Aeromonas salmonicida is a Gram-negative bacterium recognized globally as a pathogen causing significant losses throughout the aquaculture industry. This organism is the causative agent of furunculosis, a serious disease affecting both wild and cultured fish populations in freshwater and marine environments. The impact of this pathogen is widespread, affecting fisheries across North America, Europe, Asia, and other regions where fish farming is prevalent. Due to its virulence and ability to persist in aquatic systems, A. salmonicida remains a major concern for aquatic animal health management worldwide.
Biological Profile of the Bacterium
The bacterium is classified as a Gram-negative, rod-shaped organism belonging to the genus Aeromonas. It is typically non-motile and is considered a facultative anaerobe, capable of surviving with or without oxygen. A. salmonicida is subdivided into five recognized subspecies; A. salmonicida subsp. salmonicida is the strain most commonly associated with classical furunculosis in cold-water fish.
This bacterium is generally psychrophilic, growing best at lower temperatures, with many strains unable to grow above 25°C. However, atypical strains can be mesophilic, tolerating warmer conditions and showing a broader host range. A distinctive feature of many strains is the production of a brown, diffusible pigment when cultured in a laboratory setting. The organism possesses a protein outer layer called the A-layer, which helps the pathogen evade the host’s immune response and establish infection.
Clinical Signs of Furunculosis
Furunculosis is categorized as peracute, acute, subacute, or chronic, each having distinct signs and mortality rates. The peracute form is most common in young fish, causing rapid death with few external signs other than skin darkening and lethargic behavior. This form involves swift, systemic septicemia.
The disease takes its name from the characteristic external signs of the subacute and chronic forms: the furuncles, or boil-like lesions. These furuncles are abscesses involving the skin and underlying muscle that contain necrotic tissue, blood, and bacteria. When these lesions rupture, they leave open ulcers that expose the fish to secondary infections.
Other external signs include hemorrhages, appearing as reddened spots, particularly at the base of the fins, on the mouth, or in the muscle tissue. Internally, the infection spreads throughout the bloodstream, leading to systemic pathology. Examination often reveals an enlarged spleen, soft and hemorrhagic kidneys, and petechial hemorrhages (pinpoint bleeding) on internal organs and the intestinal lining. Affected fish often show a loss of appetite and may swim erratically or float listlessly near the surface before succumbing.
Spread Pathways and Susceptible Species
The dissemination of A. salmonicida primarily occurs through horizontal transmission between fish populations. The bacterium is shed into the aquatic environment via feces, urine, and discharge from ruptured skin lesions of infected fish. Water is the primary vehicle for infection, which can enter a new host through the gills, damaged skin, or the digestive tract.
The pathogen can persist outside of a host, surviving for several days in seawater and for months in freshwater environments, contaminating equipment, nets, and holding tanks. While the egg surface can become contaminated, true vertical transmission is not conclusively demonstrated. Environmental stressors, such as high stocking densities, poor water quality, high temperatures, and excessive handling, often trigger clinical outbreaks in carrier fish.
While subsp. salmonicida is most virulent for salmonids like salmon, trout, and char, the host range is broad. Brown Trout and Brook Trout are especially susceptible to the classic form of the disease. Atypical strains of A. salmonicida can infect many non-salmonid species, including Common Carp, Goldfish, and Flounder, often causing ulcerative skin diseases like carp erythrodermatitis.
Control and Treatment Measures
Prevention relies heavily on stringent biosecurity protocols to keep the pathogen out of fish facilities. This includes isolating newly acquired or potentially exposed fish and disinfecting all equipment and gear that moves between holding units. Disinfection protocols often use a two-step process: mechanical removal of organic matter followed by treatment with an approved disinfectant.
Vaccination is the most effective preventative strategy available for susceptible species. Commercial vaccines are typically whole-cell bacterins administered to provide long-term protection against the typical strain. Hatchery practices also include surface-disinfecting fish eggs, often using povidone-iodine solutions, to prevent the transfer of surface contamination.
If an outbreak occurs, treatment requires the use of aquaculture-approved antibiotics, such as florfenicol or oxytetracycline, administered under veterinary guidance, often through medicated feed. It is necessary to test the isolated bacterial strain for antibiotic resistance before starting treatment. In severe, rapid outbreaks, depopulation of the infected unit may be necessary to contain the spread, followed by thorough cleaning and disinfection.