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Bacterial infections in Koi  article by Dr Paula Reynolds Aquatic Patho-biologist

The Problem

In recent years, there has been an increase in outbreaks of bacterial disease in Koi ponds.  I am not suggesting a single strain of bacteria has suddenly become more aggressive as the infections I refer to are caused by various bacterial pathogens or harmful agents.  The number of outbreaks over the last 15 years has been considerable and more and more Koi-keepers find themselves dealing with ulcerative and other forms of bacterial infection.  Prevention is far more effective than any cure will ever be and being aware of the possible underlying causes or triggers averts disease outbreaks and alleviates the ever-increasing dependence on antibiotics that now concerns every branch of medicine.  

The Filter and bacterial disease

Modern filtration technology has changed the way hobbyists support their Koi.  Some systems make it easier to overlook the fact that pathogens or harmful agents as well as detritus are harboured in all pond filters.  When cleaning or in any way disturbing the biological part of a filter it is vital to prevent the water returning to the pond immediately.  Instead, it should be flushed to waste for several minutes and a non-return valve plumbed into the system to prevent contamination of the pond is an option in certain types of filter system. Inadequate hygiene during maintenance is a prime cause of outbreaks of Aeromonas the most common bacterium causing disease in Koi and cases can be obvious within a few days of filter maintenance.   The parts of the filter system that are mechanical that remove debris and are cleaned regularly are far less likely to be an issue although the greater the hygiene measures the lower the risk of disease.  In cases when the filter is the cause of bacterial disease several Koi are more likely to have red marks, raised scales or the early signs of ulcers at around the same time and this needs treating immediately. 

New Koi and bacterial disease   

Whilst the filter is the most common factor, there are other reasons for outbreaks of Aeromonas and other bacterial diseases and the introduction of new Koi is another possibility.  If a new Koi is, the culprit it is likely to be the first to show signs of disease that will then gradually spread to other fish as opposed to a few Koi with signs simultaneously.  However, it is also possible for a new Koi to be introduced to a pond in which disease is already in progress and it soon becomes cross-infected.  A further risk are  Koi  whose origins and medical history are unknown or that are being re-homed no matter how healthy they appear as such fish carry the risk of latent disease.  The production methods in some farms can influence the later development of disease once Koi are living in their home pond.  The volume of fish in farms leaves little choice but to resort to treatments that are not ideal during early stages of Koi development.  Antibiotics are used inappropriately as growth promoters in some fish farms that are unaware how damaging this practice is.  The false belief that antibiotics prevent future disease outbreaks rather like a vaccine when their role is curative adds to antibiotic resistance.  In some areas of Koi culture antibiotics are seen as a replacement for proper husbandry and the consequences of this are serious.  Koi are now bred in great volume and it is inevitable that some will become victims of their own popularity as a species.

Pond Conditions and bacterial disease 

Overstocking places, a huge burden on the biological capacity of the pond to support Koi adequately on a day-to-day basis and puts their health at risk.  In addition, in many ponds the over use of chemicals in attempts to cure undiagnosed aliments destroys the filter biology and weakens the Koi predisposing them to disease.  What constitutes good pond management is hard to define due to the variable nature of ponds.  Good quality water is not merely the absence of ammonia and nitrite but in many cases, the complexity of water chemistry if overlooked can leave Koi in less than optimal living conditions.  A further factor is the dissolved oxygen content of pond water that is vital to the biology of the pond as well as the health of the Koi.  The potential for disease is greater when the dissolved oxygen level is low, 6mg per litre is a basic requirement for Koi when resting, and they need more in order to be active and healthy.  

Signs of bacterial disease  

Pond water should be tested and the possibility of parasites eliminated at the first indication of any health issue, as the signs of disease are confusing.  The Koi can be in peak condition when a red area of skin or a patch of raised scales is spotted.  In other cases, circles of white film may develop on the body in one or two places as bacteria attempt to open the skin and an ulcer forms.  Whilst ulceration is a common type of bacterial disease, it presents in other forms.  One Koi may have a reddish tinge over the entire body or only the ventral region is flushed.  The fins may have lost definition, eroded at their edges or become inflamed.  Hyperplasia or fusing of the fins is common following exposure to poor water quality or chemicals and the membrane is then open to infection by water borne bacteria. However, in good pond conditions the loss of fin integrity can also be due to bacterial disease that is not waterborne. Individual Koi can develop an infection specific to that fish alone that poses no risk to other fish.  None the less, the more common types of bacterial disease are infectious and if adequate facilities are available isolation can limit cross infection. When Koi remain in the pond for treatment, early action is important. 

