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Antibiotics in the treatment of Koi

It is a fact that bacterial disease is more prevalent in Koi now that they have achieved worldwide popularity. Being cultured pets well removed from wild carp Koi are now a less hardy species. However this is not the root cause of the increase in outbreaks of bacterial disease as most cases have an environmental trigger factor and overcrowded ponds are the commonest cause of this type if health problems.  The pond is a highly conducive environment allowing bacteria to flourish undetected until it is too late and, when disease strikes, disease occurs usually in the form of ulcers.

Skin lesions
There are many bacterial species with the capacity to cause disease in Koi but it is principally strains of Aeromonas that are responsible for the ulcerative forms of disease familiar to hobbyists. The individual strains within this bacterial complex vary in their sensitivity to antibiotics which explains why no one antibiotic will ever cure all ulcerative types of disease in Koi. To add to the complexity, if for example Aeromonas and another bacterium are both isolated in a disease outbreak in the same pond, more than one antibiotic or other drugs could be required, but this situation would only be known if an investigation was carried out.

Biochemistry in action
The activities of bacteria and their metabolites on the epithelium or skin of Koi are influenced bio-chemically. Those bacteria capable of independent movement such as the motile Aeromonas species may search for nutrients in the mucus layer but this behaviour is controlled by biochemical substances which respond to stimuli in the mucus or the water or are interactive with components in the immune response of the fish. We refer to some of this activity as chemotaxis. Aeromonas hydrophila is a well-known bacterium secreting several extra cellular proteins that are responsible for some of the damage to skin and tissue. Aeromonas bacteria also produce beta lactamases which are associated with drug resistance and as newer types of antibiotics are used in Koi the difficulties created by resistance increase. Beta lactamase genes can be secreted by plasmids that are resistant to all antibiotics.

Diagnosing disease
For accurate disease diagnosis, bacteriological samples should be taken from a recently dead fish at post-mortem. A swab taken from an open ulcer is an alternative for hobbyists but this method does not always prove satisfactory and can fail to capture the primary pathogen with misleading results. A swab resembles a large cotton bud although the tip is not cotton as this can destroy bacteria. Accompanying this is a tube with preservative to keep the bacteria viable in transit.

Which antibiotic
Antibiotics are a range of natural materials which are in fact secondary metabolites and many are now produced synthetically. Enrofloxacine (Baytril) is probably the best known antibiotic used in Koi. Its broad spectrum action and its safety compared with other antibiotics that can cause side effects has increased its use, however bacteria are now increasingly isolated in cultures able to resist Enrofloxacine which limits its success. Resistance to antibiotics is a growing problem with larger implications than the Koi pond.

The Antibiogram
An antibiogram is the procedure that determines the most sensitive antibiotic to treat a specific pathogen. An aquatic laboratory has the advantage over others when it comes to Koi investigations as the pathogens and commensal organisms are understood and the incubation temperatures for cultures target the disease, for example hospital laboratories operate their incubators at temperatures appropriate to human disease.

Culturing bacteria
Bacteria are cultured to obtain colonies of the pathogen 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. This is incubated at 22 C, 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 can rapidly breed out of control in ponds.

Identifying bacteria
Bacteria possess variable properties that are utilised for their identification. They vary in shape and size and also produce substances exclusive to certain bacterial groups. These substances are exposed to various reagents to initiate a colour change reaction which identifies the bacteria. Fungus and other matter from the body of the fish, or the pond water, must not contaminate the culture, as many organisms that are secondary invaders could be present. The culture must isolate what is termed the primary pathogen in order to relate to the actual disease the Koi are suffering from or it will be of no value.

The antibiotic sensitivity test
Once grown, one or two colonies of bacteria are placed in a sterile saline solution and the resulting bacterial suspension is applied to the surface of a new type of agar. Paper discs impregnated with various antibiotics suitable for Koi are placed equally apart onto the agar, and a further incubation takes place. The result will be circular areas of variable clarity around some of the discs, and one or two that are completely clear. The antibiotics actually inhibit the growth of the bacteria, and the clearest zone around a disc is the appropriate antibiotic for use in the treatment. The discs vary in the strength of antibiotic they contain to provide dose level information.

The wisdom of using antibiotics at all
The stress of netting, sedating and injecting fish during a course of treatment must be considered at the outset. It is also vital to appreciate that antibiotics can have long-term side effects and should only be used for serious outbreaks of systemic disease. With small Koi the oral route is favoured, as injection is not an option. Impregnating feed with antibiotic is less successful and fails to provide a therapeutic dose level of the drug. With larger Koi, a course of injections is usually appropriate in most cases.

Getting help from a vet
Koi-keepers may not realise that vets do not have a statutory obligation to treat fish or supply antibiotics as fish are not covered by the Veterinary Act. If a vet is willing to help a Koi-keeper the law requires that the vet ensures there is a genuine need for antibiotics, so most will want to see infected Koi. Veterinary Surgeons may not want to take on fish as patients but they do have in depth knowledge of the disease process and the role of antibiotics and many vets will help Koi-keepers treat bacterial disease. However, repeated requests for antibiotics could suggest to the vet a failure in pond management is the root of the problem.

Managing the problem
Aeromonas outbreaks often occur in spring as water temperatures rise. An open wound or ulcer on one Koi can be the result of injury, but when several Koi all develop open skin lesions, bacterial disease is the likely cause. It is largely the Koi-keeper's ability to manage the disease that decides whether there will be a successful outcome or if the disease will re-occur later. Success should not be measured in the short-term by the evidence of healing ulcers but only when there are is no new outbreak the following spring. When only one or two Koi have ulcers there is an argument for treating only those infected. However once Aeromonas spreads to many fish there is a case for treating all the Koi in the pond with antibiotics.

Disease prevention
Biological pond cleaners will lower harmful bacteria in pond water. Such products cannot cure systemic disease but will help to prevent it and they can take the hard work out of pond and filter management. The pond filter is often the culprit when bacterial disease occurs and this has much to do with the cleaning process, always flush to waste after any filter disturbance. Quarantining new Koi is a vital preventative measure in reducing the spread of disease. Many ponds are seriously overstocked and this increases all health risks and is a common cause of Aeromonas outbreaks.

Antibiotics in perspective
Antibiotics are a short term cure not a preventative measure and cannot ever replace good husbandry. Optimum conditions are the best deterrent and an assessment of every pond parameter enables hobbyists to evaluate their pond as a hygienic home for Koi with less chance of an Aeromonas outbreak. When considering the parameters never stop at testing just ammonia, nitrite or pH but include the much underestimated dissolved oxygen levels which can be critical to good Koi health. Aeromonas is a faculative organism which means it can survive with or without oxygen but thrives in dirty areas of the pond and filter and dead water which never circulates. It would be a rarity to isolate Aeromonas from Koi living in optimum conditions in pond with plenty of water volume per fish although Koi keepers need to realise it is ever present in pond water.

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.