Furunculosis: causes, symptoms and treatment

Furunculosis (Aeromonas salmonicida) — etiology, symptoms, diagnosis, active-substance medication, recovery and prevention; mortality without treatment: very high.

Overview

Classic salmonid disease causing characteristic furuncle-like skin lesions and systemic septicemia. Also affects goldfish, koi, and other coldwater species. Causative agent: Aeromonas salmonicida. Transmission: water. Incubation: 2-14 days. Reported mortality without treatment: very high.

Symptoms

raised boil-like skin lesions that ulcerate
darkening of color
lethargy
loss of appetite
hemorrhages at fin bases
rapid mortality

Causes

Outbreaks are typically triggered by chronic stress, poor water quality, temperature swings, overcrowding, or the introduction of unquarantined fish. The pathogen spreads via free-swimming or waterborne stages in shared water.

Diagnosis

Diagnosis is based on clinical signs (lesions, hemorrhages, behaviour) combined with bacterial culture and Gram-staining where available. Differentiate from co-infections with other Gram-negative pathogens; antibiotic sensitivity testing improves treatment success against Aeromonas salmonicida.

Treatment

Effective treatment requires isolating affected fish in a quarantine tank, identifying the pathogen, administering the appropriate active substance at the correct dose and duration, and supporting recovery with stable water parameters and nutrition.

Step 1: Quarantine

Set up a bare-bottom quarantine tank with a mature sponge filter, heater, and aeration. Match temperature and pH to the display tank, and acclimate fish slowly. A bare bottom simplifies daily siphoning and prevents medication from being absorbed by substrate.

Step 2: Medication

Oxytetracycline injection or feed. Oxytetracycline 75 mg/kg fish daily x 10 days in feed; injectable for individual valuable fish. (duration: 10 days)
Florfenicol or enrofloxacin. Veterinary florfenicol 10 mg/kg in feed for 10 days; broader and effective against resistant strains. (duration: 10 days)

Step 3: Recovery

After medication, perform a 30-50% water change and run fresh activated carbon for 24-48 hours to remove residues. Continue feeding a high-quality, varied diet with vitamins and immunostimulants. Reintroduce fish to the display tank only after at least one week without recurrence of symptoms.

Prevention

vaccinate koi and salmonid stocks
avoid temperature stress
quarantine new fish
low stocking density