Columnaris is a highly contagious bacterial disease of freshwater fish caused by Flavobacterium columnare. It is often called cotton wool disease, cotton mouth, or saddleback disease because of the white, fuzzy, cotton-like patches it produces on the head, mouth, fins, and body. Columnaris is frequently mistaken for a fungal infection because of its appearance, but it is bacterial and requires antibiotic treatment. Warm-water strains can kill entire tanks in 24 to 72 hours, making columnaris one of the most dangerous diseases a fishkeeper can encounter. Early recognition and aggressive treatment are essential.

Overview

Flavobacterium columnare is a gram-negative, rod-shaped bacterium present at low levels in most aquarium systems. It becomes pathogenic when fish are stressed or injured and water conditions favor bacterial growth. Key factors include:

  • Water temperature above 75°F (24°C) — disease progresses more rapidly at higher temperatures
  • Hard, alkaline water (high pH) — Flavobacterium thrives in these conditions
  • High organic load, ammonia, or nitrite
  • Physical injuries, fin damage, or handling stress
  • Overcrowding and poor oxygenation
  • Transport stress or introduction of new fish

There are strains of varying virulence — highly virulent strains can kill within a day, while less virulent strains produce chronic, slowly progressing lesions.

Symptoms

  • White or grayish-white fuzzy patches on the skin, often on the head, mouth, back, or fins
  • Saddleback lesion — a classic pale patch extending over the dorsal fin and back
  • Mouth fungus — white cottony growth around the mouth that erodes jaw tissue
  • Frayed or rotting fins, often with a pale margin
  • Ulcerations and tissue erosion in advanced cases
  • Rapid breathing (if gills are affected) and gasping at the surface
  • Loss of appetite and lethargy
  • Sudden death, sometimes with minimal external signs in peracute cases

Columnaris on the gills is especially lethal because it can cause suffocation before external lesions appear.

Diagnosis

Columnaris is usually diagnosed presumptively based on the appearance of the lesions and the speed of disease progression. A definitive diagnosis requires microscopic examination of a wet mount or gill scrape showing characteristic long, thin, gliding rod-shaped bacteria that form columns (hence the name). True fungal infections show branching fungal hyphae instead.

Distinguishing columnaris from true fungal infection (Saprolegnia) is important because treatment differs — fungus responds to antifungals, while columnaris requires antibiotics.

Treatment

  • Lower the temperature — If species tolerate it, dropping the temperature to 75°F (24°C) or below slows bacterial growth and buys time for treatment. This is one of the few fish diseases where cooler water helps.
  • Kanaplex (kanamycin) — Broad-spectrum and effective against many columnaris strains. Can be dosed in water or bound to food.
  • Furan-2 — Combination nitrofuran antibacterial useful for early to moderate columnaris.
  • API Fin and Body Cure (doxycycline) — Another option, particularly when body involvement is present.
  • API Erythromycin — Sometimes effective but use cautiously as it can disrupt beneficial filter bacteria.
  • Aquarium salt — At 1 teaspoon per gallon may provide mild osmotic support in salt-tolerant species.
  • Medicated food — For systemic infection, antibiotic in food is more effective than water dosing if fish are still eating.

Remove activated carbon during treatment and maintain excellent oxygenation. Treatment should continue for the full recommended course, typically 7 to 10 days.

Prevention

  • Maintain stable, cool-to-moderate water temperatures for susceptible species
  • Keep dissolved oxygen high with surface agitation
  • Avoid overcrowding and aggressive stocking
  • Quarantine new arrivals for at least 4 weeks
  • Minimize handling stress
  • Test water regularly and perform consistent water changes
  • Isolate any fish with signs of stress or injury

When to Consult a Veterinarian

Because columnaris can kill so rapidly, veterinary guidance should be sought quickly when multiple fish are affected or when high-value fish are involved. An aquatic veterinarian may prescribe more potent antibiotics or guide culture-based selection when over-the-counter options fail.