-resistant strains of furunculosis are an increasing problem. Therefore, fish culturists need to be aware of the dangers of transporting fish from infected areas to areas free of the disease.

Scanning electron micrograph of Aeromonas spp. (Northwest Fisheries Science Center)

The bacterium that causes furunculosis is Aeromonas salmonicida.

Furunculosis is widespread in natural waters. The seasonal pattern of infection is dependant on water temperature, with the greatest number of outbreaks occurring in July. After this time, the numbers drop sharply. Young fish are more susceptible to furunculosis than are older fish.

Transmission of the pathogen generally occurs as a result of contact with diseased or carrier fish, but can also occur via water passed from one pond or raceway to another. Fish-eating birds may also aid in the transfer of the pathogen either by contact or by dropping infected fish into an uninfected pond. Aeromonas salmonicida may also occur on the surface of non-disinfected eggs from carrier broodstock.

Aeromonas salmonicida can enter susceptible fish through a wound or the digestive tract. From there the organism can spread, via the bloodstream, to other organs, such as the heart, spleen, kidney, gills, lower intestine, skin and muscle. The immune response of infected fish is depressed by toxins released from the bacteria. A generalized infection then occurs and infected fish typically succumb to the disease