Hikkue The word Hikkue The word Hikkue can be loosely translated as red gone and in koi this refers to the loss of red skin colour. There are several reasons why the red pigmentation fades or undergoes change and as the term Hikkue is often applied to them all this can be confusing. Many years ago, a parasite known as the “Hi-eating worm” was alleged to cause Hikkue by feeding on the red cells and it took me many years to dispel this myth by offering a reward if someone could bring me the worm and, naturally I am still waiting. Recently a koi keeper was told all his koi were suffering from a contagious disease known as Hikkue when no such disease exists. Koi keepers need to be aware that the term can be applied to various skin conditions irrespective of their actual cause. Red or Hi stability Often the loss of red pigment is a trait 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 colouration.  The pigments in the colour cells have a slightly granular texture and their retention is supported by 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 (KH), mineral content, pH, as well as the temperature range the fish live in can all influence colour retention to a degree. Acid rain and chemical pollution are factors also to be taken into account in colour maintenance. The red colouration may be a stunning feature but it is notoriously unstable even in healthy well fed koi. In some varieties colour diminishes with age and this means that many koi are at their peak for relatively few years. Koi Judges look for colour conformity whereas in my laboratory I look for koi that are off colour as, a slight decrease in overall colour intensity can suggest a health or an environmental problem. Minor problem or disease All the pigments that give koi their skin colour can redevelop following injury and are also capable of regeneration after   certain diseases depending on the degree of skin damage, and despite its vulnerability this also includes the red pigment. However, when the skin is completely normal and the red colour is gradually fading in mature koi this usually indicates a pigment loss that will be permanent although in young koi colour can still be emerging.  Koi ponds are gin clear and this can allow more of the damaging rays of the sun to penetrate the water. One koi may have a patch of blistered skin due to excessive sun exposure and this usually heals without intervention. However, koi that expose the dorsal region by swimming high in the water or sunbathing in shallow areas continuously are going to be more vulnerable to sun damage.  Only if the skin is open is there a need to treat sunburn with an iodine-based product such as Tamodine and in most cases this will heal. Skin cancer Koi are sent to my laboratory for examination with various skin abnormalities. It is when the skin fails to heal irrespective of what has been tried that a skin disease is more likely. There are various forms of cancer in koi and I apply the term Hikkue only to cases of a specific form of skin cancer. This commonly begins as a raised area and in some cases the skin develops a miliary appearance and then blisters and fails to heal. Most cancers of this type begin in the red pigment cells although not exclusively and in time spread into other areas. It is possible to test for certain cancers and this can be useful when the external signs of disease are not classic as this suggests the possibility that a lesion is benign and could respond to treatment. Skin cancers and other skin conditions can be treated with a degree of success depending on how advanced they are although a complete cure cannot be guarantee. However, koi can live with certain skin cancers for many years without any secondary complications. I see no reason to put koi through unnecessary treatment and I do not advise ending the life of any fish capable of a normal life when skin cancer poses no risk to other fish in the pond.