I just wrote a one hour researched (finding old links) and the post disappeared.
Old thread but adding information.
So going to post links only now.
It is Flexibacter [FONT="]Columnaris a gram negative bacteria which can take many forms (the original photo a "saddle back") and is found with various strains - from slow to fast (as in overnight) strains.
I've done hundreds of betta rescue so know well this bacteria, which is found every where but is more virulent in the warmer seasons, and is becoming drug resistant due to meds in our water systems. (back ground in biochemistry and more plus 45 years aquarists - never had this as a kid though!) Plus fish are often grown in horrible conditions, and weaker strains due to commercialization.
Since it is found in fisheries/commercial production there is a great deal of research on it (from which most web sites take theirs from).
There for many meds mentioned such as M2 will not work. Acriflavine [/FONT][FONT="]is only for mild cases as well. (for true fungus and is found as Fungus Cure API's) (note how one of the articles mentions [/FONT]erythromycin which is for gram positive bacteria...) Mardel.
Link to how medications work: (and many other articles - excellent)
http://www.americanaquariumproducts.com/Aquarium_Medication.html
Link to an excellant article, which discusses the various strains and much more, and importantly sites the research from where this information was obtained - a great way to learn from institutional research.
http://www.cichlid-forum.com/articles/columnaris_disease.php
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See links (which I had cut and paste from as well as provide links) originally.
Since as well there are other secondary infections, and although it starts with often unnoticed details (such as a raised scale, or slight bump or even in the gills) by the time it is "noticed" it has invaded the internal organs - clearly the case with the above shown fish and it as well has hemorrhagic septicemia (bacterial).
In this case (and I have prior brought people grade drugs from National Fish Pharmeceuitical over a broad range in my limited budget) I would have:
Spot treated with diluted 3% h2o2 (used as well in fisheries and much research). Since it is not near the eyes or gills (instant death if in gills).
And perhaps a bath (I think dips are far too harsh for most fish esp in weakened condition) take from list below (link to how fish meds works, section on this bacteria and various treatments and baths/dips).
Then I would feed it food with oxytetracycline and treat treat the water with Kanamycin [/FONT]as it is absorbed internally and is for advanced cases[FONT="]. Since Kanamycin is a gram negative med, it will help with any other secondary infections such as the clearly observed bacterial hemorrhagic septicemia as seen in the photo. (and for which one needs a med that is skin absorbed) Mardels M2 does this and is gram negative (for) but is for milder cases.
Increasing does of salt (which inhibits attachment to the fish) slowly over days (and is well tolerated even in catfish if done this way, but that said, they can not go as high as say a betta) and when done with the treatment, reduce over time in the same incremental manner.
(as well, I use a hospital tank for many reasons. Some meds interact with organic solids, act differently with different ph/hardness levels, and esp for the patient's needs... lowered lights, no buddies to disturb it, silk plants for security so on... Just like us when ill, this will help them by reducing stress. To further reduce stress, I never use nets, but clear plastic tubs - they can't see it is a "trap" and slowly change any water as needed (treatment - newer hospital tank so on - I start with water - drained over floss, in new hospital tank and add rest of fresh medicated).
http://www.americanaquariumproducts.com/Columnaris.html
Great info and photos:
[/FONT]Identification of Columnaris (AKA Saddleback Disease & Cotton Wool Disease)
(continue to the link for a great well researched article)
Columnaris along with Saprolegnia in particular (Saprolegnia is often confused with Fungus) are two of the most misunderstood and often mistreated aquariums diseases, even Wikipedia has at best marginal information (& this is being kind as of my last reading of their article) about these pathogens and their CORRECT treatment and prevention (prevention is the key with both of these unrelated but similar symptom pathogens).
Columnaris (often referred to in the past as "cotton wool disease" or "cotton mouth disease") is a warm water, gram negative
strictly aerobic and nonhalophilic (meaning they do not live in saltwater conditions) bacterium often appears like a fungus (or more correctly; Saprolegnia) however it is not a fungus, although many treatments for fungus are effective for mild cases of Columnaris (Flexibacteria), which is why Erythromycin is a strange choice of treatment as it is rarely effective for either Fungus of Columnaris (yet is often suggested, which really makes me scratch my head as to why these sites suggest this treatment).
Another great source of information:
http://www.tilapia.ws/Columnaris.php
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.
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.
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Photo search results:
http://search.yahoo.com/search?ei=utf-8&fr=slv1-&p=Photos of fish with flexibacter Columnaris&type=
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