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Columnariasis (cotton mouth disease)
Columnariasis, also called "cotton mouth disease" or "horse saddle disease", is a bacterial infection of the guppy's skin, fins and gills. It is caused by a filamentous bacteria, Flavobacterium columnare (formerly Flexibacter columnaris). T...
What is it?
Columnariasis, also called "cotton mouth disease" or "horse saddle disease", is a bacterial infection of the guppy's skin, fins and gills. It is caused by a filamentous bacteria, Flavobacterium columnare (formerly Flexibacter columnaris). This disease affects many aquarium fish and can progress very quickly, especially in poor water quality. It gets this name because it sometimes forms whitish, cotton-like deposits around the mouth.
Key point
This disease can progress very quickly (sometimes within a few days) to a serious form, especially in young guppies or if the gills are affected.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
The first signs are often discreet: the guppy becomes less active, stays away from others and loses its appetite.\n\nWe then observe whitish or grayish areas, a little cottony, around the mouth, on the body or on the fins. These lesions can spread and take on an eroded appearance, with irregular edges sometimes reminiscent of a horse saddle shape on the back of the fish.\n\nThe fins may fray or tear. The fish may rub itself against the decorations or the floor of the aquarium, as if to relieve an itch. If the gills are affected, breathing becomes difficult and the fish may remain near the surface or near the bubbler.\n\nIn severe forms, the disease progresses in just a few days and can lead to the death of the fish, particularly in young or already weakened individuals.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
Columnar wilt is caused by Flavobacterium columnare, a bacteria naturally found in many aquatic environments, including seemingly healthy aquariums. It becomes pathogenic especially when the fish is weakened or stressed, or when it is found in large quantities in the water.\n\nThis bacteria takes advantage of a breach in the fish's protective barrier: a small wound, a damaged fin, or skin irritated by poor water quality.
Several factors favor the appearance of the disease in guppies: water rich in organic waste (ammonia, high nitrites), unstable or too high water temperature, overcrowding in the tank, stress linked to transport or the introduction of new fish, injuries due to fights or sharp decorations, and an immune system weakened by another disease.
Transmission
The bacteria is transmitted in water, from one fish to another, in particular by direct contact or via contaminated aquarium water.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The aquarium fish veterinarian or an experienced aquariologist may suspect columnariasis based on the typical appearance of cotton-wool lesions. A microscopic examination of a sample of mucus or damaged tissue often confirms the presence of the characteristic filamentous bacteria.\n\nIn some cases, a bacterial culture can be performed to precisely identify the causative agent and guide treatment.
Treatment
Treatment is based on antibiotics suitable for fish, administered in water or sometimes via food, on prescription or advice from a competent professional. Topical antibacterial products can also be used in a short bath.\n\nImmediately improving the quality of the water (water changes, reducing the temperature if it is too high) is an integral part of the treatment, as this limits bacterial proliferation and supports healing.\n\nA specific salt for aquariums can sometimes be used in addition, under specialist advice, for its mild antiseptic properties.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
The best prevention is based on good aquarium management: maintaining clean water with regular changes, monitoring the parameters (ammonia, nitrites, pH, temperature), avoiding overpopulation, and properly acclimatizing new fish.\n\nIt is recommended to quarantine any new guppy before introducing it into an already populated tank, in order to observe its state of health for a few days.\n\nLimit sources of stress (transport, sudden changes in temperature, decorations aggressive) also helps reduce the risk.
Possible complications
Without treatment, lesions can spread to the gills, causing severe respiratory distress. A generalized infection can also set in, affecting several internal organs and leading to the death of the fish.\n\nFish weakened by columnariasis also become more susceptible to other secondary infections, particularly fungal ones.
When to see a veterinarian
Appearance of whitish or cottony areas on the body, mouth or fins.\nFins that fray quickly.\nFish that remains isolated, inactive or refuses to eat.\nRapid breathing or fish that stays near the surface.\nSeveral fish in the tank show similar symptoms at the same time.
Prognosis
The prognosis depends on the speed of treatment. Detected early and treated with improved water quality, columnariasis can be cured in guppies. On the other hand, an advanced infection affecting the gills or generalized throughout the body can progress very quickly to death, sometimes within a few days.
Frequently asked questions
Is columnariasis contagious to other fish in the tank?
Yes, the bacteria can be transmitted easily in the water to other fish, especially if the water quality is poor. It is advisable to isolate affected fish if possible and monitor the others.
Can we treat an affected guppy without professional advice?
It is strongly recommended to seek advice from a veterinarian or an aquarium specialist before any treatment, as the wrong dosage of antibiotic can aggravate the situation or disrupt the bacterial balance in the aquarium.
Is a water change enough to cure the disease?
Improving water quality is essential and helps the fish to resist better, but this alone is usually not enough to eliminate the infection once it is well established. Antibacterial treatment is often necessary.
How to prevent the disease from reappearing?
By maintaining consistently good water quality, avoiding overcrowding and stress, and quarantining any new fish before introducing them to the main aquarium.
References
Content reviewed by the TunisieVet editorial team · Updated 11 August 2026
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