One day, you’re faced with one of your fish, who has patches of white fuzzy stuff on him and you start thinking to yourself, ” I need some medicine quick to kill this fungus, because if it’s white and fuzzy, then it must be a fungus, right?!” Wrong! White fuzzies that occur on otherwise normal skin and fins of a fish are much more likely to be due to a Flexibacter infection, which is a bacterial infection and not a fungal one. True fungus (Saprolegnia) can appear on open, raw or wounded areas, that is, it grows on dead tissue and it is seen much less often in the hobby than a Flexibacter infection is. The majority of fish medications with the word “Fungus” in their title actually contain antibiotics to kill bacteria! The fish drug companies know that most things white and fuzzy are not due to a fungus but rather are due to Flexibacter and although they give you the correct drug to treat the problem, they perpetuate the myth that all things white and fuzzy must be due to a fungus!

Flexibacter columnaris, most recently named Flavobacterium columnare, is a bacteria that grows in layers, one on top of the other, giving it the appearance of “columns” or “haystacks” under the microscope and this is what gives us the white fuzzy look with our naked eye. Flexibacter is a thin gram-negative rod bacterium that is an ubiquitous organism, meaning that it is ever-present in ones tank water, though the organism will only cause disease in fish that are undergoing stress. The specific conditions and stressors identified as setting up the right conditions for a Flexibacter infection are:

    * Sudden increases in water temperature.
    * Overstocking.
    * Infrequent water changes.
    * Low water oxygen (O2) content.
    * Elevated nitrite levels.
    * Uneaten food in the tank, which supports growth of Flexibacter.

In general, poor water conditions will pave the way for a Flexibacter infection, so regular maintenance with partial water changes are a must for a healthy tank. The overcrowding that occurs at fish/pet stores acts as a fish stressor, making your newly acquired fish at risk for a Flexibacter infection. Therefore, keeping your newly purchased fish in quarantine for a period of two-four weeks before introducing them into your main tank is very important.

As a fish board moderator, I see a flurry of Flexibacter infections from fish keepers living in the Northern Hemisphere in the late spring and through the summer as the ambient temperatures go up. For the Southern Hemisphere keepers that rise in Flexibacter infection is seen in the late fall and through the winter. Specifically, Flexibacter grows like gangbusters at water temperatures of 77ºF (25ºC) and higher. The onslaught of Flexibacter infections that are seen as the temperatures warm up and our battle to treat them, has led this bacterium to be viewed as, “The Dreaded Flex”! It is also a dreaded infection of cultured fish worldwide and accounts for an estimated loss of $50 million annually to U.S. catfish producers alone.

A Flexibacter infection will be more severe if it is due to a sudden rise in temperature and if your fish are in hard, alkaline water. The most important factor in determining the severity of infection depends upon the virulence of the strain of Flexibacter, that is, how powerful the bacteria is at infecting the fish. One strain has a virulence that is low and it takes a few days to kill the fish. On the other hand, the highly virulent strain will kill a fish in under 24 hours. If your fish has the highly virulent strain, there is not much that you can do to stop what is already in motion and these fish often die so quickly, that there may not even be enough time for them to show any outward signs of infection.

Early diagnosis is the key to beating this infection, as delays often result in death. It is acceptable to make the diagnosis of Flexibacter infection based solely on clinical signs, as Flexibacter does not grow well on standard bacteriological culture media. A smear taken from the infected fish and observed for the characteristic “columnar” appearance under microscope can be performed to clinch the diagnosis, but this method is not available to most aquarists.

The signs of Flexibacter infection run the gamut from fuzzy lesions of the mouth (“Mouth Fungus”) to septicemia (infection in the bloodstream) to white fuzzy patches scattered over the body, sometimes so severe as to cause the skin to look as if it is shedding or peeling, with the fins disintegrating. Due to the white fuzzy patches, Flexibacter also goes by the layterm, “Cotton Wool Disease” and this infection is so common in mollies that it carries the common names of “Black Molly Disease” and “Mad Molly Disease” with mollies often exhibiting “shimmying”. Often the fuzzy patch will be located on the fishes back, surrounding the dorsal (top) fin, having the characteristic look to it of a saddle, giving rise to the term “saddle-back lesion”. These lesions may also be found on the fins (fin rot), especially on the caudal (tail) fin and on the gills. The natural course of “Mouth Fungus” is that the white fuzzies or patches are followed within a day or two by redness, ulceration and necrosis, which means that the mouth quickly turns into a gnarly mess!

The white patches often have a reddish rim around them and within 24 hours, they can quickly form an ulcer (raw open area in the skin), which is shaped like a circle and then it enlarges. The ulcers may sometimes have a yellow or orange color to them due to the piling of pigmented bacteria upon each other. If an ulcer penetrates into the body deeply, this leads to septicemia. The symptoms of a Flexibacter infection may include sluggish activity and a poor to absent appetite.

