Columnaris Fish Disease Flexibacter Columnaris
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Columnaris Fish Disease Cotton Mouth Disease


Photo courtesy of Blue Ridge Koi. Fin rot and sunken eyes on an infected fish.

Columnaris is NOT some exotic new koi disease. However, it can show up occasionally and when it does it can cause a lot of damage to your fish in a short amount of time. Unfortunately it is quite often misdiagnosed.

Flexibacter columnaris is a freshwater bacterium that belongs to a group of bacteria called cytophaga-like bacteria. Often shortened to CLBs, this group includes other pathogens, such as cytophaga and flavobacterium. Flexibacter columnaris is relatively long and slender; it is motile and tends to form large aggregates. Out breaks tend to be temperature related and usually occur at more than 60⁰F. Fishes that appear healthy can carry this disease, and outbreaks will occur when conditions suit the infection. Ideal conditions include high temperatures and large amounts of organic waste. Flexibacter have been shown to grow on uneaten food.

Often the first sign of an infection is a largish white spot somewhere on the body. Soon it will develop a reddish outer tinge as the underlying skin becomes damaged and inflamed. This white spot is actually a mucoid exudate that contains tens of thousands of bacteria all massed together. Erosions appear, often with a reddened rim. Fins are frequently targeted, with the infection eroding the fins from the outside in toward the body. The gills are also affected, with extensive damage caused by the swarms of flexibacter. Other CLBs can secondarily invade at this point. The swimming pattern of the fish often changes at this point; surface and mid-water swimmers begin a wobbling motion with exaggerated side-to-side movements of the tail. Eventually, the bacteria invade the bloodstream and there is a fatal spread to the internal organs.

Predisposing factors include water temperatures about 58 F and high levels of organic matter. Flexibacter do not appear to enjoy waters with a low pH and water hardness, although tolerances appear to vary among strains of bacteria.

What to Look for

All species are potentially susceptible. The disease is well recognized in Japanese weather fish (Misgurnus augillicaudatus), goldfish, carp, tilapia, and Poecilia live bearers, especially guppies and mollies. Large whitish patches on the body, gill covers, or fins may have a reddened rim around them. Discoloration or loss of body color is due to overgrowth of the infection. Respiratory distress, apparent by the rapid pumping of the gill covers and hanging at the surface, is a good indicator, too. Loaches may switch to alternative methods of breathing by swallowing air from the surface to be absorbed across the gut wall. Degenerate, ragged fins may be present, also known as classic “fin rot”. Finally, there may be some blood streaking in the fins.

Mouth fungus or Mouth Rot

The bacteria, along with mucus and dead skin, will often hang from the mouth area. These strands may be seen moving (like hair) as the fish breathes. In guppies there is typically a simultaneous fin and tail “rot” along with white skin patches. Small, brightly colored tetras such as neons, cardinals, and glowlights may lose their color and develop whitish-gray patches. If you have access to a microscope, placing some of the whitish exudates onto a slide will reveal huge swarms of flexibacter. In some cases they can form a haystack-like appearance.

Mouth fungus normally occurs at temperatures above 68⁰F (20⁰C) when one or more of the following contributory factors is applicable:

  • Injury to the skin, example, through netting, handling, or aggression. The bacteria infect the open wound and spread to the surrounding healthy tissue.
  • Vitamin deficiency, which can cause poor skin condition, which allows the infection to gain a foothold and/or spread.
  • Poor water quality, high levels of ammonia, nitrite, and/or nitrate; incorrect pH; low oxygen levels.

Flexibacter can be mistaken for many other bacterial infections. The whitish patches may be mimicked by a localized patch of excess mucus that is sometimes seen with some external parasitic problems. Fungal infections are another common misidentification. In small tetras, the loss of color and pale patches can lead to confusion with neon tetra disease or NTD (Plistophora).


Microscopically, these bacteria appear as long thin rods, usually clumped together to form dome shaped masses with a "hay stack" appearance.

Small off-white or grey lesions or marks concentrated on the head, fins, gills, and mouth cavity. These areas develop into off-white cottonwool-like growths, mainly around the mouth region. The more greyish color of these tufts commonly distinguishes this condition from Fungal infections, (Saprolegnia, Achyla) which are normally whiter, although the two diseases are nevertheless commonly confused. High-power microscopical investigation is required for a positive diagnosis (by the presence of rod-shaped bacteria, often seen moving in a gliding fashion, and sometimes forming aggregations; an absence of fungal hyphae excludes Saprolegnia/Achyla as the cause).

In the chronic slow-acting form of mouth fungus, with the classic symptoms of external fungus-like lesions described above, death occurs only after significant skin damage. There is, however, an acute fast acting form, which generally occurs at higher temperatures. This acute systemic infection, which incubates for less than 24 hours and kills fish in 2 or 3 days with few or no external symptoms, may well be the cause of some unexplained fish death.

Left untreated it can kill 50% of a population in 2 or 3 days.


Because of the difficulty in differentiating columnaris and fungal infections without microscopical investigation, the medication should be both bactericidal and fungicidal, to cover both eventualities. Melafix, formalin and malachite, praziquantel, and salt won't work. So look for medications with antibiotics in them. Nitofurazone, Furazolidone, Tetracycline and Oxolinic Acid have been shown to be effective. Bath immersion using a proprietary treatment containing Phenoxyethanol is normally effective. In advanced cases, especially where the disease invades internal tissues, it may be necessary to treat with Antibiotics.

While Columnaris is not something you will see very often, especially if your fish supplier properly acclimates and quarantines your fish before shipping, it can show up occasionally.

Any underlying causes such as poor water quality should be remedied, and good husbandry will minimize the likelihood of any further problem. Flexibacter columnaris prefers hard water with a pH above 6. Fish that survive infection may develop an acquired immunity which will confer some protection against any subsequent exposure to this pathogen.

Some of the information provided here is from Blue Ridge Koi

Columnaris is a symptom of disease in fish which results from an infection caused by the gram negative, aerobic, rod shaped bacterium Flavobacterium columnare. It was previously known by the names of bacillus columnaris, chondrococcus columnaris, cytophaga columnaris and flexibacter columnaris. The bacteria is ubiquitous in fresh water, and cultured fish reared in ponds or raceways are the primary concern – with disease most prevalent in air temperatures above 12–14⁰C. The disease is highly contagious and the outcome is often fatal. It is not zoonotic.

The bacterium usually enters fish through gills, mouth, or small wounds, and is prevalent where high bio-load exists, or where conditions may be stressful due to overcrowding or low dissolved oxygen levels in the water column. The bacteria can persist in water for up to 32 days when the hardness is 50 ppm or more.

From Wikipedia, the free encyclopedia


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