Infamous Aeromonas 

Members of the Aeromonas family are common isolates in disease investigations and Aeromonas Hydrophila is an infamous bacterial strain as it survives in clean ponds and is often regarded as the enemy of Koi-keeping.  In technical terms it is a gram-negative heterotrophic oxidise positive rod with a singular polar flagellum resembling a tail that gives the bacterium its motility.  Aeromonas salmonicida is also well known member of the same genus but is non-motile and slower to colonise in the laboratory.  We use the term facultative for organisms that whilst normally aerobic have the capacity to live in both aerobic and anaerobic conditions and this applies to Aeromonas.  The bacterium can therefore hide in poorly oxygenated ponds and filtration systems or harbour in decaying matter emerging only when disturbed.  It is this capacity that gives rise to my concern regarding pond filtration hygiene.  The Aeromonas family cause various forms of disease that differ in their sensitivity to antibiotics.  The motile Aeromonas species search for nutrients in the mucus to keep their colony alive and reproducing new generations.  The waste products play a major role in how the bacterium attacks and survives both externally and inside the host Koi. 

Aeromonas and Pseudomonas

Aeromonads are found in fresh water rivers, lakes and ponds and not only causes disease in fish, reptiles, frogs, and warm-blooded animals but also in humans.  Outbreaks of some strains can occur as low as 40F/5C but are usually less virulent.   Aeromonas is programmed for survival and it is this that puts Koi at risk.  The higher the temperature the faster the bacterium multiplies and more signs of disease may be observed. Over 65F /17C, the multiplication rate is amazing and in one day, a single Aeromonas organism has the capacity to generate another 30 million if the appropriate conditions prevail.  Depending on previous exposure, each new bacterium can possess genetic material enabling it to resist antibiotics. Certain strains of bacteria team up during disease outbreaks and this facilitates reproduction, ensures disease spreads to new hosts, and supports the survival of the species within the pond. Other organisms are also capable of recognising weakness in host fish and can join in an attack. Pseudomonas is a bacterial family commonly found living in soil and water and on plants, although few are actual pathogens. The most common strains of Pseudomonas isolated during disease outbreaks in Koi are opportunistic pathogens. This means they attack fish unable to mount a strong enough immune defence to repel the invading bacteria. In Koi Pseudomonas is more likely to be a secondary invader as the bacterium seldom attacks fish unless their tissues are already compromised by primary disease such as Aeromonas infection or injury. 


Getting a diagnosis   

To ensure a disease is purely bacterial and to determine a proper treatment regime Koi should be examined. However, in cases of ulcerative disease a diagnostic swab is a second best option as at the very least it prevents Koi being subjected to unnecessary injections that fail to cure them. A swab is taken from an open lesion so that only blood not scales, mucus or skin debris is collected, and this is sent to a laboratory for an antibiogram. A specialist laboratory is not required for an antibiogram although some knowledge of the commensal or common organisms in the aquatic environment and the antibiotics used in Koi is needed. The antibiogram report should be taken to a veterinary surgeon who must fulfil the legal requirements before prescribing an antibiotic. Some vets question the use of antibiotics in fish due to the re-occurring nature of bacterial disease. The concern is that one course of an antibiotic leads to further requests they may have to refuse. Using an antibiotic is a sign that disease is out of control and the problem needs managing. The bacteriology of the pond changes as does antibiotic sensitivity so no antibiotic will work not indefinitely. Koi excrete antibiotics used in their treatment thus creating a breeding ground for resistant bacterial strains in ponds and it is not surprising that in research my laboratory is looking at other disease control methods.


Laboratory techniques 

Bacteria are single-celled organisms with the most amazing survival capacities. As hobbyists  know we separate them into  “goodies” such as those that colonise   biological filtration systems and “baddies” the pathogenic or harmful bacteria that vary in the type and degree of disease they cause. Nowadays DNA sequencing is being used in some laboratories for bacterial identification. This method uses a polymerase chain reaction to multiply a small quantity of the ribosomal RNA gene commonly found in bacteria. DNA assay requires cultured bacterial cells to create primer sequences that reveal the bacterial strain, genus and species. Variations in DNA can be very small but when compared with known bacterial species or subspecies the organism can be identified.  In time, this method will replace conventional laboratory techniques and supersede the   Automated Pathogen Identification system, which utilises the unique properties of bacteria to classify them. This system has worked effectively for many years although it has its limitations.

Antibiotic Treatment 

Antibiotics are usually the only solution in serious bacterial disease outbreaks so it makes sense to use them safely. Feeding antibiotics would be an excellent alternative to injections if the gut was the appropriate distribution centre for every medicine and the dietary route achieved the same level of success as a course of injections. Whilst feeding can work in a highly controlled aquatic system, Koi fail to consume the dose level required. The variation in the size of the fish and their reduced appetite during disease makes feeding an antibiotic unsatisfactory and allows disease to continue. Antibiotic injections deliver a therapeutic dose of a drug when injected correctly. They are not multipurpose, an injection formula antibiotic should not be fed, and an oral formula should not be injected. Do not mix antibiotics together or inject two into separate locations unless this has been confirmed as safe as antibiotics do not work identically and they can conflict. Calculating the amount to be injected is based on the dose level of the antibiotic, the volume, or strength of its formulation and the weight of the fish. Accuracy is important as an overdose can initiate toxic shock and under dosing is pointless, and might inhibit further treatment. Always weigh Koi do not rely on length charts as they can be inaccurate particularly in female Koi. In addition, administering the correct number of injections at the appropriate frequency can be the difference between success and failure.