The treatment of Flexibacter is best accomplished by feeding medicated food, containing oxytetracycline, tetracycline or kanamycin. Other antibiotic-laced foods may contain antibiotics, to which Flexibacter is resistant, such as ormetoprim and sulfadimethoxine, so, substitutes are not acceptable. The unfortunate problem with this disease is that the infected fish rapidly becomes ill and stops eating, especially if it has “Mouth Fungus”, therefore, treating with medicated food may not be an option.

Antibiotics in the water would be the next treatment option. It is best to perform this type of antibiotic treatment in a separate “hospital” tank as this removes the sick fish from the healthy ones in the main tank, does not unnecessarily expose the healthy fish to antibiotics and medication costs will be lower when treating in a smaller tank. However, if multiple fish are infected in the main tank, then it may need to be treated. Kanamycin (Kanaplex, Kanacyn) daily in the water is a great first choice for treating Flexibacter and it usually does not harm the biological filter (the “good guy” bacteria). As an alternative, erythromycin (plain Maracyn) or tetracycline can be used. When using either erythromycin or tetracycline in an established (“cycled”) tank, be sure to watch your water parameters, as these two antibiotics may harm the biological filter. Likewise, nitrofurazone (Fungus Eliminator) works well against Flexibacter but can harm the bio filter. Cephalexin (Keflex, Velosef) can be used at the rate of 250 mg per 10 gallons daily preceded by a 25% water change but this cephalosporin antibiotic is harmful to the biological filter. For those fish keepers living outside of the United States, who are unable to obtain the antibiotics mentioned, then either Waterlife’s Myxazin or Interpet’s Anti-Fungus and Finrot would be the best treatment to choose. Other antibiotics, which may be effective against Flexibacter are nifurpirinol, acriflavine, chloramphenicol and malachite green. Disinfectant dips, such as with quaternary ammonium, and prolonged immersion in potassium permanganate or copper sulfate have been advocated.

Aeromonas, another gram-negative bacterium ubiquitous to ones tank, often infects a Flexibacter-infected fish at the same time (secondary bacterial infection) and is thought to be the cause of death in many cases of Flexibacter infection. Therefore, treatment should also be aimed at this bacterium. If you are already using kanamycin (Kanacyn) in the water to treat the Flexibacter, it will also treat the Aeromonas. If you are alternatively using either erythromycin (plain Maracyn) or tetracycline, then treat simultaneously with minocycline (Maracyn-Two) for the Aeromonas. Both kanamycin and minocycline are antibiotics, which are considered not to be harmful to the biological filter, but at high dosages, kanamycin can harm the bio filter.

Be sure to remove all activated carbon when treating with antibiotics in the water. All antibiotic treatment should be continued for a minimum of 10 days and the decision for treatment beyond 10 days will depend upon how well ones fish is recovering.

Bring the water temperature down in the main tank until it is at 75-76ºF (24ºC), to slow the growth of Flexibacter. The water temperature in the hospital tank must not go above 76ºF (24ºC) for the duration of treatment. Additionally, increasing aeration, which improves oxygenation in the tank, will help to provide conditions less suitable for Flexibacter.

In short, treatment for a Flexibacter infection in a hospital tank would look like the following…

    * Feed medicated food containing oxytetracycline, tetracycline or kanamycin.
    * Kanamycin (Kanaplex or Kanamycin) daily in the water for both Flexibacter and Aeromonas. As an alternate choice, Maracyn (erythromycin) for Flexibacter and Maracyn-Two (minocycline) for Aeromonas can be used in the water. Overseas fish keepers can use either Waterlife’s Myxazin or Interpet’s AntiFungus and Finrot.
    * Decrease the temperature to 75-76ºF and no higher.
    * Increase aeration.

While care should always take place when treating sick fish, this strain of Flexibacter cannot be transmitted to humans.

A recent study from the Department of Fisheries and Allied Aquacultures at Auburn University in Alabama showed that keeping some freshwater fish in water with salt of increasing concentrations can prevent Flexibacter infections, presumably by preventing adhesion of the Flexibacter to the fishes body. When fish were exposed to Flexibacter, those kept at a 0.1% salinity (one teaspoon salt per gallon) had mortality rates reduced by one third, while those at a 0.3% salinity (one tablespoon salt per gallon), experienced no deaths. In the freshwater control group (salinity= 0.03%), there was virtually a 100% mortality (except goldfish, which had a 67% mortality). Whether the findings of this study hold true for aquarium fish is unknown, but it suggests that salt at concentrations that are commonly used in the hobby may help to prevent a Flexibacter infection.