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Skin Problems

article by Dr Paula Reynolds Aquatic Patho-biologist

Carp have lived on earth far longer than humans and evolution and adaptation has enabled them to survive in new or hostile environments. Today numerous species and sub species make Carp the largest fish family widely spread around the world. However , the  cultured Carp we  know as  Koi   bear  little resemblance to their ancestors  and have been  created  by selective breeding their skin like an  artist’s canvas for the skills of the Koi producers. It is the unique appearance of individual Koi that makes them desirable and popular pets. In Koi, skin is their greatest asset and it makes sense to ensure it protects them all their lives. It can be difficult when looking at the skin of a sick fish, to know if a disease or condition is affecting the skin alone or if what is visual is actually part of a more serious disease process. Most minor issues hobbyists learn to cope with and the diseases and conditions that cannot be diagnosed at the pond side and might need laboratory help to treat are  thankfully rare in a well managed  Koi pond.

The Cuticle

In Koi, the cuticle is the term given to the outermost layer of body and it is the home of the mucus layer. Mucus provides Koi with their first line of defence as it is part of their immune system. Natural secretions team up with antibodies and proteins to deter adherence to the body by pathogens such as bacteria, viruses, and ecto-parasites. The composition of mucus can differ Koi to Koi due variations in the immunity they were born with or have acquired in life. The best way to understand immunological memory is to imagine a database storing the codes to mount an appropriate defensive attack against specific diseases. The consistency of mucus can be changed by disease and by the conditions in which the fish live and can be the first indication of any health problem. Mucus can harbour ecto-parasites that are the type that live externally on fish. Whilst some only irritate Koi and are not directly parasitic, others can damage the skin and open it to infection. Mucus biopsies or smears viewed with a microscope confirm which parasite is the culprit so the most appropriate treatment can be administered, as this is safer than using chemicals one after the other based on guesswork.


The Scales  














The skin beneath remains unexposed , only then can the development of scales way back in evolution can be seen to be a great advantage for many fish species . It is suggested that each scale in Koi equates to the human fingerprint revealing otherwise unknown details about individual fish. Scales do reveal what are inaccurately termed growth rings. These rings reflect the phases of dormancy as well as periods of growth and will not calculate the age of a fish with accuracy. Whilst mature Koi may appear to have more scales than juveniles in fact Koi have 1200 scales all their lives, they simply grow larger at the same rate as the body. When damaged by injury or disease the scales can regenerate given time and the right conditions. However, the healing capacity of the scale pockets very much depends on the degree of damage they have sustained.

The Skin  

Epithelium is the correct term for the skin that obviously forms a physical barrier between the water and the organs of the body. Scales alone could not achieve this, as it is vital that water accesses only the parts of the body, such as the gill, where it can be properly processed. Otherwise, the balancing of vital body fluids and salts could not take place and life in water would be impossible. In addition, the skin exists to prevent harmful microorganisms gaining entry to the body. Despite its protective capacity, the skin must look good, as it is their highly individual markings that set Koi apart from other species and one another. We may use the same descriptive terms when referring to the skin of humans and Koi but that is where any similarity ends. We do not share the same properties and humans would look in the mirror at an eternally youthful complexion if nature had endowed us with the skin possessed by Koi. Covering the body and fins, the Koi epidermis is capable of renewing its cells by what is termed cell division when worn out by time or damaged by injury or disease. Wrinkles would not exist if only humans shared such properties and Koi possess even more unique features. The Schreckskoff cells secrete a pheromone that is released into the water by an injured Koi to warn others of imminent danger and these cells are also located in the epidermis. Koi in the vicinity respond to this chemical message and avoid the area. This saves the lives of Carp in the wild and whilst similar support systems exist in the animal kingdom, humans have nothing comparable. Also located in the epidermis are the goblet cells that secrete the mucus that forms the defensive layer in the outer cuticle. Mucus is part of the immune system and comprised of antibodies and proteins it is there to defend the fish. Immunity to all health problems will vary fish to fish.


