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A guide to the use of antibiotics

There have been numerous discussions regarding the use of antibiotics when treating koi. Mostly the discussions are amongst those that are willing to use it and those that are totally against the use of antibiotics.

This article is not intended to promote the use of antibiotics or to convince keepers to use antibiotics. It is merely to indicate the different views regarding antibiotics and also to guide the koi keeper should he/she have to decide in a specific situation whether to use antibiotics or not.




The use of antibiotics is not new to koi treatments but it must be acknowledged that very little scientific tests were conducted to determine the exact dosages for koi. Most dosages currently published in books and on the electronic media is the result of a handful of pioneers that endured lots of criticism and heartache until the dosages were successful. The determination of correct dosages is also complicated by the fact that the dosages for mammals cannot be considered for koi. Koi are ectotherms, in other words they rely on water temperatures to dictate their metabolism. Water temperature therefore plays a crucial part in the effectiveness of antibiotic treatment and the generic dosages are only useful within a certain temperature range.


If one considers the above, antibiotic dosage for Koi, whether prescribed by a Veterinarian or not, mostly becomes a hit and miss affair and luck plays a vital role in the effectiveness of the treatment.

Although the topic of bacteria has been covered in detail in other articles, a short summary is necessary. Very few species of bacteria are disease causing. Most bacterial fish pathogens are Gram-negative bacteria. These are the bacteria that are usually involved with disease such as ulcers, fin rot, septicemia and bacterial gill disease. Bacteria have tremendous reproductive potential. Under ideal conditions one bacterium can multiply to several million within a day!

When should antibiotic treatment be considered?

It is of the utmost importance to determine when antibiotic treatments are necessary. Please remember that when a fish has injured itself for example, by jumping out of a pond, it is generally accepted that 100% of the lesions will be infected. This means that micro organisms will inoculate a wound, but they do not actively divide and harm the host. Soon after the injury, some micro organisms will start to replicate within a wound but they will also not harm the host. An infected wound that warrants treatment implies that replicating micro-organisms within the wound are having a detrimental effect on the host.

The salinity within the body of a freshwater fish is higher than the surrounding water mass and the fish is therefore in a continuous struggle to counter the effects of osmosis. When the protective layer of skin is damaged, a thin layer of skin will cover the wound in remarkably short time to prevent osmosis of water into the fish. The failure of this protective layer to form will be an indication that some pathogenic or bad bacteria are at work.


                                     Wound covered by protective layer

The signs of infection in fish will differ from mammals because necrotic tissue and dead white blood cells are constantly slouched off by water movement. Signs on koi that indicate infection are the failure of a microscopic layer of skin to cover the wound, red rims to a lesion, a steady enlargement of the lesion (progressive wound breakdown), dead scales around the wound, serrated edges of surrounding scales and also accumulated fluid in the pockets of scales surrounding the lesion.


              infection_016           damaged scale

                Tell tale signs of bacteria damaging a wound

When these symptoms are evident, the koi owner will have to make a decision about specific treatment. Mostly an appropriate topical treatment or series of treatments will be sufficient. If the treatment is ineffective, the infection may become systemic. The dilemma that the koi keeper will face is the timing of when to take a swab for culturing. The timing is important because if it is done too soon, the swab will merely indicate the bacteria on the wound, whether they are pathogenic or not. If the keeper postpone too long, chances are that the infection may become systemic. This particular decision is up to the koi keeper and practical experience in this regard will greatly assist.

 fungus_bacteria_systemic Bacterial_infection_fatal

Bacterial infection where the keeper will have no choice but to use antibiotics

Now comes the crucial part. Although not always possible, a fish should only be treated with antibiotics after culturing and sensitivity testing. This process can be approached in different ways, namely blood samples, kidney samples or a bacterial swab. Obtaining a blood sample is difficult for the general hobbyist and a kidney sample means sacrificing a fish as described elsewhere. To the general hobbyist, although not the best way, the bacterial swab remains the most viable option.

So the question remains, what is the correct way to take a swab? The methodology in taking a swab (culture) has not been without its share of controversy. The technique of using a swab for microbiological sampling in the medical profession has been practiced for more than 100 years, but the swab remains a contentious issue, with critics questioning many aspects of this technique, for instance what type of swab should be used, does superficial swabbing of the wound reflect deeper tissue cultures, does the swab miss the particular organisms that are harming the host and how should the swab be rolled across the wound itself?

The procedure is also not without its own set of problems when it comes to fish in general. Millions of bacteria are present in a drop of water and when a swab is taken from a fish the actual pathogenic bacteria may be a very small percentage of the bacteria that eventually ends up sticking to the applicator. Please take into consideration that the gel inside the tube was designed to keep the bacteria alive till it can be cultured at a laboratory. Some bacteria will multiply during transport while others, possibly those that caused the problem may remain only a few or disappear totally from the applicator. The technician in the laboratory will have to rely on the hobbyists to take the samples correctly, to ensure that strict hygiene was observed throughout the process and that the sample was not polluted with some other pathogens. In this regard the Technician will have to rely heavily on experience to firstly recognise the “bad” bacteria and then to do the sensitivity testing on those “bad” bacteria identified.

The way to approach bacterial sampling is firstly to get the correct kit through your Veterinarian, who will also explain the correct way to use the applicator. The abovementioned problems have prompted laboratories to mostly accept only bacterial samples from a Veterinarian. The kit will consist of an applicator and a gel-filled tube. Both will be sealed in a sterile packet. This packet should only be opened when the fish has been prepared and you are ready to take the sample.



Applicator and a gel-filled tube.

When doing the swab it is best to observe the ulcer/wound and determine the most likely places where the bacteria may active. Normally the most active wound breakdown will be around the edges of the lesion and in the accumulated fluid/blood of the infected scale pockets. These are the places where water and topical treatments are ineffective and you are most likely to find concentrated colonies of the pathogenic bacteria.

 Swab1 Swab2  Swab4

 With a sweeping motion of the applicator, collect blood, fluid and some tissue here

A little damage may be caused, but I do not roll the applicator over the wound. I mostly use it in short sweeping motions as if you want to clean underneath the scales. Blood and tissue sticking to the applicator is a good sign. Make sure the applicator do not come into contact with any other surface, insert it into the tube and seal it properly. The sample needs to get to the Veterinarian as soon as possible after the wound has been sealed and the fish released back into the quarantine pond.

In a few days you will receive a certificate from the laboratory listing the bacteria that have been cultured as well as the various antibiotics that have been used to determine the sensitivity of the bacteria.



Make sure that the protocol for the specific antibiotics is followed during the subsequent treatment. Other considerations are the expiry date and the fact that once opened, the expiry date will be of academic importance. Some antibiotics should be used within hours after opening or constitution.

Another perspective

As always, I felt compelled to present the other side of the coin. I have witnessed many instances where medical practitioners will operate on a patient, whether it is a human or an animal and on conclusion of the operation the patient will get a shot of broad spectrum Antibiotics as a preventative measure against infection. That dosage is given on the assumption that it will prevent infection. In various books, articles and on internet, the same practice will be followed after minor surgery on fish. Therefore the insistence on culturing a bacterial sample and to determine sensitivity with absolute certainty before injecting a koi, is most often not always practiced by the very same people that advocate such steps to be taken

I believe when a fish is in dire straits and requires urgent treatment most keepers will follow this example and inject a koi with a broad spectrum antibiotic. And if the broad spectrum does not have the desired effect, the process of culturing may well be followed.

Now also read how to inject a fish

Last Updated on Thursday, 28 May 2015 13:28