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Bacterial Gill Disease: causes, symptoms and treatment

Bacterial Gill Disease (Flavobacterium branchiophilum and mixed flora) — etiology, symptoms, diagnosis, active-substance medication, recovery and prevention; mortality without treatment: high.

Overview

Bacterial colonization of gill epithelium causing hyperplasia, fusion of lamellae, and respiratory distress. Common in salmonid fingerlings under high organic load. Causative agent: Flavobacterium branchiophilum and mixed flora. Transmission: water. Incubation: 1-5 days. Reported mortality without treatment: high.

Symptoms

  • rapid breathing
  • gasping at surface
  • swollen pale gills
  • loss of appetite
  • lethargy
  • high mortality in fry

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 Flavobacterium branchiophilum and mixed flora.

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

  1. Chloramine-T or formalin. Chloramine-T 10 ppm bath 30-60 min daily x 3 days, OR formalin flush 25 ppm; address water quality. (duration: 3 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

  • reduce stocking density
  • increase water flow
  • lower organic load
  • frequent water changes for fry tanks