Moving down a layer to the dermis we found the chromatophores or pigment cells it is their colour and type that give Koi their endless variations. The irridocytes are cells comprised of material that reflects light such as seen in the skin of Ginrin Koi varieties.  Without the composition of the chromatophores Koi would look alike and Koi-keeping would be a very different hobby if it even existed. The dermis lies beneath the epidermis and is comprised of cells found in most mammals termed malpighian cells. These cells are tough and more fibrous than others that make up the skin. The actual thickness of the dermis can vary in each individual Koi variety. Those Koi with little scalation have a thicker dermis to provide greater protection in the absence of scales and the few scales they do have are for adornment. The deepest layer of skin is the hypodermis consisting of fatty tissues that are far less compacted than the higher layers. The flaccid texture of the hypodermis means this layer makes the final connection that attaches the skin to muscle tissue and other parts of the anatomy.   


Skin is vulnerable

The skin in Koi is vulnerable and cats and herons are infamous for causing injuries and many Koi-keepers will know the damage that a mass spawning can do to the skin of the most stunning female. However, poor water conditions can cause a pink flush to the skin; nitrite for example, causes reddening and can give the skin a soapy appearance as well as putting Koi at risk.  Little attention is given to what acid rain and other airborne pollutants can inflict on Koi by upsetting the balance in the pond and triggering health problems. A minor abrasion affecting only one Koi may heal without any intervention. However, if a wound becomes deeper or wider a topical treatment is needed with an iodine based product or another Koi safe antiseptic to prevent bacteria accessing the body. A wound sealer is only required if the wound is exceptionally deep. In summer minor abrasions are less likely to need treatment as natural healing is faster at higher water temperatures. None the less, bacteria reproduce faster in warm conditions so injuries need to be closely monitored.  There is a big difference between a minor issue and a serious problem. One Koi with a minor abrasion does not automatically mean the fish is bacterially infected and needs an antibiotic. Skin stores substances that support healing and immune system rushes to the site of any injury so nature only needs a helping hand occasionally.  The purpose of an antibiotic is to kill bacteria not to heal every type of skin lesion that Koi may develop.  


Wounds ulcers and antibiotics

Once the body is breached, the disease circulates internally and antibiotics will be required so fast action is the most effective control measure. If antibiotics are needed, it is important to inject the one sensitive to the infection and to apply the right dose at the appropriate frequency or the disease will continue. If the treatment is inappropriate, the ulcer may heal only to break out again later. All fish with any sign of infection should be treated or this type of disease re-occurs. Preventing disease is far more effective than treatment and resorting to antibiotics is in reality failing to manage the pond hygienically.


Koi keepers are naturally concerned about the underlying cause of various lumps and bumps that emerge on the skin of their Koi.  Whilst tumours are often non-malignant they can be unsightly and an investigation is required to see if treatment is possible. In many cases a biopsy is carried out and hospitalisation is required. Cysts and tumours of the reproductive system are more common than others in Koi and often cause protrusions once advanced. Whilst male Koi can be affected cases are more frequently in females.  Some skin lesions can be removed by cryogenics and other surgical techniques that require little invasion of the body. However, any form is surgery is not advisable if a tumour is more deep seated than is obvious externally. All procedures should be carried out professionally as removing uninvestigated growths can spread disease around the pond. In addition, the fish undergoing treatment is at risk of secondary disease once in recovery and maintaining the balance of body fluids adequately during and after surgery is yet another concern.  All skin damage irrespective of whether it is the result of surgery or injury or disease is likely to put the kidneys under pressure and it is this fact that makes some surgery in fish less successful.  My laboratory is pleased to discuss this type of health problem and its treatment with any concerned hobbyist. However, whilst minor skin lesions can be removed with little risk we often advise allowing Koi with internal growths to live their lives able to swim and feed normally without intervention as it is quality of life that counts.


