Dermocystidium Koi Articles on the rarer diseases in koi are for interest and should not concern koi keepers. It is far more practical for hobbyists to expand their knowledge in respect of water chemistry, pond and filter hygiene, and the more common koi parasites as these are the problems koi keepers are more likely to experience, although with good pond management these are avoidable. The cysts Dermocystidium is a skin disease that can be contracted by many species of fish. However, species are always named to make differentiation clear as the disease does not present identically in every fish species. This article refers only to Dermocystidium koi. During outbreaks of the disease, cysts erupt through the skin anywhere on the body or fins and the number of cysts can vary from 3 - 4 and even 20 or more. The cysts are oval rather than round and are not perfectly shaped. They are pink in colour and create only a little inflammation in the skin layer known as the dermis as they emerge through it. The cysts initially become visible as they are breaking through the skin, and are then approximately 1 mm. They grow in size until, ultimately, they rupture releasing infectious spores into the water to seek a new host fish.  The size of each cyst at this stage varies although they seldom rupture at less than 5 – 6 mm in size. A few remain intact and in the skin and can reach 10 mm although larger cysts are rare in koi. During an outbreak most cysts are an average size although occasionally one or two will outgrow the rest.  The cysts contain spherical shaped spores that produce hyphae and create a network of fine stranded material. Internally the contents of each spore are a tightly packed 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 disease Dermocystidium koi is infectious although this depends on the ingestion of a ruptured cyst and, while this could be regarded as low risk, the isolation of infected koi is the safest way to protect the rest of the koi. When one fish is observed with a cyst, the infectious nature would not be realised initially and this allows further cases to develop.  Although the emerging cysts break the skin, it is possible for this to heal with no trace of a scar. Those koi minimally infected with no indication of secondary infection can recover without treatment. They should be left in isolation if this is possible until they are fully restored to health and eating normally.  Full recovery can take many weeks and survivors can remain vulnerable.  However, Dermocystidium koi proves fatal in many cases and survival is far less likely when there are numerous cysts. As the cysts rupture, they leave a crater-like hole in the skin and some can be deeper than others. Secondary infection is the main problem in any disease that opens the skin to invasion by bacteria, fungi, and other pathogens.  The cysts should be allowed to erupt naturally and not be removed surgically as this can complicate the recovery process. Treatment issues Treatment can be tried with expert guidance and, until this is available, infected koi can be housed in acriflavine in conjunction with salt at 0.3% (see table below) in order to reduce secondary infection. Topical treatment of the hole the cyst creates should be avoided until advice is sought as cases can vary significantly.  If a fish becomes lethargic and the skin red in colour survival is very unlikely irrespective of any medication.  Dermocystidium koi normally occurs in spring although this much depends on living conditions primarily the water temperature and, naturally, heated ponds will influence certain diseases.  There is no way of avoiding this type of disease in koi as it can be carried sub-clinically for months and the conditions during quarantine may not have provided the appropriate criteria for the disease to develop.  It is occasionally seen in fish farms due to the numbers of koi stocked and the greater potential for the spread of disease.  In farms, obviously the mortality level will be high as treatment is not practical. Caution When a growth of any kind appears on a koi, it can initiate a search for the cause and invariably some guesswork takes place.  Photographs cannot actually diagnose any disease in fish as that requires an examination. Dermocystidium koi is misdiagnosed in more cases than it is ever confirmed. LFH Laboratories can be contacted by anyone concerned about this disease. July 2020 The area of inflamation around each cyst is slight Crater-like lesions after cyst has ruptured A healing lesion following Dermocystiduim Koi