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Columnaris (mouth rot, mouth fungus, 'flex')


Early signs of this disease are greyish-white marks or patches around the mouth, or on the body or fins of the fish. These may appear thread-like, particularly around the mouth, giving rise to the incorrect name of 'mouth fungus'. Fins will deteriorate and sores may appear on the body. The gills may be affected, giving rise to bacterial gill disease. It may cause 'shimmying' behaviour in fishes like livebearers.


The disease is caused by the bacteria Flavobacterium colmnare, formerly known as Flexibacter columnaris. It has been refered to as 'mouth fungus' (even though the cause is bacterial), due to the whitish strands which may appear fungus-like. These are Gram-negative, rod shaped bacteria, which move by gliding across a surface.

As with many bacterial diseases, poor water quality is a major factor in triggering the disease. Sudden changes in conditions may also trigger it, and the disease is common in newly imported fish.

Possible cures

Anti-bacterial medications should cure this disease if used promptly. At later stages, the bacteria may invade internal organs, in which case only antibiotics may be effective. Several strains exist which vary in their virulence (how 'aggressively' they cause disease)..



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