Columnaris in tilapia

Columnaris is a disease caused by the bacterium Flavobacterium columnare. The first signs of the disease are usually frayed and ragged fins. After the first symptoms have become evident, you can expect ulcerations to appear within 48 hours. If not treated, tilapias infected with columnaris usually dies 48-72 hours later. Some specimens may survive without medical treatment, but the survival rate is normally very low.

Columnaris is a dreaded disease among tilapia farmers and it can also infect aquarium kept tilapias. The disease is highly contagious, especially among fry and fingerlings, and the disease is especially common in hatcheries. Flavobacterium columnare typically enters the fish through gills and mouth, or through small wounds.

What causes columnaris in tilapia?

The disease columnaris is caused by the bacterium Flavobacterium columnare. If you consult older sources, you may find it referred to under one of its older names, such as Flexibacter columnaris, Myxobacterium  columnare or Cytophaga columnare. It is currently placed in the genus Flavobacterium.

Flavobacterium columnare is a gram negative, rod-shaped bacterium. If you cultivate it on wet-mount preparations and watch it trough a light microscope, you will see the bacteria slowly glide into the characteristic columns from which they derive their name. The columns look somewhat similar to small hay stacks. You can see the same pattern if you arrange infected tissue in a wet mount. (Use phase contrast at 400x magnification.)

If you cultivate Flavobacterium columnare on low nutrient agar media (e.g. cytophaga  agar) the bacteria can be easily recognized on their characteristic rhizoid growth pattern. Isolating the bacteria on this type of medium is necessary to obtain a definitive diagnosis.

Symptoms of columnaris in tilapia

The first symptoms of columnaris are normally frayed and ragged fins. Another early warning sign are the brown or yellowish-brown lesions that sometimes appear on the skin of the tilapia, on the mouth, on the fins and on the gills. Eventually, it is often possible to make out the typical “saddle-pattern”, i.e. lesions concentrated to the back and sides of the tilapia. Sometimes the lesions look just like lusterless pale areas to begin with, before they develop into real lesions. The area around a lesion will normally become reddish. Lesions that form on the mouth can look like cotton or mould.

Lesions on the gills are usually necrotic and the gill filaments will eventually disintegrate. Sometimes the gills change color to light or dark brown. The damaged gills will not function properly and the infected tilapia will therefore breathe fast and laboriously.  In an effort to get more oxygen, fish suffering from columnaris can sometimes be seen close to the surface grasping for air. Farmers trying to save their tilapias from columnaris can aid the fish by increasing the oxygen content of the water.

During acute columnaris, bacteria can reach the blood system of the tilapia and cause a systemic infection. When this happens, all internal organs can be affected.

A tilapia weakened by columnaris is an easy target for opportunistic bacteria, parasites and other harmful organisms.  Winter saprolegniosis is for instance a common problem for tilapias with columnaris.

What can fish farmers do to prevent columnaris?

  1. Reduce stress

A tilapia exposed to stress is more susceptible to columnaris so if you manage to decrease the amount of stress, you will decrease the risk of columnaris. Handling is highly stressful for fish and must be kept down to a minimum. The water quality must be kept up, because high levels of organic waste and other pollutants will weaken your tilapias. Keep a watchful eye on water chemistry, water temperature and oxygen levels.

2. Do not crowd

In a crowded environment, fish typically experience more stress and it is harder to keep water quality and oxygen levels up. Flavobacterium columnare is known to thrive in organic waste and disease will also spread more quickly in a crowded environment. Exactly when a growing unit is consider crowded varies; a growing unit with extra aeration and supreme water management will for instance be able to house more fish.

3. Feed a suitable diet

A varied and nutritious diet will boost the immune system of fish and make the less susceptible to columnaris.

4. Use separate sets of gear

If you use the same equipment for several growing units you risk spreading Flavobacterium columnare to all of them. Fish doesn’t have to be housed with infested fish to catch columnaris; they can catch it from the environment as long as Flavobacterium columnare is present. The bacteria can live on equipment such as nets and cages, and will survive for up to 32 days in water if the hardness is at least 50 ppm. If you have to move equipment from one growing unit to another, disinfect them to kill all bacteria. It may to be practically feasible to disinfect them after each use (which is ideal) but it should at least be done a regular basis.

5.   Pay special attention to fish subjected to change

Sudden changes, e.g. in water temperature, is known to increase the risk of columnaris outbreaks in tilapia. Be observant and you may be able to notice early warning signs and do something before the situation goes out of hand.

6. Pay special attention to hatcheries

Young tilapia fish are especially prone to columnaris. Pay special attention and look for early symptoms. Some farmers use salt (5-10  ppt) to decrease the risk of columnaris in hatcheries. Salt is also commonly used when transporting young tilapia. Using salt will however make it possible for other pathogens, those that thrive in brackish or marine conditions, to attack your fish so you need to be vigilant.

Can I use antibiotics to treat columnaris in tilapia?

Yes, Flavobacterium columnare is sensitive to antibiotics and columnaris can therefore be treated medically. Unfortunately, sick fish tend to loose their appetite which makes it difficult to use medicated food. Another problem is that antibiotics normally only keeps the infection from developing further; the medication will rarely solve the problem with reoccurring columnaris in fish farms. You have to attack the problem at the root and follow the advice above if you wish to see any long term effects.

If your tilapias are still eating despite the infection, you can give them food with oxytetracycline. For fish that has stopped eating or only eat very little, sulfate based drug combinations is the most common approach, e.g. Triple Sulfa, TMP Sulfa or Sulfa 4 TMP. Tetracycline, erythromycin, acriflavine, nitrofurazone, chloramphenicol and nifurpirinol are other examples of antibiotics to which Flavobacterium columnare is known to be sensitive.

A side effect of using antibiotics is that you may favor the development of bacteria resistant to antibiotics. The risk is especially high if the use is prolonged and/or reoccurring. Another problem is that antibiotics can make fish less attractive among consumers due to residue concerns.

Is there anything else I can do to help my tilapia survive columnaris?

Columnaris can cause gill damage in fish which makes it hard for them to breathe. You can help you tilapia by increasing the oxygen content of the water. Since it is easier for fish to breathe in waters with a high amount of dissolved oxygen, the increased aeration may increase the survival rate.

Can tilapia develop immunity to Flavobacterium columnare?

Reports from tilapia farmers indicate that fish surviving a columnaris attack might develop some type of immunity, but the exact mechanisms or how long the immunity lasts is far from fully understood. More research is needed.

Is there a vaccine against columnaris in tilapia?

No, there is currently no columnaris vaccine available for tilapia. However, the company Intervet Inc. is producing Aquavac-COLTM, a live-attenuated immersion vaccine against columnaris for use on channel catfish. Hopefully, we will soon see a similar vaccine for tilapia.