Spots and cysts  

Skin abnormalities are difficult to discuss with Koi keepers without an examination of affected Koi. The conversation invariably involves the words white and spot yet the parasite white spot is rarely the problem. White growths or cysts in a variety of shapes and sizes appear on the skin during various diseases or conditions and are mistaken for the parasite we know as white spot. In addition such growths are suspected of being due various disease and conditions that are either extremely rare or Koi cannot contract them at all. Changes in the mucus can create white patches and parasites are a possible trigger. In the early stages of certain diseases a white film soon looks more like large spots as the mucus is shed and leaves the body and this is often interpreted for white-spot. Fungal infections can also resemble spots when observed through the water. A classic example of yet another white growth are the  glossy white tumours caused by the viral disease Cyprinid Herpes Virus  or CHV commonly called Carp pox. These are often misdiagnosed as White-spot. Carp pox concerns hobbyists as the tumours are unsightly but removal is not advisable as it spreads the disease and opens the skin to infection. Most cases clear up in warm conditions although stubborn Carp pox growths can occur in Koi never exposed to the virus as juveniles. It can take several winters for the antibodies to develop that allow the Koi to resist the virus. When cases persist for many years it is possible that a papilloma has formed and whilst this can be removed a blood test is required to investigate the status of the CHV virus.
















Dropsy is also observed during certain diseases most commonly bacterial infection. The myth that no Koi with this condition can ever be saved is not the case. Although many Koi do not survive dropsy it is delay in taking any action and applying the appropriate treatment given that dropsy has many causes that is the problem.  Immediate action and the right treatment can save some unfortunate Koi. Fish with dropsy need to be living in salt before any other chemical is used or administered. The fish may need antibiotics when the cause is bacterial in origin although this will not help in cases of dropsy that have other causes.  However, it is vital that the fish is able to excrete stored fluids before injecting the fish with a broad-spectrum antibiotic as the bacterial strains are involved will be unknown. The salt solution should be at ½ an ounce per gallon initially and after 12 hours increased to 1 ounce per gallon. The fish will excrete a lot of fluid and water changes are essential. Additional oxygen is also vital for all fish with dropsy as there will not be sufficient in circulation in the bloodstream.  The use of salt in the short term during a serious health problem utilises its medicinal properties. However, in the long term it alters the fluid balance of the body and can also dull the skin by flattening the colour cells so in Koi salt is only for short term use until the fish improves.

Rather like the tiles on a roof the scales in Koi overlay each other to afford the skin underneath protection. Scales vary amongst fish species but are termed cycloid in Koi and are composed of material resembling extremely fine bone. Each scale is embedded in a scale pocket that is located in the dermis. The arrangement of the scales allows flexibility and far greater movement than a shell or other denser material would. Many animals with a solid type of outer protection lack the great manoeuvrability that Koi possess. Just watch a Koi negotiate an object in the pond or turn at an acute angle. The subtle adjustment in the position of the scales is barely perceptible and involves only those scales where the body will bend or flex .

When several Koi all have skin damage at the same time, bacterial disease is more likely than injury. It would be unusual for many Koi to be injured simultaneously on the same sharp object, Koi are too clever for that. There are various types of bacterial disease that require different treatment although skin ulceration is the most common. An iodine based antiseptic or other product made for Koi should be applied to the ulcers and a bactericide such as Chloramine T used in the pond. This treatment should be carried out immediately the infection is observed to stop bacteria entering the body. 

Koi suffering from dropsy become bloated and the scales are raised over most of the body.   Ultimately the fish resembles a pine cone and in many cases the eyes also protrude.  Despite the myths that circulate dropsy is not a disease it is a sign that the fish is storing body fluid and this has various causes. It is often seen in spring when the kidneys are not coping with water temperature fluctuations or when the pond conditions are poor and the Koi often recover by the introduction of salt to the pond at ½ an ounce per gallon.

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