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Furunculosis: Hole in the Side Disease

By Bob Fenner

Warning, not for the weak hearted.

Aquatic Gardens

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by Robert (Bob) Fenner

Every few years there seems to be a pandemic of this problem in the industry and hobby of cool water aquatics, with many resultant losses. The late nineteen eighties were especially bad years in the United States, Europe and Japan.

Furunculosis, aka Ana aki (in Japanese), is a condition evidenced by open ulceration (furuncle sores) mainly in freshwater pond fishes, such as ornamental "koi" carp and goldfish, though it has been identified in nine families of fishes. This disease can lead to open wounds, disfiguring loss of scales and scars, septicemia, internal infection and death. Shown: Koi with beginning, chronic, advanced acute cases of Ana aki/Furunculosis

The etiology (causative factors) of hole in the side disease are poorly understood as well as it's prevention and "cure". This Section  attempts to summarize what is known and speculated concerning Furunculosis; cause and treatment.


Furuncle sores are recurring boils, ulcerations, open sores on the "outside" of fishes. These ulcers are tinged with blood but notably are non-emarginated; that is, they are non-inflammatory" ("scabless"). Some acute cases of Furunculosis may be internal only with hemorrhagic petecchia ("blood spots") in the gut wall and organs. Hemorrhagic septicemia, bacteremia (bacteria in the blood) is generally present.

You have to look closely... Hemorrhagic septicemia evidenced as red bloodiness on the skin of a koi and a hard to see sore on the underside of another individual.


As stated above there is no one cause of hole in the side. The condition appears to randomly affect one or more individuals in a system on a non-selective basis. Various predetermining factors have been advanced as important: (Wedemayer and Wood, 1974) Genetic heritage, development, several aspects of water quality (especially low pH <less than 6/5>, presence of detectable ammonia, nitrites, tannins from leaf litter, hydrogen sulfide gas), pre-existing protozoan (such as Trichodina, Costia), and fluke (Gyrodactylus, Dactylogyrus) infections, leading to secondary bacterial infection. The implicated genera of bacteria, Aeromonas, Pseudomonas, have been found within the above gill and skin flukes and are presumably passed on during their parasitic feeding.

Especially in the sub-acute cases of low virulence and low incidence, an initial physical injury, disrupting the fish's skin by jumping, striking an object, spawning activity, rough handling, is implicated.

Larger koi (more than a few inches in length) should NOT be netted at all, but directed into a submerged plastic (soft sided) bucket or bag(s) underwater and lifted there... A koi with the beginnings of a furuncular sore on its gill opercle.

It cannot be stated emphatically enough or too often that this "condition" (like most diseases) is environmentally mediated. Pre-disposing conditions, i.e. poor water quality, improper feeding, lack of maintenance coupled with breaks in the skin from chemical, physical, and/or parasitic insult are the real "causative" mechanisms of hole in the side "disease". If this is not clear, please do read over The Three Sets of Factors that Determine Livestock Health.

These hole-sites become whitish and reddened, with raised scales. They develop into larger, non-emarginated sores infested with pathogenic bacteria (Aeromonads, Pseudomonads, Flexibacter) and/or Protozoans ( e.g. Epistylis).

Not to dwell too long on chicken versus egg theories... there is another viewpoint that Furunculosis is "caused" by bacteria in the blood that become more predominant and virulent as a consequence of host immune system breakdown, as a consequence of too much stress... I discount this notion though acute, rapid onset, high mortality incidences in a system fit this "from the inside out" model. Other coldwater fishes are susceptible to this environmental disease/condition: here a Goldfish with a sore on its "belly".


If you are around pond fish long enough you will see hole in the side disease. Some suggestions for prevention:

1) Optimize your holding system.  

Design, construct and maintain your pond, tanks to reduce the presence of infectious organisms and the conditions allowing their proliferation.

Provide no rough objects for your livestock to injure themselves on.

Reduce stress by having adequate filtration, circulation and aeration. In particular, having an appropriate size and flow-rate biological filter will go a long way to maintaining a balance of microbe species and populations.

Frequent partial water changes, reducing metabolite build-up are a necessary part of maintenance.

Do not crudely handle, or eliminate handling your fishes altogether. Use nets gently, pushing the fish into a partly submerged container (photo), don't lift them into the air in coarse nets .

