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Columnaris

By the koi.wiki editorial team · Updated

Key facts: Columnaris is a bacterial disease of koi and other freshwater fish caused by the Gram-negative bacterium Flavobacterium columnare, producing cotton-like lesions on the skin, fins, mouth, and gills, often called saddleback disease due to a characteristic white band across the back.[1][2]
Columnaris disease, first described by Herbert Spencer Davis in 1922, is one of the oldest known diseases of warm water fish and has been referred to by several names for its causative bacterium, including Bacillus columnaris, Flexibacter columnaris, Cytophaga columnaris, and currently Flavobacterium columnare.[2][1] It is highly contagious and commonly fatal, though it is not zoonotic.[1]The bacteria are considered ubiquitous in fresh water and in most aquaculture environments, and disease most often develops when fish are stressed or when water temperatures are in the upper part of their preferred range.[1][2]

Cause

Columnaris disease is caused by Flavobacterium columnare, a Gram-negative, aerobic, rod-shaped bacterium.[1] The bacteria are long, thin rods, 7 to 10 millimeters in length and about 10 to 20 times longer than wide, that move by flexing and gliding and form characteristic column-like "haystacks" on tissue surfaces.[2] In laboratory culture the organism produces a yellow pigment and grows in a spreading, rhizoid (root-like) pattern rather than in discrete round colonies.[2][1]Most species of fish are susceptible following some type of environmental stress.[2] Stressful conditions that favor the disease include low dissolved oxygen, high ammonia, high nitrite, high water temperatures, rough handling, mechanical injury, and crowding.[2] The disease is most prevalent at air temperatures above 12–14 °C (about 54–57 °F) and bacteria can persist in water for up to 32 days when hardness is 50 ppm or more.[1] The bacteria usually enter fish through the gills, mouth, or small wounds.[1] Buildup of organic detritus from feces, uneaten food, and decaying organisms can serve as a reservoir for Flavobacterium columnare in ponds.[4] The organism can also increase in concentration within recirculating aquaculture systems.[5]

Symptoms and saddleback lesions

Affected fish typically develop brown to yellowish-brown lesions on the gills, skin, and fins.[2] The bacteria attach to the gill surface, spread in patches, and eventually cover individual gill filaments, eroding tissue through protein- and cartilage-degrading enzymes.[2] A characteristic lesion is a pale white band encircling the body, commonly called the saddleback condition, often with a yellowish-brown ulcer in the center of the "saddle."[2] A yellowish-brown, mucus-like growth of the bacteria may also appear inside the mouth, sometimes described as cottonmouth.[2][1]In acute cases, especially in young fish, the disease can progress so quickly that there are no superficial signs before death, which occurs through gill damage and respiratory distress.[1] Signs of acute disease include shortness of breath, gill discoloration, and flashing (rubbing against surfaces).[1] In chronic cases, fin rot and frayed, ragged fins can appear, with lesions beginning at the base of the dorsal fin and spreading over time.[1] Gills may become light or dark brown and show necrosis, and affected fish often breathe rapidly, stop eating, and become lethargic.[1] Ulcerations can develop within 24 to 48 hours, and fatality can occur between 48 and 72 hours without treatment, or within hours at higher temperatures.[1] A commercial koi health resource, the US dealer Kodama Koi Farm, similarly lists cotton-like patches, fin erosion, and ulcers as symptoms of columnaris associated with Flavobacterium columnare.[6]

Diagnosis

A presumptive diagnosis is made by scraping an affected area, preparing a wet mount on a microscope slide, and examining it at 100 to 400x magnification for the organism's characteristic shape, movement, and haystack formations, which are best seen on fresh tissue from live fish.[2] Definitive diagnosis is typically made from bacteria isolated on low-nutrient growth media kept between 25 and 30 °C, where colonies appear small, 3–4 mm in diameter, rhizoid, and pale yellow, growing within 24 hours.[1] Specialized low-nutrient media such as Ordal's medium or selective media like Selective Cytophaga Agar and Hsu-Shotts medium are used because the bacteria grow in the presence of neomycin and polymyxin B, which inhibit most other organisms.[2] More definitive identification can use specific antibodies in a slide agglutination test or molecular methods such as PCR.[2] Diagnosis and treatment decisions are generally discussed with a qualified fish health professional experienced with disease cases.[2] For koi keepers, general approaches to diagnosis including scraping and microscope examination are described in the broader context of diagnosis using scrapes, microscopes, and fish vets.

