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Antibiotics and the treatment of bacterial disease in Koi

Bacterial disease has been more prevalent in Koi in recent years.  Whilst Koi are cultured carp and a less hardy species than their wild carp cousins, this is only a facet in their susceptibility to disease.  Outbreaks of all forms of bacterial disease have increased and the prime trigger is the management of pond filtration.  In addition, low levels of dissolved oxygen and inadequate water quality and chemistry are also contributing to disease. If we then factor in the popularity of Koi that results in overstocked ponds and the possible introduce of disease by new Koi, it is obvious why some ponds are becoming more conducive to bacterial diseases.

Cause and effect
Some filter management methods allow harmful bacteria harboured in all filter systems to enter the pond after cleaning.  Flushing the water to waste for several minutes after any maintenance procedure before allowing it to return to the pond is the safest method. Many systems have a non-return valve in position to make this easy.  If bacteria have not been flushed to waste and the pond conditions are appropriate particularly water temperature several Koi may show signs of infection within a few days of the filter maintenance. However, when a new Koi is carrying disease, it is that particular fish that is more likely to develop signs first that gradually spread to other fish depending on water temperature. An open wound on one Koi can be the result of injury and may not be an ulcer. However, when several Koi all develop skin lesions bacterial disease is the likely cause not a sharp object.  These patterns apply only to certain forms of bacterial disease and ulceration is only one way in which bacteria can attack Koi. LFH manufacture Envirex Biological Pond Cleaner that controls the various bacterial species in pond water and lowers the risk of disease.  Whilst Envirex cannot cure disease, if it is used regularly it will help to prevent it and also reduces the work of pond and filter hygiene; details are on the products page.

There are many species implicated in bacterial disease although it is principally strains of Aeromonas that cause the ulcers familiar to hobbyists.  The individual strains within this bacterial complex vary in their sensitivity to antibiotics, which explains why no one antibiotic will cure all ulcerative disease.  To add to the complexity, if Aeromonas and another bacterium are isolated in the same outbreak more than one antibiotic could be required although this would only be known by investigation.  Those bacteria capable of independent movement such as the motile Aeromonas species are water borne and can reside on the skin searching for nutrients in the mucus layer. This activity is controlled by interaction with components of the Koi immune response and biochemical reactions in mucus and water. Aeromonas hydrophila is a well-known bacterium secreting proteins that aid the attack on the Koi and Aeromonas produces beta lactamases associated with antibiotic resistance.  Aeromonas outbreaks often occur in spring as water temperatures rise. It is largely the Koi-keeper's ability to manage the disease that decides the outcome or if it will re-occur. Success measured in the short-term by healing ulcers may not be a complete control of the disease as new outbreaks are possible.

Minor skin lesions
It is important that antibiotics are used for serious outbreaks of disease and not to treat minor red spots or a raised scale as this will add to antibiotic resistance as new generations of bacteria are able to survive its use.  In ponds where antibiotics are used too often, the day will arrive when they are no longer are effective.  A hobbyist can deal with a minor ulcer without an antibiotic by using an iodine-based product applied every 2-3 days on and around the ulcer.  This can stop the attack on the skin and the disease may never enter the body as it is once this happens that antibiotics will be needed.  It is well worth persisting with iodine applications as long as the ulcer is turning white around the edges and the white area is slowly getting wider towards the centre of the ulcer.  Ultimately, the area will become whiter and glossy although the skin colour can take a while to return.

Diagnosing disease
Accurate diagnosis requires bacteriological samples taken from a recently dead fish at post-mortem, or without sacrificing a fish from a live Koi in laboratory conditions.  In the case of bacterial disease causing skin ulceration, a swab taken from an open ulcer is an alternative for hobbyists who do not have seriously ill Koi. A swab is a diagnostic tool with limited uses in Koi.  It resembles a large cotton bud although the tip is not cotton as this destroys bacteria. Accompanying the swab is a tube with preservative to keep the bacteria viable in transit.  Swabs are not always satisfactory and may fail to detect the primary pathogen. This is often due to how the swab is taken, as blood alone is required not mucus or debris from the skin lesion.  Often hobbyists do not know how to get the right advice and the treatment of their Koi is based on guesswork and luck.  The main drawback is whether the problem is completely cured as in  many ponds Aeromonas outbreaks keep reoccurring without guidance.

Which antibiotic?
Antibiotics either kill or inhibit bacterial growth and Enrofloxacine (Baytril) is the best-known antibiotic used in Koi.  If administered in a proper treatment plan it can be far more successful than its current ad hoc use.  It is the need of knowledge and experience that suggests to a Koi keeper a certain drug is not working when in reality it might be effective if used correctly.  Enrofloxacine has a broad-spectrum action and is safer for Koi compared with many antibiotics. However, bacteria are now isolated able to resist it and resistance is a major problem in all branches of medicine.  Using an antibiotic with the potential to damage the major organs in Koi is something hobbyists might do without realising it as many medicines are available illegally.  Although the fish may recover initially, certain antibiotics can shorten the life of Koi and injection techniques are often poor causing abscesses and the loss of fins when an inappropriate site is used. It will not be a popular suggestion with a few Koi keepers but it is fact that it is safer to take proper advice before resorting to the use of any antibiotic.