Some people advocate the use of salt (ten pounds per thousand gallons) and periodic prophylactic use of therapeutics (see below under treatments).

2) Buy healthy stock; 

seems superfluous to say. Check out your source(s) carefully. If there is evidence of hole in the side in their systems, be suspicious. Better still, assume any and all stock is suspect and:

3) Quarantine and treat all new arrivals.  

Here's where I dive off the deep end and mention involvements in fish shows, auctions and sales.

Build into your holding system a mechanism for totally isolating "new" fish for two weeks minimum. If this is not possible/practical at least administer a sterilizing dip/bath to new arrivals. Formalin/formaldehyde, potassium permanganate are useful for this.

Please note: in defense of shows and new livestock purchases, it should be stated that even people who have not introduced new fish or entered shows for years have had severe furuncular problems.

4) Early diagnoses

observe your livestock closely! Hole in the side "takes off" in Spring and Fall, during periods of rapid temperature change. It is notable here to mention that Furunculosis was first described in 1894 as a problem of salmonid fishes linked to warming conditions. Pond fish are affected by rapid or extreme temperature change both ways.

Be aware that affected stock appears normal, swimming and eating as non-affected until the infection has spread internally to the muscle and internal organs.

5) Isolate affected individuals:  

Unless you suspect many versus one or a few fish are infected, move those fish to their own system for treatment.

Can you spot the fish with the one, beginning sore? The picture is small, hard to make out as is the disease. It's the larger white fish in the middle.


If possible, if you're treating one or two fish, keep them in a smaller, more easily controlled environment. This system must be monitored and filtration and aeration provided for; including it's own biological filter.

Now a disclaimer: As with many diseases, indeed, items of interest in a given field, there are many facts, methods, ideas and attitudes with their attendant experts. Many of the "cures" listed below have worked and not for many people under many varying conditions with many trials with many types of stock. However; what works one year with some stock may be ineffectual with the same stock or others under what might appear identical conditions. Hokay, having said all that, on with the treatments.

1) Topicals:

The actual sore sites may be daubed using cotton swabs with:

A) Mercuric, iodine compounds. Merthiolate, Merbromin, mercurochrome. Just like mommy used to do.

B) Oxidants: Hydrogen peroxide, potassium permanganate solution, copper, formalin/formaldehyde solutions.

C) Antibiotic creams ointments and sprays with non water soluble base. Bacitradium, Myaxin, Neosporin, Polysporin.

2) Dips/Baths:

The oxidants mentioned above are useful in prescribed dosages and intervals. These are most efficacious for new introduction, preventing introduction of parasites leading Furunculosis.

3) Antibiotics:

Should not be just poured into the water fro several valid reasons. For one, all antibiotics will kill the beneficial microbes in your filter to some extent, regardless of claims to the contrary.

Freshwater fish do not drink much (water) contrary to some popular beliefs and you want to get the material into the fish's bloodstream. This can be done by concentrated baths, feeding antibiotic laced food, or injection.

Many types of antibiotics have been used with varying degrees of success. Use of tetracycline, erythromycin and Nitrofurazone have no documented effect on Pseudomonas or Aeromonas.

The best results (presently, mid 1990's) involve the combination use of 1) Sulfamethoxazole and Trimethoprim (brand named Septra or Bactrim), and 2) Quinolones (tablets that must be crushed and mixed with water to make a suspension) (brand named, Ciprio, Floxin, Maxaquin, Naroxin, Penetrex)

Feeding antibiotics is done only on a preventative basis, in the case of discovering and removing/isolating other noticeably affected individuals. It is difficult to gauge the dose applied, as some fish may eat a great deal of treated food, others none.

Mix as much antibiotic powder as will stick to pelletized food in a "shake and bake" fashion in a plastic bag and feed as regular. Some folks add water or oil to the mix and/or prepare for longer periods of time. I like to make and use it fresh.

Making antibiotic laden food: the drying process.

4) Antibacterial Injections:

In severe, advanced cases injection of expensive livestock may be necessary. If you're never done this, get some experienced help to show you how.

There is much "anecdotal" (amateur) reports of "trials" utilizing different substances, concentrations, intervals... The best treatment modum in my opinion is still the 'silver bullet", Chloramphenicol. Kanamycin works well for baths outside the pond. Dosage for injection is determined by weight; about fifty milligrams per kilo or twenty-five milligrams per pound of fish.