Injecting Koi

Human insulin syringes or their equivalent are ideal to avoid a large skin puncture. Fill the syringes in advance and expel the trapped air.  Sedate the fish one at a time, clean, and treat the lesion with an iodine-based product when giving the first injection. However, try not to disturb healing when giving subsequent injections and if possible leave the lesion alone. It is not advisable to sedate Koi frequently and a 3-day interval can be appropriate if this suits the recommended injection frequency. However, cases will differ with the antibiotic and the water temperature and very sick Koi may require a different treatment entirely due to advanced disease. If the antibiotic in use is inappropriate for the disease, the gap between injections is too long or too close together or the dose incorrect the Koi are not likely to respond. Heating the water can aid healing but could also exacerbate the disease. It is safer to wait until skin healing is observed before gradually applying heat. 

Injection sites 

An injection of antibiotic is regarded as a safe procedure but this is not always the case. Reactions can occur at the injection site either to the drug or to poor injection technique. The needle creates an entry portal for infection into the body and the fish has to return to water containing pathogens so use an iodine based product on the injection site. Antibiotics are nearly always injected intra-muscularly and whilst injecting the body cavity can quicken the response time, this is only suited to certain drugs and it poses risks. I do not advise injecting the pectoral muscles as serious damage leading to mortalities can result. The pectoral muscles are underdeveloped in many Koi and cannot cope with the fluid level and the injectant often leaks out from this location creating an under dose. The safest injection site is directly into the thick back muscle accessed at the rear of the dorsal fin. Holding the long back fin erect exposes a tiny scale free triangle of skin directly behind the fin, which is easier to see in larger Koi.  

Other bacterial diseases 

Many diseases caused by bacteria that can afflict Koi are confusing due to outdated terminology and new disease classifications. For example, books often refer to slime disease and originally this was the name for a tropical fish infection caused by protozoa. However, this name has been used to describe any infection that causes excess mucus. If the mucus develops a fluffy appearance a fungal infection could be suspected  that is until skin or gill lesions appear and a new  diagnosis is needed or  guesswork  may  lead to  the  wrong treatment . Mycobacterium is from the same genus that causes tuberculosis in humans. The disease is seen in Koi in different forms and hobbyists frequently suspect a case based only on lethargy and weight loss but such signs also apply to numerous diseases and conditions. Whilst cases can escape detection, Tuberculosis is a rare disease in Koi and it does not warrant the many cases attributed to it.  

The disease barrier  

Koi can be afflicted by diseases that other fish species in the same pond cannot contract. Although not all serious diseases in Koi are host, specific many are. Some viruses afflict only a few Carp species yet many members of the large Carp family can contract others. In the case of a viral disease afflicting Koi that Orfe cannot contract, it is not safe to move the Orfe to another fishpond as even unaffected fish can act as an intermediary and spread disease. Environmental diseases will stress all the fish in a mixed species pond. If the water conditions are poor, most signs will be common to all the fish although some reactions will be the natural predisposition of each individual species. A good example is that many pond fish tolerate parasitic infections better than Koi do. There are a few fish diseases that can be passed on to humans. Tuberculosis is a possibility although it can only be contracted in certain forms making the risk to humans from Koi extremely low. Leptospirosis is an acute bacterial disease humans contract from infected animal urine, and whilst curable the symptoms are extreme. Water is a common place to find Leptospirosis and rats leave little sign of having taken in dip in a Koi pond. In addition, Aeromonas Hydrophila can cause a gastro-intestinal infection in humans if a Koi keeper ingests enough pond water harbouring the bacterium. I have also seen various allergies in humans after handling both healthy and diseased fish and wearing rubber gloves is the only option despite their drawbacks. However, it is extremely rare for healthy pet fish to pass on disease to humans, however, pond water should not be overlooked, and it is good practise to wash your hands after they have been in pond water.

Koi keepers are pond managers 

It is important for me to make a distinction between my research work into diseases and conditions Koi keepers are never going to see and the health problems they may have to deal with triggered by oversights in pond management. Whilst my work in Fish Medicine is extremely diverse, I am well aware of all the issues that can arise for Koi keepers. I know how contradictory information confuses rather than helps hobbyists and I do advise caution before taking any advice from the Internet. At LFH laboratories, my team is currently working on various projects that will help Koi-keeping in the future. However, pond management is something all hobbyists need to consider more.  It does not simply mean testing water parameters regularly or careful filter hygiene as it must include every aspect of fish welfare. Special, attention should be given to the Koi diet and a feeding regime in place for the colder months as so many Koi are not fed when active.  Monitoring the Koi daily even when it is snowing to ensure their behaviour is normal is important. How the pond is stocked, is yet another factor and this means buying new Koi from the safest possible sources. Research has taught me there is currently no better way to protect an existing healthy Koi collection from the introduction of disease than by purchasing Koi from a dealer who knows the real value of biosecure quarantine. 


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