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Dermocystidium Koi
The disease
Dermocystidium is the name applied to a skin disease that affects many fish species. However, due to the variations in the way this disease both presents and behaves during outbreaks, the name of the species is always used to establish the fact that all references are purely to the disease in that fish species alone and not to dermocystidium in other fish. Dermocystidium Koi which I will term D K for ease of reading is extremely rare in Koi. However, in the few cases I see every year, it proves to be a very an alarming disease when hobbyists see their Koi suffer an outbreak. The Koi keeper’s concerns are usually, “will my Koi survive and if so is permanent disfigurement likely following this type of disease”. In addition, the two questions all hobbyists ask during any disease outbreak are “will all the Koi in the pond contract the disease and can it be treated”.
The cysts
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A spore of Dermocystidium Koi
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Inside the Dermocystidium Koi spore
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During D K outbreaks, cysts erupt through the skin anywhere on the body or fins and the number of cysts can vary from 3 - 4 up to cases of 20 or more. The growths are oval rather than round but are not a perfect shape. Pink in colour, they create only slight inflammation in the dermis as they emerge through it. When the cysts initially become visible breaking through the skin, they are usually around 1 mm. They grow in size until ultimately they rupture releasing infectious spores they into the water to then seek a new host fish. The size of each cyst can vary but they seldom rupture at less than 6 mm in size. However, few remain intact to reach 10 mm and cysts over that size are extremely rare and usually due to two individual lesions that appear as one. In outbreaks of D K, it is common for most cysts to be of average size with only one or two that outgrow the rest but there is no rigid pattern to the disease.
The cysts contain spherical spores that produce hyphae that weave a network of fine stranded material .Whilst visually the disease has the properties of a fungal infection, scientific classification as a fungus has been debated for many years and is still unresolved. Diseases caused by fungi are common in fish but Koi-keepers should not confuse D K with the more common fungal infection saprolegnia that invades the skin of Koi if an entry portal has been created in the skin by injury, parasitic attack, or disease and is therefore a secondary disease. When viewed microscopically, the contents of each D K spore is a tightly packaged unit comprised of a vacuole or fluid sac so compressed that the granular material and the nucleus or powerhouse for the spore that keeps it active and reproducing has to live at the outer edge of the cell in the cytoplasm that surrounds the vacuole.
The facts
Although the cross-infective, risk of this disease is low and involves ingestion of a previously released cyst. Isolation of affected fish to limit the spread of the disease is the normal policy. Usually only one or two Koi are unlucky enough to be affected at any one time but further outbreaks can occur later in other fish. D K normally occurs in spring but has been known at other times of year depending on temperature related factors and the increase in numbers of heated ponds which is influencing the disease.
Although the skin is broken by the emerging cyst, it can heal with no trace of a scar. Infected Koi may recover totally without any treatment or intervention by the hobbyist. However, secondary infections are very common in any disease that opens the skin to invasion by bacteria and fungus. In fact, any disease-causing agent can gain access to the body when the skin is open and we term the site for this the entry portal. Antibiotics and topical treatments are often required to help Koi to recover from any skin disease. In the case of D K, the ruptured cyst leaves a deep hole best described as a crater in the skin although the fins can also be affected. Given the probability of several cysts over the body, the potential for secondary disease is high. D K can prove fatal for Koi in which secondary problems occur and full recovery in my experience is much less likely the more cysts an individual Koi develops. In addition, the areas of the skin in which the cysts emerge also play a part in the chances of survival. Due to the risk of secondary disease, removing the cysts surgically is not a serious option.
The treatment
Fish infected by D K can be housed in a mild bactericidal product that can safely be used in conjunction with salt in order to reduce the secondary risks of fungus and bacteria into the lesions and this can lower the level of cross-infection. Topical treatment can be applied after the cyst has ruptured but this can be discontinued once the crater or hole the cyst creates develops a slight gloss that indicates that healing has begun. Secondary bacterial infection may require antibiotics. The recovery period is temperature related and therefore it is not possible to be accurate about how long this will take but weeks, rather than days, should be anticipated.
The recovery
Koi with a low level of cysts suffering very little secondary complications usually make a complete recovery. Occasionally a few Koi fail to do so and my assumption is that there may be other aspects of the disease in some fish. A sudden rather than gradual decline in Koi that appeared to have coped well with D K has been observed repeatedly in outbreaks and seem to affect certain Koi varieties more intensely. Initially they seem to recover quickly and respond to treatment only to succumb to unknown factors long after the disease appears to be over. This tendency involving only two varieties suggests that genetics are at work. It may be that the parent fish have passed on to future generations of Koi a predisposition to secondary complications that are occurring sub-clinically or their offspring have inherited a weak immune response or even a low threshold for the toleration of chemicals. It is amazing how health problems can vary in different Koi varieties and there will always be Koi who fail to cope with disease or its treatment that all other fish in the pond survive.
The risk is low
There is no way of avoiding this type of disease in your Koi pond as it can be carried sub-clinically for some time and quarantine may not reveal it. It is naturally more common in fish farms due to the numbers of Koi stocked and the far greater potential for the spread of disease. In farms, obviously the mortality level will be higher as treatment is not usually practical even if it was available.
A rare disease
Articles on rare or unusual health problems are for interest not to concern the hobbyist. I see many Koi with skin cysts or growths that have other causes yet are easily mistaken for Dermocystidium Koi which is a disease that is extremely unlikely to afflict the average Koi pond. Poor water quality, a lack of pond and filter hygiene and overstocking increase the risks posed by parasites, viruses, fungi, and bacteria. All these factors pose a far greater threat to life in the Koi pond than any disease ever will and are still the areas in which hobbyists should expand their knowledge.
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