AquairiLearn

Columnaris (Flavobacterium columnare) Treatment Guide

Columnaris is caused by the Gram-negative filamentous bacterium Flavobacterium columnare, often misidentified as fungus. Mortality can be high in acute outbreaks.

Overview

Columnaris is caused by Flavobacterium columnare, a Gram-negative filamentous rod bacterium named after its tendency to form rhizoid columnar colonies. It is one of the most economically important warm-water fish diseases worldwide and is the second leading cause of mortality in US pond-raised catfish. Acute outbreaks can kill fish within 24-72 hours of first visible signs.

Symptoms

  • Cottony white-grey patches on body, fins or mouth
  • Saddle-shaped lesion behind the dorsal fin
  • Fraying mouth and fin edges (often called 'mouth fungus')
  • Rapid breathing and gill discoloration
  • Lethargy and refusal to eat
  • Rapid death in acute hyper-virulent strains

Causes

F. columnare is opportunistic and present at low levels in many systems. Outbreaks are triggered by warm temperatures (above 25 C), low dissolved oxygen, organically rich water, overstocking, abrasions, transport stress, and the introduction of asymptomatic carriers.

Diagnosis

Diagnosis is presumptive in the field: cottony patches under good lighting, often with a yellowish tinge at the edges (yellow pigment is characteristic of F. columnare colonies). Definitive diagnosis requires microscopic examination of skin scrapings or culture on neomycin-polymyxin selective media. Differentiate from Saprolegnia, whose tufts are denser and grow on existing wounds rather than spreading rapidly across healthy skin.

Treatment

Step 1: Quarantine

Move affected fish into a quarantine system with strong aeration. Lower the temperature gradually to 24 C (75 F) — F. columnare slows considerably below this threshold. Remove activated carbon and any UV sterilizers during medication.

Step 2: Medication

  1. Aquarium salt (sodium chloride) at 1 teaspoon per 4 L slows the bacterium and supports gill function.
  2. Nitrofuran antibiotic (nitrofurazone / furazolidone, e.g. Furan-2) per package for 7-10 days against external infection. Restricted in some jurisdictions.
  3. Aminoglycoside antibiotic (kanamycin sulfate, e.g. Kanaplex) per package for 7-10 days for systemic infection.
  4. Methylene blue at 3 ppm as a 30-minute bath for early external lesions only — not for systemic cases.
  5. Potassium permanganate baths under veterinary guidance for advanced outbreaks.

Step 3: Recovery

Continue antibiotics for the full course even if symptoms disappear after 2-3 days. Perform 25% water changes daily during treatment to keep dissolved oxygen high. Restore filter and carbon after the course is complete and gradually raise temperature only once the tank is clear of new lesions.

Prevention

  • Maintain strong surface agitation and dissolved oxygen above 6 mg/L
  • Avoid overstocking and overfeeding
  • Remove uneaten food and detritus promptly
  • Lower temperatures for sensitive or stressed species
  • Quarantine all new fish, plants and equipment