Columnaris Disease in Tilapia: Symptoms, Causes & Treatment

Columnaris is a bacterial disease of tilapia caused by Flavobacterium columnare, a long, thin rod that attacks the gills, skin and fins. It is the rot you see as greyish-white or yellow-brown patches with a slimy, cotton-like fuzz on the edge — which is why farmers call it “cotton wool disease,” and why a band of dead skin across the back is called “saddleback.” Left alone in a warm, dirty pond it moves fast: gills go necrotic, fish stop breathing, and a fingerling tank can crash in a couple of days.

If you raise tilapia in the tropics, columnaris is one of the diseases you will meet most often. It is right up there with streptococcus and Aeromonas on the farms our customers run in Ghana, Egypt, Indonesia and across the warm belt — and like those, it almost never strikes a clean, well-run system. It waits for warm water, crowding and a layer of organic muck on the bottom, then takes the fish the handling net or a parasite already weakened. That pattern is the whole story, and we’ll come back to it.

What columnaris actually is

Columnaris is caused by Flavobacterium columnare (older names: Flexibacter columnaris, Cytophaga columnaris), a Gram-negative filamentous bacterium found in nearly all fresh and brackish water. Under a microscope from a fresh lesion the bacteria gather into the haystacks or “columns” that give the disease its name — long rods piled together, often with a slow gliding, flexing movement.

It is an opportunist that lives in the water and on the fish’s slime coat all the time at low levels. It does not need a wound to start, but it loves one: a scrape from the net, a bite, a patch of gill already chewed by Trichodina or gill flukes is exactly where it takes hold. From there it spreads fish to fish through the water — no intermediate host — which is why one neglected corner of a crowded pond can seed the whole batch.

Symptoms: how to recognise columnaris

The bacterium digests skin and gill tissue, so the signs are erosion and rot rather than the swelling and dark colour you see with septicaemia:

• Greyish-white or yellow-brown patches on the skin — often starting on the back or near the dorsal fin. As they spread sideways they form the classic “saddleback” — a band of pale, dead skin across the back.
• A cottony or fuzzy edge to the lesions — fine, mould-like strands at the rim. These are bacterial mats, not true fungus, but they look like cotton wool, hence the name. (True fungus, Saprolegnia, is woollier and usually secondary.)
• Frayed, eroded fins and a rotting, ulcerated mouth — “mouth rot” or “mouth fungus.” The lips and jaw go pale and ragged.
• Pale, swollen, necrotic gills — the most dangerous and most common site in tilapia. Gill columnaris kills fast and may show almost no skin signs before the fish die.
• Off feed, lethargy, gasping at the surface — wrecked gills can’t take up oxygen, so the fish piles up at the surface or the inlet, especially at dawn.

In gill-form outbreaks you may lose fish before you see a single skin lesion — so a sudden die-off in warm water with pale, ragged gills should put columnaris near the top of your list.

Diagnosis: confirm before you treat

You can suspect columnaris from the saddleback lesions and cottony edges, but several problems look alike — true fungus, Aeromonas ulcers, parasite damage — so confirm it before you spend on antibiotics:

1. Take a fresh fish (or one dying, not long dead — these bacteria are overgrown by others within minutes of death).
2. Scrape the edge of a lesion, or snip a piece of affected gill, and make a wet mount with a drop of pond water.
3. Look at 200–400×. Columnaris shows as long, thin rods gathered into haystacks or columns, often along the tissue edge, with a slow flexing or gliding motion. That column-forming habit is the giveaway.

A lab can confirm it on selective (Cytophaga) agar, where it grows as flat, yellow, rhizoid colonies that stick to the plate. For routine farm work the wet mount is usually enough to tell columnaris from fungus and from a parasite problem.

Why it flares up: warm, dirty, crowded water

Here is the part the “what antibiotic do I use” articles skip. Flavobacterium columnare is always present; an outbreak means the pond has tipped in its favour. The triggers are consistent, and the first one is the one tilapia farmers underrate:

• High water temperature — columnaris is a warm-water disease. It gets sharply more aggressive above about 25–28 °C and is most dangerous in the high 20s and into the 30s — exactly the range a tropical tilapia pond sits in. A heat spell can turn a few lesions into a wipe-out.
• High organic load — uneaten feed, faeces and dead algae on the bottom feed the bacteria directly. A dirty pond is a culture flask.
• Overcrowding and handling stress — high density means more contact and more waste; grading, netting and transport scrape the slime coat and open the door.
• Low dissolved oxygen and poor water quality — high ammonia and nitrite, low DO and a stressed immune system let a normal bacterial load become a lethal one.
• Existing skin or gill damage — a parasite burden (Trichodina, gill flukes) or a net injury is the wound columnaris waits for. Treat the parasite and you remove the doorway.