Treatment

External columnaris infections are commonly treated by treating culture water with therapeutic chemicals legal for use on food fish, including potassium permanganate (KMnO4), which is a commonly used therapy, though as of 1998 it was considered on deferred status by the US FDA while still permitted.[2] A traditional approach involves dissolving KMnO4 in water and monitoring that a red color is maintained for a full 4-hour treatment period, with dosing guided by a 15-minute KMnO4 demand test.[2] According to Wikipedia, a medicated bath using aquarium merbromin, methylene blue, or potassium permanganate and salt is generally used as a first step, and lowering aquarium temperature to about 75 °F (24 °C) is advised since the disease is much more virulent at higher temperatures, particularly 85–90 °F.[1]Medicated feeds containing oxytetracycline, such as Terramycin, have been reported to give better results when affected fish are treated soon after disease is detected, especially since diseased fish often eat less.[2] Oxytetracycline still works adequately against most cases of Flavobacterium columnare, though resistance from misuse has developed in some facilities, and tetracyclines are light sensitive and turn brown when decomposing, requiring a water change after bath treatment.[7] Romet medicated feeds have also shown effectiveness against columnaris in recent laboratory antibiotic sensitivity tests, although this use is technically extra-label since the product is labeled for Edwardsiella ictaluri infections.[2]The FDA-approved product 35% PEROX-AID (hydrogen peroxide) is approved for control of mortality due to external columnaris disease in freshwater-reared coolwater and warmwater finfish, with no withdrawal period required when used according to labeling.[3] Wikipedia also notes that potassium permanganate, copper sulfate, and hydrogen peroxide can be applied externally to adult fish and fry but may be toxic at high concentrations, and that a live attenuated vaccine against columnaris disease exists for catfish.[1]

Economic considerations in treatment decisions

In commercial pond aquaculture, treatment decisions weigh the cost of treatment against the value of fish likely to be lost.[2] One example describes a 2-acre, 10 acre-foot pond stocked with 6,000 half-pound catfish losing about 40 fish a day over a week; a KMnO4 treatment estimated at roughly $108 was considered economical against estimated losses of about $20 a day, or about $280 if mortality continued for two more weeks.[2] The same source notes that if losses were much smaller, for example five fish per day, treatment costs could exceed the expected losses from the disease, making treatment less justified.[2]Because columnaris bacteria are considered ubiquitous in most aquaculture environments, prevention centers on minimizing stress to fish, since stocking and feeding at levels needed for production can itself increase stressful conditions and disease likelihood.[2] Reducing organic detritus buildup from feces, uneaten food, and dead organisms in ponds is recommended, since this material serves as a reservoir for Flavobacterium columnare and other disease-causing organisms while also depressing dissolved oxygen.[4]Columnaris frequently occurs alongside or precedes other conditions. One study found that 80 percent of catfish ponds with winter saprolegniosis (fungus) also had a preceding columnaris infection, and columnaris was isolated internally in 40 percent of cases where it was also found externally.[2] Related topics covered elsewhere include Saprolegnia (fungus), fin rot, and Aeromonas and ulcers, as well as general signs of illness and quarantine practices that can reduce introduction of stressed or diseased stock.

Common questions

What causes columnaris in koi?
Columnaris is caused by the Gram-negative bacterium Flavobacterium columnare, which is considered ubiquitous in fresh water and most aquaculture environments and tends to cause disease when fish are stressed.[1][2]
Why is columnaris called saddleback disease?
A characteristic lesion is a pale white band encircling the body with a yellowish-brown ulcer at its center, resembling a saddle, which is why the condition is also called saddleback disease.[2][1]
How is columnaris treated?
Reported treatments include potassium permanganate baths, medicated feeds containing oxytetracycline (such as Terramycin) or Romet, hydrogen peroxide products approved for external columnaris control, and lowering water temperature, since the bacteria are more virulent at higher temperatures.[2][3][1]
Can columnaris be confused with a fungal infection?
Yes; due to the appearance of bacterial clumps, columnaris can be mistaken for a fungal infection, which is why it is sometimes called cottonmouth.[1]
How quickly can columnaris kill fish?
Ulcerations can develop within 24 to 48 hours, and fatality may occur between 48 and 72 hours without treatment, or even within hours at higher water temperatures.[1]

References

  1. ^ Columnaris (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Columnaris. Retrieved November 2026.
  2. ^ SRAC Publication No. 479b: Columnaris Disease. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/479b%20Columnaris%20Disease-%20…. Retrieved November 2026.
  3. ^ UF/IFAS EDIS FA157: Use of Hydrogen Peroxide in Finfish Aquaculture. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA157. Retrieved November 2026.
  4. ^ SRAC Publication No. 4712: Biosecurity in Aquaculture, Part 3: Ponds. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4712%20Biosecurity%20in%20Aquac…. Retrieved November 2026.
  5. ^ UF/IFAS EDIS FA100: Fish Health Management Considerations in Recirculating Aquaculture Systems—Part 2: Pathogens. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA100. Retrieved November 2026.
  6. ^ HELP, Is My Koi Sick?!? Diagnose Symptoms & Fish Diseases - FREE Health Checklist. Kodama Koi Farm (koi dealer). kodamakoifarm.com/koi-sick-health-diagnosis-symptoms-diseases/. Retrieved November 2026.
  7. ^ UF/IFAS EDIS FA084: Use of Antibiotics in Ornamental Fish Aquaculture. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA084. Retrieved November 2026.