Columnaris is a major problem for farmed tilapia and it can also affect aquarium kept tilapia. The disease is highly contagious, especially at the fry and fingerling stages, and will often be fatal. Columnaris is especially common in hatcheries, but can be a problem for larger tilapia as well. The bacteria behind columnaris will normally enter the fish through gills, mouth, or small wounds. The first signs of disease are normally frayed and ragged fins. Ulcerations will then develop within two days and without treatment it is common for fish to die within 48-72 hours. It is possible for tilapia fish to survive columnaris outbreaks without medication, but the fatality rate is usually high.

What causes columnaris in tilapia?

The causative agent behind columnaris in tilapia is a bacterium named Flavobacterium columnare. This bacterium has been known under several different names in the past, including Myxobacterium columnare, Cytophaga columnare and Flexibacter columnaris.

Flavobacterium columnare is a gram negative, rod-shaped bacterium. When cultivated on wet-mount preparations, the bacteria will form typical columns, hence the name columnare. The columns look a bit like tiny hay stacks. When grown on low nutrient agar media, Flavobacterium columnare is easily recognized on its characteristic rhizoid growth pattern.

Symptoms and diagnosis of columnaris in tilapia

• One of the first symptoms of columnaris in tilapia is usually frayed and ragged fins.

• Another early warning sign is brown or yellowish-brown lesions that appear on the skin, the mouth, the fins and the gills of the fish. When a lesion first appears, it can be in the form of a paler area that lacks lustre. The area around a lesion will normally become reddish.

The skin lesions are often found on the back and the sides of the fish, in a typical “saddle-pattern”.
Lesions that appear on the mouth can look like mould or cotton. 

Lesions that appear on the gills are normally necrotic and the bacteria will cause disintegration of the gill filaments. The gills can change from their natural colour to light or dark brown. Since it is difficult to absorb enough oxygen from the water using damaged gills, the fish will start breathing rapidly and laboriously, and it might also swim up to the surface gasping for air. (You can help you fish by increasing aeration, since a high oxygen level makes it easier for it to obtain oxygen from the water with its damaged gills.)  

• During acute columnaris, bacteria can reach the blood system and cause a systemic infection.

• When a fish has been weakened by columnaris, other pathogens can take advantage of the situation and attack the fish. Winter saprolegniosis is for instance known to frequently occur in tilapia weakened by Flavobacterium columnare.

• When infected tissue is arranged in a wet mount and observed under light microscopy, it is possible to see how the bacteria slowly glide into characteristic columns (“hay-stacks”). Use phase contrast at 400x magnification. To obtain a definitive diagnosis, bacteria must be isolated on a low nutrient medium (e.g. cytophaga agar).

 What can I do to prevent columnaris in tilapia?

• Stress will make your tilapia more susceptible to columnaris and the best way of reducing the risk of columnaris outbreaks is therefore to minimize the amount of stress that your fish has to deal with. Keep handling down to a minimum since being handled is highly stressful for fish. Keep water quality, water chemistry, oxygen levels and water temperature within the recommended ranges, feed your fish a suitable diet and avoid overcrowding. Don’t forget that Flavobacterium columnare thrives in organic waste and is prevalent in environments with high bio-loads.
  
  
• Be especially vigilant right after sudden changes in water temperature, since such changes are known to promote columnaris in tilapia and accelerate existing problems. Also pay extra attention to hatcheries because young fishes are particularly prone to columnaris.
  
  
• In order to prevent Flavobacterium columnare from spreading, it is safest to use separate equipment for separate growing units. As mentioned above, this is a highly contagious disease and the fish can catch it from the environment as well as from infected fish. The bacteria can for instance be present on nets, cages and food. To be extra safe, disinfect all equipment after each use or at least on a regular basis. In water, Flavobacterium columnare can survive for up to 32 days if the hardness is at least 50 ppm.
  
  
• Many farmers use salt (5-10 ppt) to reduce the risk of columnaris in hatcheries. Salt can also be used during transportation. You should however keep in mind that an altered salinity can open up the field for a new set of pathogens, e.g. parasites that belong in brackish or marine environments.
  
  
• If you fish is already infected with columnaris and suffering from gill damage, you might be able to reduce the fatality rate by increasing aeration. A high level of dissolved oxygen in the water will make the environment better for fish with damaged gills.

Can I use antibiotics to treat columnaris in tilapia?   

Flavobacterium columnare can be killed with antibiotics, but using antibiotics to treat columnaris in farmed tilapia is problematic. Infected fish will often loose their appetite, which makes is difficult to use oral antibiotics. In addition to this, antibiotics will normally only prevent the infection from developing further, they will not solve the problem. Reducing stress and creating a beneficial environment for the fish is therefore usually the best course of action in the long run. Reports from tilapia farmers also suggest that fish that manages to survive a columnaris infection might develop some type of immunity. Prolonged use of antibiotics will favour the development of bacteria resistant to antibiotics and the fish can also become less attractive on the food market due to residue concerns.

If you choose to medicate your fish, there are several alternatives to choose among. If your fish is still eating, you can try food with oxytetracycline. If not, sulfate based drug combinations such as Triple Sulfa, TMP Sulfa or Sulfa 4 TMP can be tried. Other examples of antibiotics known to kill Flavobacterium columnare are tetracycline, acriflavine, erythromycin, nitrofurazone, nifurpirinol and chloramphenicol.

Is there a vaccine against columnaris in tilapia?

There is currently no vaccine available that will prevent columnaris in tilapia. Intervet Inc. is however producing a live-attenuated immersion vaccine against columnaris in channel catfish under the name Aquavac-COLTM. Hopefully, a vaccine will be available for tilapia as well in the near future.