The Antibiogram
An antibiogram is a process that identifies a specific bacterial pathogen and the most sensitive antibiotic to treat the disease.  An aquatic laboratory has the advantage in Koi investigations as the diseases and commensal organisms are understood, and the incubation temperatures target Koi diseases.  Hospital laboratories for example operate their incubators at temperatures appropriate to human disease.  Bacteria are cultured to obtain colonies of the bacteria on a sterile growth medium called agar. The choice of agar depends on the organism suspected and the temperature at which incubation will take place. Tryptone soya agar is commonly used in fish microbiology and this is incubated at 22C, a low temperature compared with human pathology. Each individual bacterium can produce a colony of organisms after just 24 hours in incubation so it is not surprising they breed out of control in ponds.

The next stage
The methods used in all laboratories for identifying bacteria are being superseded by equipment that provides a faster result and at LFH we have the option of a PCR machine.  For years, the properties bacteria possess have been used to identify them as they vary in shape and size and produce substances exclusive to their various groups. When exposed to special reagents these substances initiate a colour change that codes for a specific bacterial strain.  Once cultured complete colonies of bacteria are placed in a sterile saline solution and the resulting bacterial suspension is applied to the surface of a different type of agar.  Paper discs impregnated with various antibiotics safe for Koi are spaced apart on the agar and incubated.  The result will be clear circular areas around the discs that have inhibited the Aeromonas and as the discs vary in antibiotic volume this also suggests the dose level required.  An antibiogram report is then issued with all the results.

Obtaining an antibiotic
The law requires a veterinary surgeon to ensure all requests for an antibiotic are appropriate, as their use has to be controlled. Hobbyists unwilling to stress sick Koi by taking them to a veterinary surgery may not realise vets do not have a statutory obligation to supply antibiotics for fish.  It is a fact that the need for an antibiotic might suggest an underlying pond problem that no amount of antibiotics is ever going to cure and vets are aware of this fact. At LFH, we issue a laboratory report on cases we have dealt with and discuss our findings with vets and this can help.  A vet has a duty of care and will help in cases in which they have confidence in the Koi keeper so forming a relationship with a local vet is the best approach.

Large and small
With adult Koi a course of injections is the most effective method of treatment. With very small Koi or when there is a very large volume of fish to be treated the oral route may be the only option. However, impregnating Koi food with an antibiotic is less successful as it fails to provide a therapeutic dose level of the drug in every fish and the disease is far more likely to be ongoing. Oral antibiotics are not suitable for injection and those for injection should not be used in food.  It is inadvisable to use two different antibiotics at the same time without proper advice, as there can be conflicts.  To use antibiotics effectively  follow a planned approach to treatment, this helps to avoid resistance and abuse issues and is in the best interests of all the Koi in your pond not just those that are sick.

The treatment
When a hobbyist obtains an antibiotic, a course of treatment should be outlined. Administering one injection only works with specific types of antibiotic so a course of several is the norm. Whilst there are various routes for other drugs all antibiotic injections should be into the scale less triangle immediately behind the dorsal or back fin, this is easier to see on larger Koi.  There are many reasons why this is the safest site the important one being that less injectant leaks out. The syringes need to be equivalent to those used for human insulin.  When only one or two Koi have ulcers, there is an argument for treating only those infected.  However, once Aeromonas spreads widely there is a case for treating all the Koi.  Antibiotics cure disease not prevent it like a vaccine, however, in situations with a high potential for cross-infection the unaffected Koi are often injected. There are types of bacterial disease in which this protocol would not be advisable and I am referring to some cases of ulcer disease. It is better not to over treat an ulcer while antibiotics are being administered unless it worsens and applying iodine at the first injection may be all that is required. Only deep ulcers need a sealer and sealing an infected lesion not treated with iodine is often counterproductive. The stress of netting, sedating and injecting fish must be considered at the outset, as extremely sick fish are less likely it is to survive and in some cases, euthanasia is more humane.

Disease in perspective
Antibiotics cannot ever replace good husbandry and without Koi keepers being aware, reliance on them is becoming an acceptable practise. In reality, optimum pond conditions are the best disease deterrent and this is not simply water free of the toxic ammonia and nitrite. Every aspect of water chemistry should be assessed and the underestimated dissolved oxygen levels monitored, as this is critical to both to Koi health and pond biology. Aeromonas is a facultative organism, which means it can survive with or without oxygen and it thrives in dirty areas of the pond and filter and in dead water that never circulates. We humans have relied on antibiotics for too long and they have become one of the most abused substances on the planet creating the current major concern. If despite all precautions bacterial disease breaks out, by seeking sound advice in many cases antibiotics can be avoided. Koi keepers need to create a pond in which harmful bacteria cannot breed and flourish.

March 2016

Antibiotics: Prescription only treatment for bacterial disease.
Lesions: Any abnormality, swelling, wound, ulcer.
Pathogen; Any agent likely to cause harm to fish.
Systemic: Inside the body rather than external.
Bacterial culture: Growing bacteria.
Commensal organisms: Common life forms on fish and in water.
Antibiogram: Antibiotic sensitivity test.
Chemotaxis: The movement of an organism for a purpose.
Bacterium: A single bacterial organism.
Bacteria: Plural of bacterium.
Metabolites: By-products.