So you can dose antibiotics and knock it back, and it will return next warm spell if the water is still hot, dirty and crowded. The durable fix is environmental. This is where farm equipment stops being optional:

• You can’t manage what you can’t measure. A multi-parameter water quality meter tells you the temperature, ammonia, dissolved oxygen and pH behind the outbreak — in a columnaris pond the water is the diagnosis, and temperature is the first number to read.
• Heat is the trigger you can sometimes control. In hatcheries, nursery tanks and RAS, tank heating and temperature equipment lets you hold water below the danger zone and avoid the sudden swings that set columnaris off — a few degrees down buys you time and slows the bacteria.
• Outbreaks track low oxygen. Reliable aeration — a root blower feeding the pond, or a dissolved oxygen cone where you need to push DO hard in intensive systems — keeps gills healthy and fish strong enough to fight back.
• Organic load is the bacteria’s food. An automatic rotary drum filter strips out the suspended solids — uneaten feed and faeces — that feed a columnaris bloom.
• A UV pass: a UV steriliser on a recirculating loop knocks down the free Flavobacterium and secondary bacteria moving through the water column.

Treatment: act early, then fix the cause

Columnaris is treatable when you catch it early and superficial. Once it is systemic — internal organs involved, septicaemia — external baths won’t reach it and you need in-feed antibiotics. Standard, evidence-based options for tilapia:

• Salt (NaCl) — a useful first line and supportive treatment. F. columnare is sensitive to salt; a prolonged low dose in the pond, or a stronger short bath in a separate tank, helps the fish hold osmotic balance while the skin heals. Tilapia tolerate salt well and it leaves no residue.
• Potassium permanganate (KMnO₄) — about 2 ppm as a prolonged pond treatment, or a stronger short bath, is a long-standing external treatment for early columnaris. Dose against the pond’s organic demand, watch the colour, and never mix it with salt — salt makes permanganate far more toxic to the fish.
• Lower the temperature if the system allows it. Dropping the water a few degrees (where the fish can take it) directly slows the bacterium — one of the few “treatments” that costs nothing but is impossible in an open tropical pond and only practical in tanks, nurseries and RAS.
• In-feed antibiotics for systemic cases — florfenicol, or oxytetracycline at roughly 55–83 mg/kg fish/day for 10 days, are the standard choices once the infection is internal and fish are still eating. Use them on a confirmed diagnosis, ideally with sensitivity testing, observe the withdrawal period for food fish, and never as a routine preventive — blanket dosing is how resistance builds.

Two rules from experience. First, aerate hard during any external treatment — KMnO₄ and a heavy organic load both pull oxygen, and a columnaris fish with rotting gills is already short of it. Second, re-test the water and remove the cause — drop the load, ease the density, control the parasites, cool the system if you can — the moment fish are stable, or the next heat spell brings it straight back.

For the bigger picture on how warm, dirty water drives bacterial disease across the farm, see our guide to common tilapia diseases and the role of water quality, and the related write-up on motile Aeromonas septicaemia in tilapia — the other warm-water bacterial killer that rides in on the same conditions. If you’re moving toward a system that controls water quality by design, our biofloc water management guide covers the bacterial approach.

Prevention beats treatment

The farms that don’t fight columnaris every hot season do the same boring things:

• Keep stocking density sane for the aeration and filtration you actually have.
• Don’t overfeed; clear waste and dead algae off the bottom.
• Handle, grade and transport gently — every scrape is a doorway, so net less and net softer in warm weather.
• Stay on top of parasites (Trichodina, gill flukes); treat them before they open wounds for the bacteria.
• Watch the thermometer in the hot months and hold DO above ~5 mg/L, especially through the dawn dip.
• Quarantine and screen new fingerlings before they go in the main pond.

Columnaris is, in the end, a report card on warm-water management. Read the temperature, the load and the handling — treat the pond, not just the fish.