There are several other antibiotics in the classes of aminoglycosides, beta-lactamase inhibitors, and cephalosporins that have proven to be efficacious against the gram-negative rod-shaped bacteria in question (if injected consistently and persistently), but as these change in usefulness, availability with time, I will stick with my "sure fire winner". Do not inject tetracyclines.

I suggest intramuscular injections, under a scale in the flank or near the vent with a small (diabetic) gauge syringe. Some people endorse interperitoneal (into the gut cavity) injection as safer, more effective, more long term, less damaging. Unless you've done it before and/or the fish is not valuable...

Showing proper technique: Quick, steady, under a scale, toward the head, keep the animal secure, moist.

Injections should be done three times three days apart. To reduce handling damage you may inject the fish through a partly filled fished bag or wet towel. It takes at least two people working together to do injections properly.

If an anesthetic is desired to calm the fish down, club soda or an Alka-Seltzer (tm) may be administered to the water being used for injecting.

5) Temperature:

Is sometimes touted as an effective treatment. Raising and holding it to seventy five to eighty degrees Fahrenheit with a thermostatic mechanism will effect a quick and permanent cure in some cases. Be careful to slowly lower the temperature back down before returning the fish to the main system; about two degrees a day.

Regarding Pouring "Miracle Drugs" Directly Into Your System

Most times this is a very poor idea; it can cost a lot of money and do a lot more harm than good. Biological filtration may be interrupted and you may outright kill your fish.

The one exception is non-iodized rock salt at twenty pounds per thousand gallons. This may yellow your plants, but will not bother your fish and is a popular tonic as well as a useful adjunct to other treatments.

Ridding the System of Parasite Loads:

This is an integral, and absolutely necessary step in effecting a lasting "cure". Please see the Sections on Koi Selection, Acclimation, Ultraviolet Sterilization Use in Ponds, Disease, Medications, and use of DTHP  (click on for more) for details on treatments and cautions. At the very least progressively adding non-iodized salt at twenty pounds per thousand gallons will eliminate most parasites.

Comments on Other Adjuncts to "Treatment":

Filtration add-ons like ultra-violet sterilizers, protein skimmers, ozonizers are very useful devices for suppressing waste build-up and microbial populations and improving water quality in general. They will not by themselves cure Furunculosis but do decrease incidence and aid in treatment.


Prophylaxis in design, construction and maintenance is valuable, but experience with furuncular conditions is inevitable in even the most ideal systems. Early diagnosis, isolation and treatment of the bacterial "involvement" and predisposing parasites will reduce and hopefully eliminate scarring and livestock loss. Observe your fish carefully. Be especially on the look out for ulcers on the underside, protruding scales, bulging eyes, defects or holes in the gill cartilage, and unpaired fins.


Amlacher, Erwin. 1970. Textbook of Fish Diseases. Tropical Fish Hobbyist. PP 143-146.

Fenner, Bob. 2000. Furunculosis: Hole in the side disease. FAMA 7/00. 

Fujita, Grant. 1976. Nishikigoi Textbook. self-published.

Hansen, Galen. 1993. Aeromonas infection of koi. Koi USA 11-12/93.

Johnson, Erik L. 1993. Koi ulcer disease: practical management. TFH 5/93.

Liles, Tom. 1975. Hole in the Side. Koi USA. Vol. 1:5.

McDaniel, D. (Editor), 1979. Procedures for Detection and Identification of Certain Fish. Pathogens, American Fisheries Society. PP 42-45.

Sniezsko, S.F. 1971. Bacterial Disease of Fishes. T.F.H. Publications, pp 10-20.

Takeo, Kuroki. 1985. Manual to Nishikigoi, Shin-Nippon-Kyoiku-Tosho Co. Ltd., Japan.

Wedemayer, G.A. & J.W. Wood. 1974. Stress As a Pre-disposing Factor in Fish Diseases. U.S. Fish Wildl. Serv., Fish. Leafl. No. 38, PP 7.

Aquatic Gardens

Ponds, Streams, Waterfalls & Fountains:
Volume 1. Design & Construction
Volume 2. Maintenance, Stocking, Examples

V. 1 Print and eBook on Amazon
V. 2 Print and eBook on Amazon 

by Robert (Bob) Fenner
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