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Is it Hikkue? 

article by Dr Paula Reynolds Aquatic Patho-biologist


Hikkue can be loosely translated as red gone and when applied to Koi refers to the loss of red or hi skin colouration. There is speculation in Koi circles as to whether certain skin lesions are Hikkue or not and these discussions largely depend on what everyone believes the term Hikkue currently refers to. The loss of red pigmentation due to genetic weakness is a possibility although abnormality of the skin is rare in such cases. Many years ago, a parasite known as the “hi-eating worm” was alleged to cause Hikkue by feeding on the red cells despite ample evidence to the contrary. The organism that triggered this concern is extremely rare in imported Koi now and thankfully, that particular myth no longer circulates. My laboratory has seen many cases of skin cancer and a percentage do involve the red pigment cells. None the less, in recent years various skin lesions in Koi have been examined that were thought to be cases of Hikkue by their owners. In some cases, the fish refused food and mortalities had occurred and it is not surprising various unrelated pathogens were isolated. It is likely that the similar appearance of certain types of skin lesion attracts the label Hikkue.



Often the loss of red colour is a genetic predisposition inherited from the parent fish although extremely stressful events can also be a trigger. Several stunning Tancho Kohaku handpicked in Japan surprised their owner by surviving overnight in no more than a puddle when their pond water accidentally drained away. His shock was greater when he realised his prized Koi no longer possessed any distinctive red pattern as it was  either gone already or fading fast. The pigments in the colour cells have a slightly granular texture and their retention requires carotene and spirulina in the Koi diet. Water parameters also play a role in colour preservation, and each Koi variety has its own unique needs. The carbonate hardness, mineral content, p.H, as well as the temperature range the fish live within can all influence colour retention to a degree. Acid rain and chemical pollution are also factors that have to be taken into account in colour maintenance. The red colouration may be a stunning feature but it is notoriously unstable even in healthy Koi. The capacity of cells to produce or withhold pigment means Koi may be in their middle years before their skin pattern and colour is regarded as stable. In some varieties colour diminishes in their later years and this means that some Koi will be at their peak for relatively few years. Koi Judges look for stability of body shape, skin quality, colour conformity, pattern, and lustre whereas I look for Koi that are off colour as a slight decrease in the overall colour intensity can suggest a health or environmental problem.  



All the pigments seen in Koi colouration can redevelop following injury depending on the degree of skin damage, and despite its vulnerability, this includes the red pigment. However, if the skin is normal and the red colour gradually fades that type of pigment loss is invariably permanent and improving the diet or living conditions is unlikely to help. A koi in which the red pigment alone is targeted and the skin slightly raised is enough for the term Hikkue to be suggested. However, the human eye sees only external signs and this is not sufficient to confirm the nature of any disease. Treatments often have little success for the obvious reason that skin lesions have differing underlying causes and the only common factor is one or more skin lesions. The experience of Koi keepers who believe they have a single Koi with Hikkue is likely to differ considerably from hobbyists who suspect they too have Hikkue although several Koi are infected and mortalities are taking place.



In some ponds, one Koi may have a patch of blistered skin due to excessive sun exposure and this usually heals without intervention. Koi that expose the dorsal region by swimming high in the water or sunbathing in a shallow area are more vulnerable to sunburn. Only if the skin is open is there a need to treat with an iodine based product. At the other extreme, skin blisters can be the first stage in a form of skin cancer. In certain cases, this is treatable although success depends on how advanced the disease is, it is impossible to generalise. Koi can live with this type of cancer for some time without secondary complications despite the serious nature of the disease. In the more common form of this type of cancer, the skin becomes slightly raised and later has a miliary or rash like appearance. A tissue sample can be subjected for tests to detect antigens in the cells and as antibodies bind to specific antigens in tissue the  test reveals more about the nature of the disease under investigation and ultimately if it is cancer or another disease entirely. In cases in which skin cancer is confirmed, the disease is specific to the infected Koi and is not contagious to other fish. In cases were several Koi have skin lesions the underlying cause may need investigation.



The reaction of any fish to disease varies with the nature of the pathogen or harmful agent, and the immune response in the exposed fish. Skin changes are not an aspect of every disease, and whilst ulceration is a classic sign of bacterial infection, there are strains of bacteria that create very different external signs or none at all. Hikkue was the word used by a Koi importer to describe skin lesions in some newly imported Koi. However, when the pathogen was isolated in my laboratory, a strain of bacteria very rare in Koi was the culprit and measures were taken to prevent further cases in the farm. The use of Hikkue when the problem may be undiagnosed has parallels throughout Fish Medicine. A good example is Slime Disease, which is described to me regularly by hobbyists for the obvious reason shedding mucus is a very common sign in numerous Koi health problems. Humans are also guilty we have a cold and tell everyone its influenza.



In reality, serious disease is rare in a well-managed pond particularly if new Koi are bought from a biosecure dealer. Hobbyists cope with day-to-day issues it is the diagnosis of serious disease that cannot take place at the pond side. Photographs do not help I receive many and even those actually in focus cannot help. The question arises in a pond with a health problem as to whether a disease is contagious to other Koi. Invariably the clue is the number of fish with signs of infection. However, disease does not always cross-infect in the manner or period hobbyists expect, and water temperature is influential on disease. Without pathology, assumptions have to be made and euthanasia is a humane option for Koi that are suffering when treatment is impossible. However, I favour giving every Koi the chance of life as long as it can swim around and feed normally, and it poses no risk of passing on disease to other fish. Confusion about the nature of any health problem can lead to euthanasing Koi unnecessarily and that is not good Koi keeping.


Anyone with concerns about this topic is welcome to ring LFH Laboratories.

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